EPA 340/1-76-005
JUNE 1976
Stationary Source Enforcement Series
APPLICATION OF
REMOTE TECHNIQUES IN
STATIONARY SOURCE
AIR EMISSION MONITORING
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Enforcement
Office of General Enforcement
Washington, D. C. 20460
-------
APPLICATION OF REMOTE TECHNIQUES
IN STATIONARY SOURCE AIR
EMISSION MONITORING
by
C. B. Ludwig and M. Griggs
SCIENCE APPLICATIONS, INC.
La Jolla, California 92037
Contract No. EPA 68-03-2137
Project Officers: Dr. F. J. Biros
Office of Enforcement
Dr. S. H. Melfi
Environmental Monitoring and
Support Laboratory/Las Vegas
Prepared for:
U. S. Environmental Protection Agency
Washington, D. C. 20460
June 1976
-------
EPA REVIEW NOTICE
This report has been reviewed by the Environmental Protection Agency
and approved for publication with some modification. Approval does not
signify that the contents necessarily reflect the views and policies of the
Agency, nor does mention of trade names or commercial products con-
stitute endorsement or recommendation for use.
The Stationary Source Enforcement series of reports is issued by the
Office of Enforcement, Environmental Protection Agency, to assist the
Regional Offices in activities related to enforcement of implementation
plans, new source emission standards, and hazardous emission standards
to be developed under the Clean Air Act. Copies of Stationary Source
Enforcement reports are available—as supplies permit—from Air Pollution
Technical Information Center, Environmental Protection Agency, Research
Triangle Park, North Carolina 27711, or may be obtained, for a nominal
cost, from the National Technical Information Service, 5285 Port Royal
Road, Springfield, Virginia 22161.
11
-------
FOREWORD
This report documents the work performed under Contract No.
EPA 68-03-2137 between September 1974 and March 1975, and updated
during April/May 1976. The work was sponsored by the Environmental
Protection Agency, Office of Enforcement. The objective of this program
was to assess the application of remote techniques to monitor stationary
source air emissions.
The authors wish to thank the many researchers who have pro-
vided valuable information about on-going programs, practical experience
in the field and have given freely their opinions about the role of remote
monitoring techniques. The authors are also indebted to their co-workers
at Science Applications, Inc., W. Malkmus, J. Myer, E. R. Bartle,
L. Acton and G. Hall for contributions in the different topics, and to
J. Wang for many calculations. In particular we wish to thank K, Arnone
for her dedication in preparing the manuscript.
in
-------
CONTENTS
EXECUTIVE SUMMARY
Page
1. 1 Background and Scope 1
1. 2 Performance Specifications for Remote Monitoring 3
1. 3 Present Development and Analysis of Remote
Monitoring Techniques 4
1.4 Advantages and Disadvantages 7
1. 5 Major Conclusion and Recommendations 9
INTRODUCTION 11
OVERVIEW OF THE REQUIREMENTS FOR: THE PER-
FORMANCE SPECIFICATIONS OF REMOTE MONITORS 15
3. 1 Standards of Performance for New
Stationary Sources 16
3.2 National Emission Standards for
Hazardous Air Pollutants . . 25
3. 3 National Ambient Air Quality Standards 26
3. 4 Outline of Performance and Test Procedures
for the Use of Remote Monitors 34
PRESENT DEVELOPMENT OF REMOTE MONITORING
TECHNIQUES 39
4. 1 Overview 39
4.2 Theory 43
4. 2. 1 CW Infrared System 43
4. 2.2 Pulsed Laser Systems 48
4. 2. 3 Perimeter Monitoring 53
4.3 Review of Active Systems 54
4. 3. 1 Differential Absorption 54
4. 3. 2 Raman Scattering 57
4. 3. 3 Resonance Raman 61
-------
4. 3. 4 Fluorescence 63
4.3.5 Lidar 64
4. 3. 6 Intercomparison of Measurements of Gases 68
4. 3. 7 Stack Effluent Velocity Measurements 70
4.4 Review of Passive Systems 71
4.4.1 Passive Opacity Techniques 72
4.4.2 Matched-Filter Spectrometer 74
4. 4. 3 Gas Filter Correlation 77
4. 4. 4 Medium Resolution Dispersive Spectro-
meter System 81
4.4.5 Interferometer-Spectrometer 83
4.4.6 Filter Wheel Sensor ;' 89
4. 4. 7 Laser Heterodyne Technique 89
4. 4. 8 Dispersive Hadamard Transform
Spectrometer 95
4. 4. 9 Passive Vidicon Instrumentation 97
4.5 Area Surveys . 97
4.6 On-Going and Planned Research Programs 101
PRACTICAL CONSIDERATIONS ; 105
5. 1 Overview 105
5.2 Eye Safety Hazards from Laser Systems 106
5. 2. 1 The Maximum Permissible Exposure
and Minimum Safety Range 107
5. 2.2 Required Modifications 108
5. 2. 3 Final System Parameters 109
5. 3 Error Analysis of Active Systems 111
5. 3. 1 DAS Systems . Ill
5.3.2 Raman Systems 114
5. 3. 3 Lidar Systems 116
5. 3. 4 Intercomparison of Laser Techniques 118
5. 4 Error Analysis of Passive Infrared .Systems 126
5.4.1 Source Strength 126
5.4. 2 The Measurements of Signal Differences 129
5. 4. 3 Perimeter Monitoring Data Analysis
(Infrared) 134
VI
-------
6 ADVANTAGES AND DISADVANTAGES OF REMOTE
MONITORING TECHNIQUES 135
6. 1 Overview 135
6.2 Cost Effectiveness 136
6. 2.1 Opacity 137
6. 2. 2 Gas Concentration 138
6. 3 Measurements of Mass Emission Rates 139
6.4 Unannounced and Non-Interfering Monitoring 139
6. 5 Survey of Wide Geographical Areas 140
6.6 Rapid Response in Pollution Episodes 141
6. 7 Limitation Under Certain Atmospheric Conditions 143
6.8 Increased Requirements in Calibration 143
6.9 Safety Hazard 143
7 CONCLUSIONS AND RECOMMENDATIONS 145
7.1 General Conclusions 145
7.2 Recommendations 149
8 REFERENCES 165
VH
-------
1
EXECUTIVE SUMMARY
1.1 BACKGROUND AND SCOPE
The Clean Air Act of 1967 authorized the development of mea-
surement methods to meet the requirements of reducing air pollution
to levels considered safe for human health and welfare. During the
last decade, many of the required techniques for measuring stack
emissions and ambient air quality were developed by Federal agencies
and private industry. The techniques were based on many methodologies,
such as manual, automatic, extractive, point or integrated sampling,
and remote (visual observations). The purpose of the present study is
to evaluate the status of remote monitoring instruments (i. e_., not visual
observations) and to provide a working document for the different regional
offices and laboratories of EPA and state agencies. The intent is that
this document may serve as a guide for the application of remote moni-
toring techniques in the following areas:
• Surveillance and compliance monitoring of smoke-
stacks and extended sources;
• Monitoring to support EPA's research studies for
the evaluation of control equipment and development
of performance standards;
• Surveys of emissions for validation of idispersion
models;
• Surveys to determine the representativeness of
ambient air point measurements and to assist in
developing optimum contact monitoring networks;
and
• Quick response in air pollution episodes.
-------
In the present study remote monitoring is defined as sensing, by an
electro-optical technique, specific chemical and/or physical parameters
of the environment where the monitoring instrument and the parameter
under investigation are separated by a distance. At present, EPA, NASA,
NOAA, DOT and other federal agencies are sponsbringThe"development of
instruments and/or techniques to remotely monitor the environment,
utilizing active or passive systems. Active systems consist of two
basic units: a transmitter and a receiver, while passive systems
consist only of a receiver. The transmitter in an active system emits
a beam of energy, which interacts with the plume/atmosphere by
scattering, absorption and/or stimulated emission,', which is subsequently
observed by the receiver. In the passive system, the receiver merely
observes the radiation from the plume/atmosphere; which may be emitted
thermal radiation, or scattered solar radiation.
Several modes in the application of remote monitoring were addressed
(see Figure 1-1): direct observation of the plume by passive or active
monitors, perimeter monitoring by uplooking from. van-based platforms
PaHlve/Actlve
« Area Monitoring
Passive/Active
Monitoring \
(Perimeter)
Passive/Active
Monitoring
(Perimeter)
1-1. Modes of active and passive remote techniques in
monitoring emissions from stationary sources:
Direct observation of stack plume, perimeter
monitoring (ground and airborne) and area monitoring.
-------
or down-looking from airborne platforms, and horizontally by active long-
path systems, although the latter application is limited to cases where
the horizontal long-path is representative of a given situation (Section 4. 5).
In addition, area monitoring using an aircraft, can be used in surveillance.
These surveys of wider geographical areas can also assist in the deter-
mination of environmental quality degradation, plant site selection, the
design of contact monitoring networks, and the tracking of plumes to study
atmospheric dispersion, diffusion and fate of pollutants.
1. 2 PERFORMANCE SPECIFICATIONS
FOR REMOTE MONITORING
The introduction of remote monitors as tools in EPA's enforcement
programs requires the adaptation of performance specifications and test
procedures for these instruments. We have reviewed the codified re-
ference methods, test methods, measurement principles and performance
specifications as listed in the appendices of Parts 50, 51, 53, 60 and 61
of Title 40 of the Code of Federal Regulations, as to their adaptability to
remote monitors (Section 3. 0). We concluded that the performance speci-
fications as presently given in the Appendices A and B of Part 60 for the
continuous monitors can be adapted to the remote monitors, but that the
calibration test procedures require major revisions. This is so because
the interferences of the intervening atmosphere and of the sky background
are significant, and provisions during calibration testing must be made to
simulate different meteorological conditions. This simulation is judged
to be important since the different remote monitors react differently to
changes in meteorological conditions. Some of the provisions could pro-
bably be patterned after those contained in Reference Method 9 for the
.visual determinations of the opacity of emissions from stationary sources.
-------
1. 3 PRESENT DEVELOPMENT AND ANALYSIS OF
REMOTE MONITORING TECHNIQUES
A review of the recent development of remote monitors sponsored
by Federal and other agencies, as well as by private industry, both in the
U. S. A. and other countries, revealed that several'active and passive
techniques were worthy of further analysis. The active techniques include
the pulsed laser systems that involve differential absorption, Raman,
resonance Raman, fluorescence, and ;lidar, as well as the continuous
wave (CW) systems that utilize laser, dispersive and non-dispersive com-
ponents (Section 4. 3). The passive techniques include dispersive and non-
dispersive correlation systems, spectrometers, interferometer-spectro-
meters, radiometers, heterodyne radiometers, photography and vidicons
(Section 4.4). Overviews of these techniques and their main characteristics
are given in Tables 1-1 and 1-2, together with their present status. It
can be seen that for all the remote techniques judged to be useful for
monitoring stack emissions, only two actual systems are commercially
available and even they have limited applications.
In general, we conclude that the active syste.ms have been demon-
strated to be more sensitive than the passive systems. In addition, the
passive systems are more influenced by interfering parameters, such
as background radiation. On the other hand, passive systems are less
expensive than the active systems and can be made.into cost-effective
tools for compliance monitoring, as long as the measured deviation
from the standard in pollutant concentration is larger than the instru-
ment errors.
i
The air pollutants (previously identified as important to monitor)
that are judged to be amenable to remote monitoring in the near-term
include particles/opacity, SO2, NO2, CO, light hydrocarbons, HC1, HF,
NH-j, NQx, H2S, HNO3, O3 and vinyl chloride; and in the long-term include
heavy hydrocarbons, oxides of sulfur, certain specific trace elements
(asbestos, beryllium, mercury), and chlorinated hydrocarbons. In
addition, many newly identified pollutants of interest can probably be
monitored by remote monitors, after their spectral characteristics are
identified.
-------
TABLE 1-1. Overview of Active Systems Under Development
Technique/ Spectral
Instrument Region
Differential Vis/UV
Absorption
IR
Lidar Vis
Laser Doppler IR
Velocimcter
Long-Path Vis/UV
/IR
Raman Vis/UV
Resonance Vis/UV
Raman
Fluorescence Vis/UV
Fabry-Perot Vis/UV
Raman
Species/
Parameter
SO2,NO2 \
S02, N02
°3
Many
Gases
Opacity
Particles
Velocity
Mass Flow
Many
Gases
so2
Many
Gases
Many
Gases
Many
Gases
Some
Gases
Mode
Stack/Area
Perimeter/
Area
Slack
Perimeter/'
Area
Stack
Area
Stack/
Perimeter
Stack
Area
Stack
Stack/
Area
Stack/
Area
Stack/
Area
Stack/
Area
Development Status
Prototype
Field Tested
Prototype
Field Tested
Prototype Under
Development
Under Development
for Aircraft
Application
Prototype
Field Tested
Prototype on Air-
craft Field Tested
Advanced Prototype
Field Tested
Demonstrated in
Field Tests
Several Techniques
Field Tested, Some
Prototypes Developed;
COSPEC Commer-
cially Available
Field Tests Not
Encouraging
Theoretical
Laboratory Study
Laboratory Study
Laboratory Study
Remarks
Ground-based - Present instrumentation
as used not eye-safe (Section 4.3. 1)
'Has been done for SO2 (Section 4. 3. 1)
Development is primarily for ambient
air (see Table 4-15)
'See Table 4-15, at present for ozone
Not eye safe yet, (Section 4. 3. 5) eye-
safe system being developed
Gives 3-dimensional mapping of relative
concentrations (Section 4.5)
Necessary for some emission standards
(Section 4. 3. 7 and Table 3-1)
Relates opacity and mass concentration
(Section 4. 3. 7)
Using remote transmitter or retro-
reflector; can be laser, dispersive or
non-dispersive systems (Table 3- 1C):
useful mainly for ambient air monitoring
Limited In range, especially during dav
(Sections 4. 3. 2 and 5. 3. 2)
Usefulness limited (Section 4. 3. 2 and
5.3.2)
Needs to be demonstrated in field:
possible interference due to fluorescence
by gases and other species (Section 4. 3. 3)
Looks doubtful in terms of sensitivity
and specificity; (Section 4. 3. 4)
Provides increased sensitivity over
vibrational Raman: still limited in range.
especially during day (Section 4. 3. 2)
-------
TABLE 1-2. Overview of Passive Systems Under Development
Technique/
Instrument
Spectral Species/
Region Parameter
Mode
Development Status
Remarks
Matched Filter UV/Vis SO,, NO,
Correlation
Gas Filter
Correlation
IR
Photography Vis
Vidicon UV
IR
Heterodyne IR
Radiometer
Dispersive IR
Spectrometer
so2
CO
so2
Opacity
so2
so2
Many
Gases
Many
Gases
Stack/ COSPI1C Commercially
Perimeter Available, Field Tested,
Not Encouraging for
Stacks; Main Emphasis
on Perimeter Monitoring
Quantitative interpretation difficult due
to varying aerosols (Section 4. 4. 2 and
4.5)
JRB Sensor Commercially Limited in concentration and tempera-
Available, Being Field ture range (Section 4.4.3), but tom-
perature effect reduced
No quantitative data reported as yet
(Section 4. 5)
See Table 4-14
Needs further development for quantita-
tive analysis; nighttime observations
feasible with image intensifies (Section
4.4.1)
'Quantitative interpretation difficult due
to varying aerosols: has potential as a
velocimeter (Section 4. 4. 9)
Independent knowledge of plume tempera-
ture required for quantitative analysis:
has potential as a velocimeter (Section
4.4.9)
Achieves high specificity: has yet to be
demonstrated in field (Section 4. 4. 7)
Includes scanning spectrometer and in-
terferometer-spectrometer; requires
high spectral resolution for specifirity
and requires knowledge of plume tem-
perature (Section 4.4. 4 and 4.4. 5)
Stack
Perimeter Prototype Field Tested
Perimeter Prototype Under
Development
Stack Being Field Tested on
Ground and From
Aircraft
Stack Prototype field Tested
Stack Prototype Field Tested
Stack/ Laboratory Study and
Perimeter/ Aircraft Based Proto-
Area type Under Develop-
ment
Stack Several Techniques
Field Tested, Some
Prototypes Developed
-------
1.4 ADVANTAGES AND DISADVANTAGES
Based upon the review of the remote monitoring techniques, we
have identified the following advantages which apply in varying degrees
to the different enforcement and research and development programs.
• Cost Effectiveness
Although the initial capital costs are higher for remote instruments
than for point samplers, the operational costs are less because of
their mobility allowing rapid coverage of more sources and areas.
Our estimate is that for some remote monitoring applications the
operational costs are less by as much as about a factor of 10.
Remote techniques provide also a cost-effective method to monitor
and survey wide geographical areas for the purpose of assisting in
the determination of environmental quality degradation, plant site
selection, design of contact monitoring networks, and in tracking
plumes.
• Measurements of Mass Emission Rates
!
The signals of the laser Doppler velocimeter (LDV) are not only
related to the particle velocity, but also to the opacity of the plume.
When the relationship between the opacity and mass loading for
each type of source has been established, the LDV will provide
the particulate mass emission rate from a source.
• Unannounced and Non-Interfering Monitoring
The remote technique provides a most effective tool for compliance
monitoring, even at night, without requiring entry into the facility
premises. In addition, remote monitoring does not interfere with
the normal plant operations.
• Rapid Response
In cases of air pollution episodes, the highly mobile and flexible
remote monitors can be used to assess the extent, trend, and
required response to counteractions more rapidly than the stationary
in-situ sensors.
-------
The following disadvantages have been identified:
Possible High Initial Capital Costs
At the present time, the purchase price of ail remote instruments
are more expensive than the extractive or in-situ devices. The
active systems are more so than the passive systems. However,
as more and more instruments are built, one can assume that the
purchase price will come down.
Limited Applications Under Certain Conditions
Adverse weather conditions such as fog, heavy rain or extremely
high participate content in the atmosphere can affect the measure-
ment capability of the remote techniques.
Calibration Procedures More Complicated
Indications are that the remote techniques will be more difficult
to calibrate than the extractive or in-situ devices because of the
atmospheric influence. It is anticipated that—similar to the cali-
brated smokestack used in smoke schools—a test range with a remote
stack source must be provided in which several atmospheric con-
ditions can be reliably simulated.
Possible Eye Safety Hazard
Caution must be exercised in using lasers so that they comply
with the Federal Regulations. Thus, some of the laser applications
which demand an increase in emitted power in order to meet the
sensitivity and range requirements, had to be eliminated from the
list of useful systems.
-------
1. 5 MAJOR CONCLUSION AND RECOMMENDATIONS
Based upon our review and analysis of the.remote techniques being
developed, our major conclusion is that remote monitoring can play a
significant role in EPA's enforcement and R&D activities. However, our
interaction with the private sector has led us to the conclusion that ade-
quate incentives do not exist for the instrument companies to develop many
of the promising remote monitors any further, since the costs to achieve
a production field instrument are too high and the potential market is too small.
Thus, it is necessary for EPA to provide increased funding for the develop-
ment of field instruments for use by the regional offices and state agencies.
Our analysis shows that not all techniques/instruments presently
developed are equally well suited for monitoring of smokestack emissions.
For direct observation of hot plumes and indirect observation of complex
sources through perimeter monitoring, the less expensive passive systems
will suffice, while the direct observation of cool plumes will require the
application of the more expensive active laser systems. The laser systems
that are most promising for near-term operational use are differential
absorption, lidar and laser Doppler velocimeter. The most promising
passive systems are correlation instruments, vidicons and aircraft photo-
graphic techniques. For area surveys, both active and passive systems
(ground-based and airborne) have application.
Based upon the above conclusion, we make the following recommenda-
tions:
• Increase the support for the development of remote techniques.
In particular, the following tasks are judged to be of about equal
priority:
--make the most promising active systems (differential absorption,
Lidar and laser Doppler velocimeter) operational for van-based
and airborne applications for the gaseous pollutants SO2 and NOj,
and particulate pollutants,
—make the most promising passive systems (correlation instruments,
and vidicons) operational for smokestack monitoring for the gaseous
pollutants SO2 and NO2,
--continue to develop the aircraft photographic technique for opacity
measurements,
—extend the active differential absorption techniques to other species,
including hazardous air pollutants utilizing the UV and IR spectra,
--continue the development of heterodyne radiometry,
--extend the passive correlation instruments to other species,
--continue passive perimeter monitoring;
9
ITI
-------
Continue—with somewhat less priority than the above—the
development of techniques whose practical applications have
yet to be demonstrated, in particular, resonance Raman
and fluorescence;
Consider the establishment of one or more test ranges, in which
remote monitors can be calibrated;
Adapt existing performance specifications of continuous monitors
to remote monitoring application and initiate the formulation of
additional test procedures required in the application of remote
monitors.
10
-------
2
INTRODUCTION
Presently, EPA, NASA, NOAA and other federal agencies are
sponsoring the development of remote monitoring instruments for sensing
air quality. Remote monitoring, for the purpose of this document, is
defined as sensing a specific chemical and/or physical parameter of the
environment where the monitoring instrument and the parameter under
investigation are separated by a distance. Some of these instruments
have been developed to the point where they are presently useful to EPA
in its monitoring program; other instruments have not been developed
to this stage.
The purpose of the present study is to evaluate the status of
remote monitoring techniques and to provide a working document for
the different offices and laboratories of EPA and state agencies. It
is the intent that this document may serve as a guide for the application
of remote monitoring techniques in (1) surveillance and compliance
monitoring of smokestacks and extended sources; (2) monitoring to
support EPA's research studies for the evaluation of control equipment
and development of performance standards; (3) surveys of emissions
for input to dispersion models and their validation; (4) surveys to
determine the representativeness of ambient air point measurements
and. to assist in developing optimum contact monitoring networks; and
(5) quick response in air pollution episodes.
The Clean Air Act (42 U. S. C. 1857) of 1963 encouraged state,
regional and local regulatory control programs by providing matching
funds for research and technical assistance, and at the same time, gave
authority to the Federal Government to intervene in enforcing the abate-
ment of air pollution in interstate problem areas. The specific abate-
ment procedures included—a) conference with the cognizant official
agencies, b) public hearing, and c) court action. These procedures
could take up several years, involving complex procedures to prove
non-compliance.
11
-------
The Clean Air Act was amended in 1967 to establish air quality
control regions for the purpose of setting ambient air quality standards,
based upon criteria that reflect the latest scientific and medical know-
ledge of effects of air contaminants on health and welfare. It also called
for the development and issuance of information on recommended pollu-
tion control techniques, and the initiation of joint government-industry
research to develop and demonstrate improved emission control techno-
logy to the degree required to prevent and abate air pollution. It is
clear that monitoring devices had to be available for control assist.
The Clean Air Act as amended in 1970 extended the geographical
coverage of the Federal program for the prevention, control and abate-
ment of air pollution from both stationary and moving sources. In par-
ticular, emission regulations for environmental control were established.
It was also established that research and development for measurement
techniques of the following air pollutants should be conducted as listed
in Table 2-1.
TABLE 2-1. Pollutants to be Monitored (Ellison 1974)
1. Guides of suitor
1. Glides of nitrogen
(NO^ NO, NOj, HN03)
3. Paniculate matter (alze distribution
chemical composition)
4. Asbestos
5. Mercury
(. Beryllium
7. Carbon monoxide
6. Nonmethane hydrocarbons
(. Certain specific hydrocarbons
10. Pdychlorlnaled blphenyls
II. Polynucleir organic matter
12. HeacUm hydrocarbons
11. Hydrogen chloride
M. Manganese
IS. Selenium
U. Arsenic
IT. Phosphoric acid
18. Chlorine
19. Hydrogen fluoride
to. Hydrogen aiilfide
11. Mereaptaiu
li. Ammonia and amines
II. Organic acids
M. Aldehyde*
IS. Odor
la. Photochemical oxidants
17. Copper
18. Zinc
19. Boron
90. Tin
81. Lithium
11. Chromium
18. Vanadium
M. Cadmium
IS. Lead
M. Aeroallergens
12
-------
Basically, two regulatory parameters for each of these pollutants
need be established: an emission standard and a measurement method.
EPA is proceeding along these lines and standards and measurement
methods have been established for some of the above pollutants. A pro-
gress report about the status of these developments was written by Ellison
(1974). In the same report, a brief statement about the remote measure-
ment of air pollutants was made. Ellison states that "... the potential
effectiveness of air pollution measurement and monitoring by means of
these techniques justifies a reasonable effort to investigate their usefulness. "
The earliest method of remote sensing of smokestack plumes was
by way of visual observations, comparing the blackness of the plume with
a chart of graduated shades of blackness (Ringelmann, method). This method
was extended to non-black plumes by training observers to determine the
plume opacity. The opacity method was adopted by EPA and was promul-
gated as a reference method. The present study is not concerned with
visual observations of plumes, however.
The present study is concerned with remote sensing and monitoring
by means of instruments. One of the earliest instruments for monitoring
the atmosphere remotely was a modified searchlight as pioneered by
Hulburt (1946) and refined by Elterman (1966). By observing the back-
scattered signal, the concentration distribution of theiparticles could be
determined up to altitudes as high as 35 km. This system was double-
ended and not readily applicable to the monitoring of smokestack plumes.
However, with the advent of laser systems that could produce pulses of
short duration, the single-ended probing of plumes became feasible. The
method of using lidar (light detection and ranging) to determine plume
opacity was first developed at SRI TEvans 1967). About the same time, the
utilization of the movement of the particles for the measurement of the
velocity was realized (Foreman et al. 1965) and the development of the
LDV (Laser Doppler Velocimeter) was begun.
Efforts to measure gaseous constituents in the plume were intially
restricted to passive methods. The thermal emission of hot plumes pro-
vides a distinct source that can be observed by infrared sensors against
the cold sky background (Low and Clancy 1967). The. major problem with
the passive method is the fact that the emission is a function of both the
gaseous concentration as, well as the plume temperature. The difficulty
in remotely determining the plume ^temperature has prevented the early
application of the passive methods, and only now it appears that through
the application of various ratioing techniques, eliminating the temperature
dependency, has it become feasible to measure gaseous pollutants.
13
-------
The application of active systems to the measurement of gaseous
species in plumes was slow in developing. Although the theoretical
feasibility of utilizing scattering methods (Raman, resonant Raman and
fluorescence) was recognized rather early, the experimental demon-
stration was much more difficult than anticipated. The reasons for this
are the small Raman cross sections and/or the interference effects for
the resonant scattering methods. Many attempts have been made to
overcome the difficulties. The avenue of using more powerful lasers
to overcome the weak Raman signals has been restricted by stringent
eye safety rules, and the elimination of the interference effects (broad-
band fluorescence by the aerosols and some commonly occurring gaseous
pollutants such as nitrogen dioxide and hydrocarbons) has not been achieved
as yet.
In general, molecular absorption techniques do not have these
limitations; DAS (Differential Absorption by Scattering) systems are
being widely developed for several species in the visible and infrared.
The absorption cross sections are large, and the narrow band width of
laser lines minimizes the interference effects.
A discussion of the theory and state-of-the-art of the above-
mentioned active and passive methods is presented in this report,
together with an overview of the New Source Performance Standards
(NSPS), the National Emission Standards for Hazardous Air Pollutants
(NESHAPS) and the National Ambient Air Quality Standards (NAAQS).
14
-------
OVERVIEW OF THE REQUIREMENTS FOR THE
PERFORMANCE SPECIFICATIONS OF
REMOTE MONITORS
In this section, we review the basis for the performance speci-
fication and operating parameter for the use of remote monitors. For
the application to stack compliance monitoring, the specifications can
be based upon the ones proposed or promulgated by EPA for the contin-
uous emission monitors of stationary sources. For the application of
monitoring the ambient air quality, the specifications must be consistent
with the ones promulgated for the ambient air monitors.
In December 1971, the standards of performance for new sta-
tionary sources were codified (40 CFR Part 60) for five industries
together with several reference test methods to be used during the
performance tests to demonstrate compliance with the standards.
The reference methods may be altered if it can be demonstrated that
the alteration does not affect the results significantly: These methods
fall into the category of alternative methods. No equivalent methods
have yet been specified. The reference methods are for the in-stack
measurements of sample and velocity traverses, gas velocity and
volumetric flow rate, CO2 and excess air, moisture, .particles, SO2,
NOX, sulfuric acid mist and for the remote visual observations of the
opacity of emissions. Since that time, standards of performance for
nineteen more industries have oeen promulgated, together with additional
reference methods that apply to the in-stack measurements of CO, H2S,
SO2, and total fluorides.
EPA has promulgated in 1975 provisions for the continuous com-
pliance monitoring of some gases and opacity, together with performance
specifications and specification test procedures of instruments that monitor
those gases.
15
-------
National Ambient Air Quality Standards were promulgated in
1971 (40 CFR Part 50). Subsequently, the requirements for the prep-
aration, adoption and submittal of implementation plans, including
the requirements for emission monitoring of stationary sources
(40 CFR Part 51) and the approval and promulgation of the imple-
mentation plans (40 CFR Part 52) were codified.
In 1973, national emission standards for some hazardous air
pollutants were promulgated, together with several reference methods,
for the determination of emissions from stationary sources (40 CFR 61).
In February 1975, rules and regulations for ambient air monitoring
references methods and .equivalent methods were promulgated (40
CFR Part 53).
These rules establish the concept of measurement principle
and calibration procedure for continuous/automated analyzers that
measure certain air pollutants. Except for the visual observations
of emissions, no test methods for remote sensing.techniques have
yet been proposed for either the emissions from stationary sources
or the ambient air quality.
In the following subsections, we give first an overview of the
new source performance standards, including the performance specifi-
cations for continuous monitors (Section 3.1), the emission standards
for hazardous air pollutants (Section 3. 2), and the national primary and
secondary ambient air quality standards including the test procedures
that establish reference and equivalent methods (Section 3. 3). Finally
in Section 3. 4 we outline a set of general requirements for the use of
remote monitors in stack compliance monitoring.
3. 1 STANDARDS OF PERFORMANCE FOR
NEW STATIONARY SOURCES
In order to assess the sensitivity remote monitoring instruments
must have, we have listed the new source performance standards of 24
industrial plants in Table 3-1. It can be seen that the performance
standards for the opacity can be lower than 10 percent, and for the con-
centrations of SO2 and CO lower than 650 ppm and 550 ppm by volume,
respectively. All other standards are given in pounds per fuel or in
pounds per product, a measure which requires additional information
16
-------
TABLE 3-1. New Source Performance Standards
Plant- Status*
(1) Fossil fueled
steam generators Pm
(2) Incinerators Pm
(3) Portland Cement Pm
Plants
(4) Nitric Acid Plants Pm
(5) Sulfurir Acid Plants Pm
(G) Asphalt Cum-rC'li; Pm
Plants
(7) Petroleum Refineries Pm
(8) Storage Vessels for Pm
Petroleum Liquids
(9) Secondary Lead Pm
Smelters
(10) Secondary Brass Pm
and Bronze Ingot
Production Plant
(11) Iron and Steel Pm
Plant
(12) Sewage Treatment Pm
Plant
(13) Primary Copper Pm
Smelters
(14) Primary Zinc Pin
Smelters
(15) Primary Lead Pm
Smelt ITS
Ref.
40CFRGO ,
(Subpart D)
40CFR60
(Subpart E)
40CFR60
(Subpart F)
40CFR60
(Subpart G)
40CFR60
(Kubparl II)
40CKHGO
(Subparl 1)
40CFR60
(Subpart J)
40CFR60
(Subpart K)
40CFR60
(Subpart L)
40CFR60
(Subpart M)
40CFR60
(Subpart N)
40CFR60
(Subpart O)
41 FR
2338
(Subpart P)
41 FR
2340
(Subpart Q)
41 FK
2340
(Subpart R)
Standards
Opacity: 20ft, 40% max. for
2 rfiln. in any hour
SO2: 0. 8 lb/106BTU (liquid fuel)
1. 2 lb/106BTU (solid fuel)
NOX: 0. 2 lb/106BTU (gas fuel)
0. 3 lb/106BTU (liquid fuel)
0.7 lb/106BTU (solid fuel)
Particle: 0. 18 g/m3(corr. to 12% COg)
Opacity: 20% (kiln)
10% opacity
NO : 3 Ib/ton acid produced
10% opacity
Acid Mist: 0. 15 Ih/lon : 78 torr, vapor
recovery system required
Opacity: « 20% (blast furnace)
* 10% (pot furnace)
Opacity: s 20% (rcverbatory furnace)
s 10% (blast or electric furnace)
(excluding uncombined water)
Particles: * 50 mg/dscm
Opacity: ' 20% (except for uncombined
water)
Opacity: * 20%
SO2: * 650 ppm v
Opacity: « 20%
SO2: f G50 ppra v
Opacity: = 20%
: * 650 ppm v
17
-------
Plant
Status*
Rcf.
Standards
(16) Steel Plant: Pm
Electric Arc Furnaces •
40 FR
43853
(Subpart AA)
Opacity: * 3% (exit from a control
device)
* 0% (exit' from the shop)
* 20% (ex'.t from the shop
during charging a
furnace)
* 40% (exit from the shop
during tapping a
furnace)
* 10% (from dust handling
equipment)
(17) Ferro-alloy Pm . 40 FR Opacity: * 0% (except for 0% from
Production 18501 control system)
Facimies * 0% from control system
(after tapping at least
60% of each tapping
period)
* 10% (from dust handling
equipment)
CO: s 200,000 ppm v
(18) Primary Aluminum Pm
Reduction Plants
(19) Coal Preparation Pm
Plants
(20) Phosphate Fertilizer
Industry Wet Process
Phosphoric Acid
Plants
Pm
41 FR
3828
(Subpart S)
41 FR
2234
(Subpart Y)
40 FR
33154
(Subpart T)
Total fluorides: < 1 kg/ton of aluminum
produced
Opacity: * 10% (from pot room)
* 20% (from anode bake plant)
Opacity: * 20% from any thermal
dryer gases
* 10% fr.om any pneumatic
coal cleaning
equipment gases
' * 20% from any coal processing
and conveying equipment,
coal storage systems, or
coal transfer and loading
ByBtctr.s gases.
Total Fluorides: * 10 g/ton of
equivalent P20S
feed * 5
(21) Phosphate Fertilizer Pm
Industry: Super-
phosphoric Acid
Plants
(22) Phosphate Fertilizer Pm
Industry Diammonium
Phosphate Plants
(23) Phosphate Fertilizer Pm
Industry Triple Super-
phosphate Plants
(24) Phosphate Fertilizer
Industry: Gra.iular
Triple Superphosphate
Storage Facilities
Pm
40 FH
33155
(Subpart U)
40 FR
33155
(Subpart V)
40 FR
33156
(Subpart W)
40 FR
33156
(SubpailX)
Total Fluorides: * 5 g/ton
Total Fluorides: * 30 g/ton
Total Fluorides: * 100 g/ton
Total Fluorides: * 0.25 g/hr/ton
'of equivalent
PG stored
• Pm means promulgated anrf ]>p means proposed.
18
-------
to determine compliance. We find that the units "pounds per product"
or "pounds per fuel" pose problems in relating dire'ctly the quantities
obtained by remote sensors tp_the standards.
As a basis for the formulation of performance specifications
of the remote techniques for enforcement monitoring, we have reviewed
the continuous emission monitoring requirements and performance
testing methods codified in 40CFR60.
EPA has promulgated or proposed standards of performance for
twenty-four source categories. These standards include emission standards
for
• particulate matter
• sulfur dioxide
• nitrogen oxides
• sulfuric acid mist
• carbon monoxide
• hydrocarbons
• fluorides
They also include test methods and procedures, using reference methods
as given in Appendix A of 40CFR60. These reference methods include
Method 1 - Sample and Velocity Traverses
for Stationary Sources
2 - Determination of Stack Gas Velocity
and Volumetric Flow Rate
3 - Gas Analyzers for CQ2, Excess Air
and Dry Molecular Weight
4 - Determination of Moisture in Stack Gases
5 - Determination of Particulate Emissions
for Stationary Sources
6 - Determination of SO, Emissions from
Stationary Sources
7 - Determination of NOX Emissions from
Stationary Sources
8 - Determination of Sulfuric Acid Mist and
SO, Emissions from Stationary Sources
19
-------
9 - Visual Determination of the Opacity of
Emissions from Stationary Sources
10 - Determination of CO Emissions from
Stationary Sources ';
11 - Determination of H^S Emissions from
Stationary Sources
12 - Reserved
13 A - Determination of Total Fluoride Emissions
from Stationary Sources - SPADNS
Zirconium Lake Method
13 B - Determination of Total Fluoride Emissions
from Stationary Sources - Specific Ion
Electrode Method
14 - Determination of Fluoride Emissions of
Primary Aluminum Plants
In order that sources which have demonstrated compliance with
applicable standards during the initial performance tests remain in
compliance, provisions were included in 40CFR60 that require the in-
stallation of continuous monitors.
Initially EPA intended that the selection and use of continuous
instruments be in the form of instrument specifications and calibration
procedures, i. e., a formal EPA instrumentation certification program.
Once certified, an instrument from a particular product line of a given
instrument vendor could be installed in fulfillment of the monitoring
requirements for that source.
However, the "product line certification" approach was soon
considered undesirable because an instrument so certified may not
perform properly at the source due to
• unexpected or disregarded interferences
• improper installation
• undetected damages from handling.
It was realized that no one instrument model would be expected
to provide accurate results for each of the wide variety of (new) sources.
Therefore, a different approach has been adopted which requires the
-------
source owner to demonstrate at each source the capability of continuous
monitoring systems to meet the performance specifications for systems
which monitor
1. Opacity
2. SO0 and NO
ft X
3. 02
Specifications for systems which monitor
H2S
CO
will be proposed by EPA at a later date. The validity of these speci-
fications for the above systems has been evaluated in depth by extensive
testing programs at steam generating facilities. The technical support
for these specifications are contained in the report "Performance
Specifications for Stationary Source Monitoring Systems for Gases and
Visible Emissions" (EPA-650/2-74-013, January 1974).
The provisions specifying general continuous-1 emission monitoring
requirements (40CFR60, §60.13 and App. B) have been promulgated in
Oct. 1975 (40FR46254). These include the performance specifications
and specification test procedures (1) for transmissometer systems for
continuous measurement of the opacity of stack effluents, (2) for monitors
of SO2 and NOx from stationary sources, and (3) for; monitors of 02 and
CO2 from stationary sources. The provisions for (2) and (3) are generally
broken down into
Principle and Applicability
Apparatus
Definitions
Installation Specification
Continuous Monitoring System.
Performance Specifications
Performance Specification Test Procedures
Calculations, Data Analysis and Reporting
References^..
Additional provisions for (1) are given for the installation, op-
tional design specifications and determination of conformance with
design specifications. It is noteworthy that provisions for (3) are made
21
-------
to include alternative procedures due to the variation of existing
analyzers. The provisions state that any such alternative procedures
must fulfill the same purpose (verify response, drift and accuracy)
as the given performance specifications and test procedures.
As an illustration of the proposed performance specifications,
the requirements for the transmissometer (1), the SOg and NO^
monitors (2) and the CO2 and NO2 (3) are summarized in Tables 3-2
through 3-4, respectively.
A report of all emission monitoring and a summary of excess
emissions must be given quarterly by the owner or operator of the
stationary source, to be used by the Administrator to determine whether
acceptable operating and maintenance procedures afe being used. The
continuous monitors are required on affected facilities where
• they can assist in minimizing pollutant emissions,
• the technical feasibility exists using currently
available continuous monitoring technology,
:
• the cost of the monitors is reasonable.
Initially, only a limited amount of cost data was available. In
the meantime, more cost data have been, accumulated. In the promul-
gation of the continuous emission monitoring requirements, EPA is
quoting the following values. For opacity monitoring alone, investment
costs including data reduction equipment and performance tests are ap-
proximately $20, 000, and annual operating costs are approximately
$8, 500. For power plants that are required to install opacity, nitrogen
oxides, sulfur dioxide, and diluent (O2 or CO2) monitoring systems,
the investment cost is approximately $55,000, and the operating cost
is approximately $30, 000. EPA states that "These are significant costs
but are not unreasonable in comparison to the approximately seven
million dollar investment cost for the smallest steam generation facility
affected by these regulations. " (40 FR 46254).
The parameters listed in the Tables 3-2 through 3-4 with an
asterisk are expressed as the sum of the absolute mean value plus 95
percent confidence interval of a series of tests, according to
j n
x = - S x>
where Xj are the individual values, £ is the sum of the individual
values, x" is the mean value, and n are the number of data points.
The 95 percent confidence interval (two-sided) is calculated according
to
22
-------
TABLE 3-2.
Performance Specifications for
Transmissometer
Parameter
Specifications
Calibration error
Zero drift (24 hour)
Calibration drift (24 hour)
Response time
Operational period
* 3% of lest filler value*
£ 2% of emission standard*
£ 2% of emission standard*
10 seconds (maximum)
168 hours
TABLE 3-3. Performance Specifications for
SO0 and NOV Monitors
Parameter
Specifications
Accuracy
Calibration error'
Zero drift (2 hours)*
Zero drift (24 hour)*
Calibration drift (2 hour)*
Calibration drift (24 hour)*
Response time '
Operational period
s 20% of reference mean value
* 5% of each (50%, 90rc) calibration
gas mixture value
* 2% of span
s 2% of span
s 2% of span
s 2. 5% of span
15 minutes maximum
168 hours minimum
TABLE 3-4. Performance Specifications for
COg and O2 Monitors
Parameter
Specifications
Zero drift (2 hour)*
Zero drift (24 hour)*
Calibration drift (2 hour)*
Calibration drift (24 hour)*
Operational period
Response lime
s 0.4^ O2 or C02
* 0. 5% O2 or CO2
* 0. 4% O2 or CO2
* 0. 5% O2 or CO2
!68 hour minimum
10 minutes
23
-------
C-^.gS = t;97; yn(Sx.2)-(£x.)2
nVn-il
where Ejq is the sum of all data points, t 975 = tj -a/2, and C. L 95
is the 95 percent confidence interval estimate of the average mean value.
VALVES FOB »:976
n ' • .. , - •fft
3 -—.'.: . — 13J708
3 : ::—'..;_•—: 4303
4 ; ;.....'. 8.188
5 ;.J-.'.i.; . i.—- aiTTe
B ;-.'i'—._--__™™-._._' 3.871
' ' ' .---.,. :. .8.447
;_„ i.v8.388
7
8
g.
10
11 „:. 1—. .—'JL 3338
13 i- - li ... 3301
13 i i "3.179-
14 '. ..'— 8'.180
16 1 : 1 --::' i 3.148
16 ——:.'.... 3.131
The values In this, table are already cor-
rected for n-1 degrees of freedom. Use n equal
to the number of samples as data points.
The above provisions are not applicable to sources for which
monitoring equipment has already been contracted for. However,
this equipment must have the capability to monitor emission levels
to within +20 percent with a confidence level of 95 percent. Com-
pliance with the requirements is determined by subjecting the con-
tinuous monitoring system to evaluation with a variety of known
opacity neutral density filters or a variety of known concentration
calibration gases as applicable. In addition, owners or operators
of all continuous monitoring systems are required to.record the
zero and span drift and to adjust them at least once daily, ( or more
often if prescribed by the instrument manufacturer), using zero and
span gas introduced into the measurement system for extractive
methods and certified calibration gas cells for non-extractive
methods.
In addition, records must be maintained that include the occur-
rence and duration of any startup, shutdown, or malfunction of the
facility as well as the air pollution control equipment. Also, files
must be kept as to the results of any tests conducted in accordance with
the procedures listed in the monitoring performance specifications.
24
-------
3. 2 NATIONAL EMISSION STANDARDS FOR
HAZARDOUS AIR POLLUTANTS
In addition to the regulations prescribing emission rates of air
pollutants (SO2, NOx, particles, etc.) from new stationary sources,
EPA has codified national emission standards in 40CFR61 for hazardous
air pollutants, such as asbestos, beryllium, and mercury, and has
proposed standards for vinyl chloride (40FR59532). Together with
these standards; which are summarized in Table 3-5, test methods
have been promulgated or proposed (App. B of 40CFR61). These
include:
Method 101: Reference method for determination of particulate
and gaseous mercury emissions from stationary
sources (air. streams).
Method 102:. Reference method for determination of particulate
and gaseous mercury emissions from stationary
sources (hydrogen streams). .
Method 103:
Method 104:
Method 105:
Method 106:
Method 107:
Beryllium screening method.
Reference method for determination of beryllium
emissions from stationary sources.
Method for determination of mercury in waste-
water treatment plant sewage sludges.
Determination of vinyl chloride from stationary
sources.
Determination of vinyl chloride content of in-
process wastewater samples, and vinyl chloride
content of polyvinyl chloride resin, slurry, wet
cake and latex samples.
-------
TABLE 3-5. National Emission Standards for
Hazardous Air Pollutants
Species
Status*
Ref.
' Standards
Asbestos
Pm
40CFR6.1
(Subpart B)
Beryllium
Pm
40CFR61
(Subpart C)
Mercury
Vinyl
chloride
Pm
Pp
40CFR61
(Subpart D)
40CFR61
(Subpart E)
40FR59544
(Subpart F)
No visible emission from
asbestos mills, manufacturing
(textile1, cement, insulating
material, friction products,
paper, ': etc. , floor tile, paints,
etc. , plastics and rubber
materials chlorine), spraying.
Roadway surfacing prohibited,
precautions in demolition.
o£ beryllium over a 24
hour period from stationary
sources; or ambient air limit
of 0..01 g/m* averaged over
a 30-day period; open burning of
beryllium -containing waste
(except propellants) is pro-
hibited:
o
^75ug minutes/m of air within
the limits of 10 to 60 minutes
for beryllium rocket motor
firings.
^2, 300g of mercury per 24
hour period from stationary
sources.
of vinyl chloride for
vinyl chloride plants.
£10ppm of poly vinyl chloride
for poly vinyl chloride plants;
special requirements for reactor
openings, strippers, holding
containers, relief valves, etc.
Pm means promulgated
Pp means proposed
-------
3.3 NATIONAL AMBIENT AIR QUALITY STANDARDS
When the remote techniques are to be applied to establish the
ambient air quality trends, the performance of the instruments must
be consistent with the National Ambient Air Quality Standards, which
have been codified in 40CFR50. :
These standards are divided into primary and secondary standards.
The primary ones are for the protection of human health, while the sec-
ondary ones are for the prevention of all other undesirable effects. In
Table 3-6, the standards for six pollutants [CO, NOg, NMHC (non-
methane hydrocarbons), particulate matter, SO2, and oxidants] are
shown. Included in 40CFR 50 (Appendix A) are the following reference
methods:
A - Reference Method for SOg (pararosaniline method)
B - Reference Method for Total Suspended Particles
(high volume method)
C - Measurement Principle and Calibration Procedure
for CO (NDIR method)
D - Measurement Principle and Calibration Procedure
for Photochemical Oxidants
E - Reference Method for Hydrocarbons
F - Measurement Principle and Calibration Procedure
for NO« (Gas Chemiluminescence)—(Proposed)
To implement these standards, the U. S. A. has been divided into
some 240 Air Quality Control Regions (EPA, 1972 ), the boundaries of
which are based on considerations of urban-industrial distribution, topo-
graphical and meteorological factors, etc. In accordance with the pro-
visions of Section 110 of the Clean Air Act, the States have submitted
plans that provide for the implementation, maintenance, and enforcement
of the National Air Quality Standards on a regional (air quality region)
basis. The State Implementation Plan (SIP) for each region must provide
for attainment of the primary standards in 3-5 years depending on
whether an extension has been granted. The State plan is required to
set forth the procedure for attaining the secondary standards within a
reasonable amount of time. In the meantime, EPA has promulgated
27
-------
TABLE 3-6. National Ambient Air Quality Standards
Pollutant
Carbon monoxide
(Primary and secondary
standards are the same)
Nitrogen dioxide
(Primary and secondary
standards are the same)
Hydrocarbons (non-methane)
(Primary and secondary
standards are the same)
Participate matter
Primary standard
Secondary standard
Sulfur dioxide
Primary standard
Secondary standard
Oxidant
(Primary and secondary
standards are the same)
Standard Description
- 10 milligrams per cubic meter (9 ppm), maximum
8-hour concentration not to be exceeded more than
once per year.
- 40 milligrams per cubic mejter (35 ppm), maximum
1-hour concentration not to be exceeded more than
once per year.
- 100 micrograms per cubic meter (0.05 ppm), annual
arithmetic mean.
- 160 micrograms per cubic meter (0.24 ppm), maximum
3-hour concentration (6-9 a.m.) not to be exceeded
more than once per year. 'For use as a guide in
devising implementation plans to meet the oxidant
standards. ;
- 75 micrograms per cubic meter, annual geometric
mean.
- 260 micrograms per cubic meter, maximum 24-hour
concentration not to be exceeded more than once per
year.
- 60 micro^rams per cubic meter, annual geometric
mean, as a guide to be used in assessing implementa
tion plans to achieve the ^4 -hour standard.
- 150 micrograms per cubic me'ter, maximum 24-hour
concentration not to be exceeded more than once per
year.
- 80 micrograms per cubic meter, annual arithmetic
mean .
- 365 micrograms per cubic meter, maximum 24-hour
concentration not to be exceeded more than once per
year. ;
- 1300 micrograms per cubic meter, maximum 3-hour con-
centration not to be exceeded more than once per
year.
- 160 micrograms per cubic meter, maximum 1-hour con-
centration, not to be exceeded more than once per
year.
28
-------
the requirements of emission monitoring of stationary sources to be
included in the SIP's [40CFR51, §51. 19(e) and Appendix P]. These
requirements include the source categories to be affected and emission
monitoring, recording, and reporting requirements for those sources.
Performance specifications for accuracy, reliability, and durability
of acceptable monitoring systems have to comply with the performance
specifications 1, 2 and 3 as given in Appendix B of Part 60. Techniques
to convert emission data to units of the applicable State emission
standards must be given. Such data must be reported, to the State as
an indication of whether proper maintenance and operating procedures
are being utilized by source operators to maintain emission levels at
or below emission standards. Such data may be used directly or in-
directly for compliance determination. Though the monitoring require-
ments are specified in detail, States are given some flexibility to re-
solve difficulties that may arise during the implementation of these
regulations.
For monitoring the NAAQS, EPA has introduced the concepts of
"reference methods" and "equivalent methods" to standardize measure-
ment methodologies. Although not yet specified, remote monitors may
eventually be included in these measurement methodologies. Originally,
reference methods were specified for manual instruments that embodied
a given technique (Appendices A through F of 40CFR 50). Subsequently,
EPA has adopted a new concept for some of these pollutants, which is
described in Part 53 of Title 40, Chapter I. Instead of specifying reference
methods, measurement principles and calibration procedures are desig-
nated. For example, the NDIR becomes the measurement principle for
CO. Any analyzer using the NDIR principle and calibration procedure and
meeting the appropriate performance tests is designated as a reference
method. As a consequence, there could be as many reference methods
for CO as there are models of CO analyzers utilizing the NDIR measure-
ment principle (Hoffman et al., 1975). Other methods, utilizing a different
measurement principle, can be designated as equivalent method as long as
equivalency to the reference methods can be demonstrated. In the case
of CO, an equivalent method could be a manual or automatic flame ioniza-
tion method or a remote long-path method.
29
-------
Procedures for determining reference and equivalent methods
were promulgated by EPA [49FR7042 (18 Feb. 1975)], affecting Parts
50 and 53 of Title 40 of the Code of Federal Regulations. An overview
of the content of these rules is given in Table 3-7. How these rules
apply to the pollutants is shown in Table 3-8.
There are three special cases in Table 3-8. The first two deal
with reference methods that are specified both for total suspended
particles (TSP) and SO2, instead of leaving them unspecified and the
third case deals with the equivalent method for TSP. ;The reason for
specifying reference methods for SC>2 and TSP is that the designated
measurement principles are manual methods consisting of a series of
operations to be performed by an operator. The reference method is
defined by a series of explicit manual operations, and.thus there
can be only one reference method. An equivalent method for TSP is
not possible since in this case the pollutant is defined-as the material
collected by the high-volume sampler during the sample-collection
phase of the reference method.
In any application of designating a candidate method as reference
or equivalent determination, the following information must be supplied:
A clear identification of the candidate method which will distinguish it
from all other methods and by which it may be referred to unambigu-
ously. A detailed description of the candidate method. The measure-
ment principle, manufacturer, name, model number, and other forms •
of identification; a listing of the significant components; schematic
diagrams; and a detailed description of the apparatus and measurement
procedures. A comprehensive operation or instruction manual providing
a complete and detailed description of the operational and calibration
procedures prescribed for field use of the candidate method and all in-
struments utilized as part of that method, adequate warning of potential
safety hazards that may result from normal use, or (if the method is
automated) from normal use or malfunction of the method and a des-
cription of necessary safety precaution, a clear description of installa-
tion and operation procedures and of necessary periodic maintenance,
as well as comprehensive trouble-shooting and corrective maintenance
procedures and parts identification diagrams. A statement that the
candidate method has been tested in accordance with the procedures
described in Subpart B afnd/or Subpart C of Part 53, as applicable,
including test data, records calculations, and test results as specified.
A statement to the effect that the method or analysis tested is repre-
sentative and that a quality control program will be followed.
30
-------
TABLE 3-7. Applicability of 40CFR50
and 53 (40FR7042)
Reference Method
Equivalent Method
Manual
Those methods as given in the
Appendices to Part 50, except
at present, App. C (for CO)
and App. D (for oxidants)
Any method that satisfies
the requirements in Sub-
part C of Part 53.
Automatic Those methods whose
"measurement principles and
calibration procedures" are
specified in the Appendices
of Part 50 (namely App. C
and D at present).
Any method that satisfies
the requirements in Sub-
parts B and C of Part 53.
TABLE 3-8. Summary of Reference and
Equivalent Methods
(Hoffman et al., 1975)
Pollutant
TSI>
SO,
CO
o,
NO,
Measurement principle
or method
Mich-volume -ampler
(manual method)
Puran»aiiiline
(m:inu:il method)
Ntindispcrsivc inflated
C'hcniilumincsccncc
r-i ; • **
Clieniiluinmcsccncc
Reference
melhud
High-volume
sampler
Pararosnnilinc
•
•
•
bquivalcnl
methods
None possible
M:inu;il or continuous
Manual or continuous
Manual or continuous
Manual or continuous
•None specified. Miinuf.ii-liircr muM Mihmii /er mtcls pcrfominntt
-------
Subpart B (40CFR53) contains the performance specifications,
which are summarized in Table 3-9 and 3-10 (41FR 11252 - 11266).
TABLE 3-9. Performance Specifications For
Automated Methods
Sulfur Photo- Carbon Nitrogen Definitions
Performance parameter 0oils' dloilde chemical monoilde dloildu and test
oildauu procedures
1. Range Faruper 0-0.6 0-0.4 0-60 0-0.{, Bcc. 63.23(a).
mlluoD;
3. Noise do 005 .COS .60 .006 Bee. 63.23(b).
3. Lower detectable Ural I do 01 .01 1.0 .01 Bee. 68.23
-------
TABLE 3-10.
Interferant Test Concentration,
Parts Per Million
1
I'ollnl-
111 1 1
Sd,
SMI
Sc'i,
so,
Sli,
Hl*|
"l
Oi
Oi
co
C'J
00
CO
CO
CO
NOi
N'II
Ntn
NOi
, Hydro- Hydrogen Sulfur NIlroBP.n
Annlyier lypo." chloric Ammonia sulfldo dloilde dlonldo
acid
Hume pholomplric
tins chiiMnnlfigrnphy- . .
reur.ilnn).
Klpflrnchi-mlrnl .2
Ciiinlili'llvllv .2
H|M>einiphotnineiric- ...
KIM plNUP.
('liKiiilliiiiilnespe.nl
Hlcr.lrnrlleniiail
Kpeelropliuloinnlrlc-
(liolnSNlum Iodide
iPliClinil). '
R| will nipliotoinetrlc- '.
Infrared
Ct.is rhromnlography ...
with Manic lonita-
Hon detector.
Klecirnchptnirnl
('•ulnlyllr. combustion-
llipnnal detection.
IK fluorescence
Mercury rephicement-
UV pholnmctrlc.
Cheinlluniliipsppnt
(rv7O-dyo leaotion).
Electrochemical
Sppclropholomptrlc- ...
gas phase
0.1 '0.14
1 '.14
•0.1 .1 ".14 0.5...
'.1 .1 '.14 .5
«. | '14 5
1.14 .5
1. 1 5 ' 5
•.1 5 .5 "
5 .5
.1
'.1 5 «.l
».l 5 '.1
«.l 5 '.1
Nllrlc Caibon Eth- M- Water Carbon
oildo dloilde ylene Ozone Xylene vapor monotlde Methane Ethane
750 120.000 50
750 120,000 50
750 0.5....;
0-5 ---- 0.2 .5 120.000
.5 5 ' 02
750 .- '.03 120.000
'•5 . ' ' 08
i. 5 . - * 08
750 .-:.:...:...;...:..; 20.000 « 10
- 20.000 '10.... 6"5
•8 .2:.-. 20.000 ' 10
750 .2 20,000 '10 ' BO 5
7SO :.: 80.000 1 10 . •. g
•S 20.000 :
•8 6 20.000 50
•8 5 20.000 50
(.nnrpnirollons of Intprforant llslod must be prepared and controlled lo ±10 percent of Ihe slate raluo.
> Annlyior iyiws nul listed will bo considered by iho administrator as special cases.
> Do not ml> with pollutnnl.
1 Concenirnllon ol pollulanl uspd for trsl. Tlicsp pollutant concrnlrallons must bp prepared to ±10 percent of Iho staled value.
TABLE 3-11.
Test Concentration Ranges, Number
Of Measurements Required, and
Maximum Discrepancy Specification
Pollutant
Concentration range.
. . pans per million
Simultaneous measurements required
1-ur .
. -. 34-hr
Ma
..'?.
• " • ' 'First set. Second set First set Second set per
Total....
Carbon
monoxide.
Total....
Etilfnr dlu\i 0.05
Med 010 to 0.15
•• High 0.30 to 0.50
• High 0.25 to 0.35
. S
l
14
5
1
• 14
7
7
8
. ' 6
6
18 ....
8
A
8
18 ....
8
8
3
3
7- •
3
2
2
7
3
3
8
3
S
8 ....
repancy
ecluca-
i, parts
million
0.02
" .03
.04
1.5
2.0
3.0
0.02
.03
.04
0.02
.02
.03
33
-------
3. 4 OUTLINE OF PERFORMANCE AND
TEST PROCEDURES FOR THE
USE OF REMOTE MONITORS
The generalized performance specifications and specification
test procedures outlined here for the use of remote ^monitors in en-
forcement monitoring follow the format given in 40GFR60, App. B. ;
Nader (1975) has suggested that the present specifications can be worded
such as to apply not only to the extractive and in-situ instruments, but
also to the remote monitors. Nader states that an evaluation of present
specifications may result in some modifications but-preferably after the
regulation is appropriately modified. It appears that, in many para-
graphs of the Performance Specifications, this can be accomplished
without great difficulty. On the other hand, the calibration procedures
required for a remote monitor are quite different and the regulations
would require—in our opinion—significant modifications. In the following
we briefly discuss the suggested performance specifications and para-
meters that would apply to remote sensors.
(1) Principle and Applicability
Must include statements to the effect that opacity and/or gases
are sampled remotely, that specifications are given' in terms of
performance specifications and that test procedures are given to
determine the capability of the remote monitors to conform to the
performance specifications prior to approving the system for evi-
dentiary monitoring in case developments.
(2) Apparatus
Must include a listing of all auxiliary equipment needed to
perform the test procedures to determine conformance with the
performance specification and to perform the field measurements.
(3) Definitions
All definitions for the measurements of the opacity and gases
are essentially applicable, as given in 40CFR60, App. A and B.
34
-------
Definitions of subsystems must include receiver, transmitter and/or
remote reflector units, where appropriate. Maximum output of mea-
surement system must take account the specific source, stack diameter,
and source temperatures. Reference must be made to the performance
standards for laser products (DHEW 1975).
(4) Measurement System Performance Specifications
Statement needed to the effect that the measurement system
(remote monitor) must meet certain performance specifications to
be considered acceptable under this method. These performance
specifications can be patterned after the ones proposed in 40CFR60
App. A and B., giving considerations to the performance standards
for laser products in terms of eye and skin safety. .However, since
the measurements are influenced by the interfering atmosphere, these
performance specifications are only valid under a specified set of
conditions given in the performance specification test procedures as
outlined below. For field measurements made under conditions sig-
nificantly different from the ones given in the performance specifica-
tion test procedures, well described and documented "field measure-
ments procedures" must be developed.
(5) Performance Specification Test Procedures
These procedures involve the calibration tests, field tests
for accuracy (relative), drift and response time. :
Some of the calibration tests may be conducted in the
laboratory. Relatively large calibration cells will be required
whose wall effects on the pollutant concentration and on the light
transmission (reflections) must be either negligible or accountable.
For passive remote monitors, the cells must be heated to simulate
the stack emission at elevated temperatures. The self emission of
the windows must be accounted for. The simulation of atmospheric
interference and sky background is very difficult, and, in the case
of water vapor interference, impractible. In the case of opacity
measurements, special "neutral density filters" simulating specific
particulate emission must be used; some simulation: of field conditions
must be made such as illumination by direct or indirect sunlight.
All tests that cannot be simulated in the laboratory must be
performed during the field tests conducted for accuracy, drift and
response time. The procedures should follow the certification
procedures developed by EPA for Reference Method 9 (visual
35
-------
determination of the opacity of emissions for stationary sources). In
these procedures, the candidate remote monitor must be tested and
demonstrate the ability to measure the opacity and/or pollutant gas
concentrations under different conditions in the plume, intervening
atmosphere, and sky background. Similarly, as in Reference
Method 9, a "stack emission generator" must be specified; it must be
equipped with in-situ meters for smoke and gases that can be related
to reference methods. In addition to the specification of a "stack
emission generator, " the sky background and the atmosphere slant
path between the stack and the candidate remote monitor must be
specified. This can be accomplished by monitoring the atmosphere
and sky background by ambient air monitors that include measurements
of interfering species (H2O, CO2, CO, CH4, N2O, particles, etc.)
and meteorological parameters (temperature, wind, clouds, etc.)
Instead of using a "stack emission generator, " it may be possible
to utilize well-instrumented actual smoke stacks, whose behavior under
different fuel and load conditions are known. Of course, certain tests
such as the one for response time, may be impractical to conduct with
an actual smokestack.
/•
In addition to the above special requirements for the use of
remote monitors, the performance specification test procedures must
include provisions to account for the expected change,in sky back-
ground and intervening atmospheric conditions between the "test
range" and the field. These additional provisions must include the
determination of the effect of interference by "normal" and/or addi-
tional atmospheric species present at the time of the observation.
This interference arises not only from the path between source and
sensor, but also from the sky background. In the UV and visible
spectrum, scattering of sunlight presents an interference, while
the thermal radiation from the sky and/or clouds (total or partial
cover) cause an interference in the infrared. For active systems,
narrow band optical filtering minimizes the background effects, and
for passive methods, measurements adjacent to the target plume
provides the necessary information about the background radiation,
although care must be taken that the background is uniform for these
two measurements.
(6) Calculations, Data Analysis and Reporting
In addition to the procedures for determination of mean values
36
-------
and confidence intervals, analysis of accuracy (relative), calibration
error, zero drift, calibration drift, response time and operational
test period, as required for extractive or in-situ continuous monitors,
field records must be kept which indicate the
time
observer location (distance, direction, height)
background description
sky conditions (cloud cover, sun position)
weather conditions (wind direction and speed, temperature)
plume description (if visible).
(7) References
Adequate references must be provided to document the con-
sistent relationship of remote monitors with in-situ or extractive
devices, as well as the consistent relationship of remotely measured
pollution levels during the performance specification tests and the
field applications.
37
-------
4
PRESENT DEVELOPMENT OF REMOTE
MONITORING TECHNIQUES
4.1 OVERVIEW
Considerable work has been done in recent years on the develop-
ment of active and passive systems to remotely measure gaseous and
particulate pollutants in emissions from stationary sources, around the
perimeter of multiple stationary sources, and in the ambient air. Some
developments are still in the theoretical feasibility stage; others have
been demonstrated in the laboratory, and some have 'been used in the field.
Passive techniques for gases rely on measurements of infrared,
visible or ultraviolet radiation containing the absorption or emission
spectrum of the gas of interest. In the UV/visible region the radiation
is either scattered sunlight or direct sunlight; in the infrared the radiation
is emitted by the .gas itself and its background. For .particulate measure-
ments, visible observations of sunlight scattered by the particles are
made by trained visual observers or instruments. Infrared techniques
have the great advantage of working during both day and night, whereas,
in general, the UV/visible techniques are useful only during the day.
An exception to this latter statement could occur for opacity measure-
ments if low-light-level image enhancement instrumentation is used, or
if the stack plume is artificially illuminated for visual observation.
For gaseous measurements the techniques generally require high
spectral resolution to distinguish the spectral characteristics of the gas
of interest from those of interfering species. This high resolution may
presently be achieved by high resolution interferometer spectrometers,
by conventional spectrometers, by gas filter correlation, or by laser
heterodyne detection. Other methods, not relying on high resolution,
utilize the matched-filter technique or several narrow band filters to-
gether with a computer for interpretation of the data. Other techniques
39
-------
which appear not to have been applied to remote sensing of air pollution
include derivative spectroscopy and Hadamard spectroscopy.
The main problem with passive techniques is the data interpre-
tation. In the infrared there is generally spectral interference from
gases in the source of pollution and in the atmosphere. This spectral
overlap together with the dependence of the radiant intensity on the tem-
perature of the source and of the intervening atmosphere makes the
inversion of the radiative.transfer equation to obtain the pollutant con-
centration very difficult. In the UV/visible the data 'interpretation for
gases is complex due to scattering by atmospheric aerosols and particles
in the source of pollution. The scattering depends on the particle type,
number, size distribution and spatial distribution, all of which are
generally unknown, and on the sun angle. For observations of particles,
in a plume or in the ambient air, the same problems due to the com-
plexities of scattering apply.
j.
The techniques presently being investigated for active laser
systems are Raman scattering, resonance Raman scattering, fluores-
cence scattering, and differential absorption, for gaseous pollutants;
and elastic backscattering for particulate pollutants. All methods involve
the projection of laser pulses into the atmosphere, and measurement of
an inelastic or elastic backscattered signal from the pollutant, and, in
theory, are capable of giving range resolution, and hence the spatial
distribution of the pollutants.
In Raman scattering the laser radiation is changed in frequency
when it is scattered by the molecule; this shift is characteristic of the
molecule, and the amplitude of the scattered signal is related to the
number density of that molecule. This technique has been demonstrated
in the field, in both stack emissions and ambient air. However, its
applicability is restricted in concentration range and/or distance.
Increased sensitivity may be obtained by using resonance Raman
scattering, in which the laser frequency is chosen to be close to an
absorption line of the molecule. The theoretical resonance enhancement
of the Raman scattering cross-section is of the order of 10^; this en-
hancement has been observed in laboratory experiments.
Sensitivity similar to that of resonance Raman is theoretically
possible with fluorescence. In this method the laser radiation is ab-
sorbed by the molecule which re-radiates over several lines. The life-
time of this phenomenon is relatively long, and collisional quenching
40
-------
at atmospheric pressures reduces the effective crpss-section, but it
is still comparable to that of resonance Raman. Fluorescence has been
observed in the laboratory, in a closed chamber, and possibly in a
stack plume. Quantitative interpretation of fluorescence signals is
difficult, and there is considerable interference by. aerosol fluorescence.
(
The laser systems with the greatest potential (both in terms of
sensitivity and specificity) appear to be the technique of differential
absorption by scattering (DAS) for the single-ended measurement of
gaseous pollutants. DAS is also known as DIAL (differential absorption
lidar) and DASE (differential absorption via scattered energy). In this
method, two signals at frequencies on and off a pollutant absorption line
are projected into the atmosphere. The difference in the signals back-
scattered from atmospheric molecules and aerosols or solid surfaces
is related to the pollutant absorption. This technique has been demon-
strated in the field, in both stack emissions and ambient air.
The elastic (Mie) backscattering lidar technique for detecting
particulate pollution has long been used semi-quantitatively to map the
relative concentrations of particles as a function df time and space.
The method has been more recently used in the field to measure the
opacity of a smoke plume by comparing the return signal from the near
side with that from the far side of the plume.
An overview of the tuning characteristics of known broadly tunable
coherent light sources was given recently by Kuhl and Schmidt (1974).
Their figure is reproduced here as Figure 4-la, where the solid lines
indicate the regions for which reliable sources are available and have
proved their potential, and the dashed lines indicate the regions for
which sources are being developed.
More details of the diode laser regime is shown in Figure 4-lb,
which is taken from Hinkley and Calawa (1974).
b
A particular application of both passive and active techniques is
the so-called "perimeter monitoring", in which the emission from a
stack or a complex of stacks can be determined without direct observa-
tion of the plume at the stack. In this technique, an up-looking instru-
ment measures the vertical loading of the pollutant continuously along
a closed path around the source. These measurements together with
knowledge of the wind velocity allow the mass flow rate of the pollutant
from the source to be calculated.
41
-------
Power [mat]
Diode laser
Off hlVI
Hir/h pressure gas laser '•
a :: c===a > :
I'aitimt'lrii oscillator , 1
Sum and difference frequency \ '• ^.
Four-wave parametric miting
Spin-flip Roman laser !
H B=) H
! Potoriton laser
Oye laser pumped Raman laser
1 -I
cw
Id'
K)'
id3
m1
1
—
pulse
10
to5
K)'?
10'
<»'
;o'
3
10'
01
100
Wavelength [um]
1000
Figure 4-la. Summary of Tuning Characteristics
of Known Laser Sources (Kuhl and Schmidt, 1974)
WAVENUMBER (cm"')
5000 3000 2000 1500 1000 900 800
T
~r
pb,-,sn»Te
PbS. Se
PVxCd«S
HO 0.
H2CO
' .' i
3 34
CS2H20
CO NO
CH,
NO,
_1 I
NH3
S°2
i •• i
6 6
WAVELENGTH
10
14
Figure 4-lb.
Nominal wavelength regions covered by
three Pb-salt semiconductors at 12°K.
Strongly-absorbing wavelengths of some
common gaseous pollutants are also indicated
(Hinkley and Calawa, 1974).
42
-------
Measurement of the vertical loading may be,made with both
matched filter correlation (MFC) and gas filter correlation (GFC) in-
struments mounted in a truck. Another method of measuring the vertical
loading from the ground could be with a differential absorption laser
system pointed vertically up, with the range being set above the maxi-
mum effluent altitude.
Possible airborne methods include the MFC instrument, the GFC
instrument, and the differential absorption using laser radiation back-
scattered from the earth's surface. The MFC is useful only during the
day, and the data interpretation is complicated by aerosols and by the
varying surface reflectivity. The GFC method can be used both day and
night, but depends on there being a difference in temperature between the
pollution and the ground, and thus would be limited in many instances.
The best airborne method appears to be the differential absorption using
a laser system.
4.2 THEORY
4.2.1 CW Infrared System
The ability of any detect or-noise-limited electro-optical instru-
ment to detect the change in environmental parameters can be quantified
generally by the signal-to-noise ratio (SNR)
SNR = -%- = --_ (4-1)
where S\ is the signal voltage per unit wavelength, N is the rms-
noise voltage, RX is the instrument responsivity (Volts/Watt) at the
wavelength X and P\ the radiant power per unit wavelength. It is
customary in infrared systems to combine the responsivity and the noise
voltage into the detectivity:
Dx = RX/N (4-2)
43
-------
which is equivalent to the inverse of the noise-equivalent-power:
(4-3)
where A^ is the detector area, Af is the bandpass, D^(f, TB) is the
wavelength -dependent specific detectivity at the chopping frequency f, and
background temperature
The available radiant power from an extended source (filling the
field -of -view) is proportional to the throughput of the system and the
spectral radiance seen by the detector:
Pxd\ = (TjxAono)dX/N°(T,x)dTx(x) (4-4)
where TJX is the efficiency of the instrument, Ao is the entrance area,
O0 is the solid angle, N£(T, x) is the blackbody radiance (W/cm2sr-mn)
at the temperature T(K), TX(X) is the transmission, and x is the position
along the line-of-sight. The general expression for the transmissivity is
given by
TX = Vs
where ra is the transmissivity of gases, and TS is the transmissivity
of particles.
Then,
Ta = " rai(X) = exP [-/? *i(X) CiW PtWdx] (4-5)
x
where x^(X) is the spectral absorption coefficient, Cj(x) is the concen-
tration (mixing ratio) of species i at location x, and pj is the total
pressure, and
TS = exp[-/Ea.n.(x)dx] (4-6)
44
-------
where a; is the sum of the absorption and scattering cross sections,
and nj(x) is the number density of species i at location x .
The Equation (4-4) can be rewritten for the special cases of
interest. In Figure 4-2, we show the principal applications of the
passive (a and c) and the active CW systems (b and d).
For the case (a), Equation (4-4) is the proper expression, where
the integration limits go from x = O(T(O) = 1) to x = °° .
For the case (b), Equation (4-4) can be writ'ten (in general,
assuming a homogeneous atmosphere between the transmitter and
receiver) as
where ef is the spectral emissivity of the source, which has the tem-
perature TS , T\ is the transmission of the atmosphere between source
and receiver and TA is the atmospheric temperature. The solid angle
of the receiver should be larger than Ag/R2 , where Ag is the aperture
of the transmitter and R is the distance, in order to reduce scintillation
effects. However, the effective throughput is given by AoAg/R2. By
chopping the source, the second term in the square brackets in Equation
(4-7) is eliminated. Thus, in this case
For the case (c), Equation (4-7) can be written as
(4-9)
x=o
/-!
where h is the altitude of the aircraft [with r(h) = 1], c\ is the spectral
emissivity of the ground, which has the temperature TQ and T\ is the
integrated atmospheric transmissivity. At the shorter wavelengths
(X < 5 urn), the sun reflection must be accounted for, which is given by
-------
///fff fff/fffff/
Figure 4-2. The principal CW applications
46
-------
pxdx
(4-10)
/~i
where p\ is the ground diffuse reflectivity, 0 is the sun zenith angle,
is the sun irradiance at the top of the atmosphere, r\(h) is the
vertical atmospheric transmission from the ground to the height of the
aircraft and T\(") is the atmospheric transmission to through the entire
atmosphere traversed by the sun rays.
For the case d, Equation (4-7) can be written as
where P is the spectral radiant power transmitted by the laser source,
R is the height of the aircraft, A is the surface ar'ea viewed by the re-
ceiver (thus A/R2 = QO) and AL is .the area of the laser beam at the
surface. It is assumed that A £ AL ; for optimum signal to background
ratio A =
A special expression is used for a laser heterodyne radiometer.
The thermal radiance Nj in a spectral channel of bandwidth )3 centered
at frequency i>j is mixed with a laser signal at t/£ to provide a signal
at the difference frequency f D = kj - vjj (Seals 1.974). . The system
responds to the sum of signals in the channels centered at "L ± f D •
For a quantum-limited system with no excess noise, the SNR becomes
SNR =
where c is the velocity of light, h is the Planck constant, v is the
spectral frequency (in Hz), t is the postdetection integration time, and
j8 is the spectral bandwidth (in Hz). N is the radiant power per unit
emitter area and solid angle for the signal channel and its image.
47
-------
4. 2.2 Pulsed Laser Systems
The basic concept of single-ended laser probing of ambient air
and stationary source emissions is illustrated in Figure 4-3 and Figure
4-4 for the case of a Raman system (Inaba and Kobayasi, 1972). The
laser is transmitted through a collimating telescope and scattered from
mixtures of particulate and molecular scatterers in the atmosphere.
The spectrum of the backscattered energy consists of Rayleigh and Mie
scattering components of the frequency, centered at VQ identical with
the transmitted laser frequency, and the Raman-shifted frequencies at
"1 > V2 > • • •» vn due to various Raman active gases in the atmosphere.
These spectral components are analyzed and detected simultaneously
through a spectrum analyzer in conjunction with optical filtering devices
(narrow band filters or a monochromator) and an array of sensitive
photodetectors. Then, via a data processor, the multichannel informa-
tion such as location and concentration of gaseous contaminants, and
their correlation with the particulate matters may be displayed in real
time, and at the same time, if necessary, sent to the pollution alarm/
control system.
The systems for the fluorescence, differential absorption and
lidar methods are basically the same, but with differences in the fre-
quencies and in the data processing. All these laser systems are
sophisticated and expensive, but may be made mobile for enforcement
monitoring purposes.
The sensitivity of these techniques, of course, is determined
by the design of the system and by the signal strength relative to the
system noise and background noise. The latter, due. to scattered solar
radiation, is generally the dominant noise in daytime operation.
In considering laser systems we can write the basic equation in
a form common to all the techniques discussed in the following
P(R) = ePt LN(R)0AR~2TA(R)TG(R) per pulse (4-12)
where P(R) is instantaneous received energy from resolution element
L at range R, Pt is transmitted energy (generally given in joules or
photons) at tg, L is effective pulse length, (L = cAt/2, where c is
the velocity of light and At is pulse duration; it is the range interval
48
-------
V-..V-. V,, • • • -,V.
"•''' : '' ''1'
SCATTERERS
DATA
PROCESSOR
DISPLAY
DETECTORS
TO POLLUTION
ALARM/CONTROL
SYSTEM
Figure 4-3. System diagram showing the basic concept of the laser-
Raman radar as a single-ended and range-resolved probe
for constituent analysis of air pollution and ordinary at-
mosphere in real time (Inaba and Kobayasi, 1972).
RAMAN SCATTERING
SCHEME
Figure 4-4. Schematic of the Raman scattering scheme showing the
transmitting laser, receiving optics, and spectrometer.
(Kildal and Byer, 1971).
49
-------
from which signals are simultaneously received at time t), R is
range R = c(t-t0)/2, where t0 is the time of transmission of pulse,
A is the effective receiver aperture, e is the optical efficiency of
the system, N(R) is the number density of the relevant pollutant,
0 is the backscattering cross-section appropriate!to the scattering
phenomenon under consideration, and T^(R) is the atmospheric
attenuation by interferents along the laser path, and TQ(R] is the
pollutant attenuation, both at range R .
The noise (N) in a well-designed laser system should be due
only to the photomultiplier current caused by signal photons, back-
ground photons and the dark current. Thus
N = (T,P(R)+T,PB+ND)1/2 (4-13)
where rj is the quantum efficiency, P(R) is the signal photons, Pg
is the background photons, and ND is the dark current equivalent noise
input. The background radiance collected by the receiver is given by
= BAOAA (watt)
(4-14)
BAOAX , . . , JV
3 h "^ (photons/second-)
where B is the radiance in Wcm~~sr~, h is the Planck constant,
and Aft is the throughput of the receiver in sq cm sr.
Thus, at the cathode, the number of background photons in a
gated system, with a gate width tg sec, is given by
P - w t - - BAOAX. . (4
B WBlg - e hc/X g (
The dark current noise may be neglected in comparison with
background noise, particularly when the photomultiplier is cooled.
Thus, the signal-to-noise ratio becomes
50
-------
SNR =
T)P(R)
(UP(R)
o/2
per pulse
(4-16a)
For n pulses,
SNR =
TjP(R) • n1/2
(T?P(R)+7}PB)1/2
(4-16b)
Assuming that no data processing noise is added to the detector
noise, then the ultimate sensitivity of the system is determined by
Equation (4-16), and is obtained when SNR = 1.
o
The daytime background noise is typically about 10° greater than
the nighttime noise, i. e., PB(day) = 106 PB (night). The significance of
this background noise varies with the technique, system, and range, as
illustrated in Figure 4-5 and 4-6.
Care must be taken in estimating the atmospheric attenuation
since the outgoing and return pulses are at different frequencies for the
Raman and fluorescence techniques.
Thus, knowing the system parameters, the atmospheric atten-
uation, and the relevant scattering cross-section, the number density
(concentration) of the pollutant may be determined from the return signal.
Typical cross-sections for the different phenomena are given in Table 4-1
(Derr and Little 1970).
TABLE 4-1. Typical Optical Scattering
Cross Sections (Per Particle)
Process
Mie
Rayleigh
Raman
Resonance Raman
Fluorescence *
Cross section (cm /sr)
1
-------
z
<
10 Vf IO*
Ronge in Meiers (P)
Figure 4-5. Fluorescence, Raman and DAS signals
versus range for sulphur dioxide
(Measures and Pilon, 1972).
Night
Noise
IO"1
10 10* 10s
Range in Meters (R)
Figure 4-6. Fluorescence and DAS signals
versus range for nitrogen dioxide
(Measures and Pilon, 1972).
-------
Based on Equation (4-12), a comparison of the techniques may be
made for detecting gaseous pollutants. 'The results of such a comparison
for SC>2 and NO2 in ambient air by Measures and Pilon (1972) are shown
in Figures 4-5 and 4-6, where DAS refers to the differential absorption
method. There is no Raman calculation for NC>2 since this molecule
apparently fluoresces at all visible wavelengths, thus obscuring any
Raman effect. It is clear that the differential absorption method shows
the highest sensitivity, with fluorescence being better than Raman.
Some of the system parameters used in the calculations are given in
Table 4-2. These comparisons are strictly on a theoretical basis without
considerations of problems such as fluorescence of aerosols and inter-
fering species, imprecise knowledge of quenching effects on the fluores-
cence cross -section, and the practical problem of optically filtering out
unwanted frequencies. A more recent comparison .by Byer (1975) also
shows DAS to be most sensitive and that future systems should operate
in the infrared where more pollutant spectral bands exist, and where
eye safety requirements can be more readily met.
TABLE 4-2. System Parameters
Total Output Power
Pulse Duration
Collector Diameter
Optical Efficiency
Filter Bandwidth
Quantum Efficiency
Resolution (ARRES)
X (Diff. Abs. )
X (Fluorescence)
100 kW
10 nsec
25 cm
75%
10 A (NO,, SO,)
18% Z Z
6 m
3020 A (SOo); 4480 A (NO2);
3020 A (S02); 4544 A (NO2);
4. 2. 3 Perimeter Monitoring
Using optical techniques to measure the vertical loading provides
a measure of the vertical optical thickness which can be converted to
ppm-m, which in turn can be converted to gm~2.
The mass flow (M g sec'1) of pollutant across a line is given by
M = IT v s sin 9
(4-17)
53
-------
where s is the line length in meters, L is the mean vertical loading
of pollutant in grams per square meter, and v islthe mean wind velocity
across the line in meters per sec., and 6 is the angle between the wind
direction and the line.
If this vertical burden is measured continuously along a closed
path around a source or a complex of sources, the net mass flow out of
this closed path gives the source emission rate (Ms) , viz.
Mg = vyL(s)sin6(s)ds. (4-18)
If the emitted pollutant is known to be of relatively low concentration in
the ambient atmosphere, then a single traverse across the plume, 90°
to the wind direction, can be made, and the mass flow rate determined
by Equation (4-17). The dimension of the plume(s) can be determined
from the shape of the signal recorded by the up-looking sensor (and the
sensor's ground speed), and the velocity (v) determined by some inde
pendent method (laser doppler, UV or IR imagery,: or anemometer).
4. 3 REVIEW OF ACTIVE SYSTEMS
Theoretical comparisons of single-ended active systems for
monitoring pollutants have been made by several authors (Kildal and
Byer 1971, Measures and Pilon 1972, Ahmed 1973, and Hirschfeld 1974).
These studies suggest that differential absorption is the most sensitive,
followed by fluorescence and resonance Raman, and finally Raman.
However, in reviewing the literature, field measurements appear to have
been achieved only with the differential absorption and the Raman methods,
and these field measurements were all for NO2 and SO2. The fluores-
cence and resonance Raman methods appear not to have developed beyond
the laboratory, and the differential absorption and Raman methods have
not been developed for other species of interest in stack emissions.
4. 3.1 Differential Absorption
The differential absorption method has been used to measure NC>2
in urban atmospheres (Rothe et al. 1974a) and in stack emissions (Rothe
et al. 1974b). They used a tunable dye laser in the 4600 A region. For
-------
the urban measurements, a mean concentration of 0. 23 ppm of NO£ was
found over the distance 1. 73 to 3. 74 km. The measurement was made
at nighttime and took about 13 minutes; no comparison was made with
other data. The same laser system was used to measure NC>2 in stack
emissions in a chemical factory. Measurements were made 300 m from
the stack exit at a range of 750 m. The distribution across the plume
indicates that 1 ppm was detectable. Again the measurement time was
long, being about 33 minutes. The same publication shows ambient air
contours of NO2 over the chemical factory, and indicate a measurement
sensitivity of 0. 5 ppm at a range of 500 m with each data point requiring
20 seconds measurement time. No comparison with other data or a
discussion of accuracy is given.
Grant et al. (1974) using a tunable dye laser in the 4500 A region
made nighttime measurements of NO2 in a closed chamber at a range of
365 m. A sensitivity equivalent to 5 ppm with a 10 m resolution element
at this range was determined, but no discussion of; the integration time
required was given. The results showed good agreement with independent
transmissometer readings across the chamber. \
Igarashi (1973) reported a differential absorption measurement
of NO2 using the 4480 I region, and SO2 using the 3000 A region. Details
of the experiment are not given, but 0. 1 ppm of the gases were measured
at a range of 300 m, with a resolution element of 100 m, using a 1 mJ
laser output.
An interesting variation of this technique due to Zaromb (1974)
does not require a tunable laser. It uses the Raman backscattered signals
from oxygen and nitrogen, which coincide with absorption bands of NC>2
and SO2- A system is presently being built to measure 20-70 ppm of SC>2
and 5-25 ppm of NO2 at a range of 200 m. The same system will also
measure 1000 ppm of CO2 using the direct Raman signal of CO2- No
further details are presently available.
Another similar technique (Granatstein et al. 1973) uses the back-
scatter of laser radiation from droplets (or smoke particles) in a stack
effluent. Laboratory measurements of CH4 and CC>2 were made by tuning
a He-Ne laser at either 3. 391 jum or 4. 217 /jm, respectively. Measure-
ments were made using gas concentrations greater.than 1000 ppm, and it
was concluded that the attenuation properties of the scatterers are needed
to interpret the data. It would appear that two adjacent wavelengths are
needed for each gas, instead of just one, and then of course, the method
is the same as the other differential absorption methods which use the
visible/UV.
r>5
-------
Feasibility studies have been conducted by Ahmed (1973) and
Byer and Garbuny (1973). Ahmed estimates a sensitivity of 0.4 ppm
for NO2 in a 100 m resolution element at a range of 1 km for a 1 mJ
laser in the 4000 A region. Byer and Garbuny, for NC>2, estimate
. 85 ppm in 15 m, at 1 km range, using 1 pulse from a 100 mJ laser;
they also estimate that . 14 ppm of CO can be determined under the same
conditions. Schotland (1974) discusses the errors involved in the dif-
ferential absorption, and using the example of measuring vertical water
vapor profiles estimates errors of less than 6% for altitudes below 3 km.
Byer and Garbuny (1973) also considered differential absorption
using topographic reflectors. This, of course, does not give range
information; it gives only the mean concentration between the laser system
and the reflector. They estimate that pollutant concentrations of . 01 ppm
over a range of 100 m or 10"^ ppm over 10 km can be measured. The
same technique is being developed by Melfi (1974) to measure tropospheric
ozone from aircraft using the earth's surface as the reflector. The system
uses two tuned CO2 lasers and will be used to conduct rapid area surveys.
Grant and Hake (1975) have recently made DAS measurements of
SO2 and 03 at 2900 A using a 15 mJ dye laser which can meet the Federal
eye safety requirements with the addition of a 2 in. diameter beam ex-
pander. The gas was contained in a 2. 5 m chamber at about 250 m range.
Averaging eight pulses, they found an uncertainty of + 60 ppm-m for SO2
and + 130 ppm-m for 03. These large errors are attributable to variations
in the atmospheric backscatter since the measurements were spread over
one hour. They estimate that + 10 ppm-m for SO2 and + 13 ppm-m for 03
can be achieved by taking all the data within one minute.
Two preliminary reports from the Radio Research Laboratories
in Tokyo (Inomata and Igarashi 1974, Asai and Igarashi 1974) discussed
the development of two systems suitable for making the two DAS wave-
length measurements simultaneously or within 1 msec of each other,
to minimize scintillation effects. The simultaneous measurement
uses two linearly polarized components (independently tunable) of a dye
laser cavity. The results of laboratory NO2 measurements with this
system are discussed in detail in Section 5. 3.1. The other report (Asai
and Igarashi 1974) concerned the measurement of 03 using a CO2 laser
and a retroreflector in a long-path measurement. The P(14) and R(14)
lines of a single laser were used, and alternately transmitted through
tin* atmosphere at 130 H/,. In controlled tests, they determined a sensi-
tivity of 12. 4 ppm-m, but by improving the stability of the laser and by
using digital processing, they anticipate a sensitivity of about 5 ppm-m.
-------
Hoell et al. (1975) recently made nighttime measurements of
SO2 downwind using a '0. 1 mJ dye laser. They measured the total column
of SO2 as a function of range out to 1. 9 km, by averaging over 100 suc-
cessive pulses for first one wavelength and then the second wavelength,
taking a total time of about 15 seconds. The estimated sensitivity of
28 ppb in 900 m could be improved by simultaneous wavelength measure-
ments.
The properties of the various differential absorption systems dis-
cussed above are summarized in Table 4-3.
!
A new approach has been analyzed theoretically by Kobayasi and
Inaba (1975), using an infrared heterodyne laser scheme. This approach
is of importance because most pollutants have absorption bands in the
infrared, while only SO2 and NO2 have bands in the UV. The sensitivity
in the IR is, of course, lower than in the visible, but the authors state
that the heterodyne sensitivity is 10^ to 10° times better than that achieved
by the conventional direct detection. They conclude that this method
permits the spatially-resolved detection of pollutants (less than ppm) up
to several km with an average laser power of 100 W. Heterodyne de-
tection of radiation is discussed in Section 4.4. 7.
Murray et al. (1976) recently reported the first measurements
with an IR DAS system. This system used a 1 J CO2 TEA laser to
measure water vapor at ranges up to about 1 km. The authors state
that the range can be extended by system improvements, and that
several other gases, including SO2, vinyl chloride, Freon 11 and
Freon 12, could be measured in the 10 urn region. Baumgartner et al.
(1976) also describe an IR DAS system operating in the 1.4-4. 0 um
region. The system consists of a 30 mJ tunable diode source, a 16 in.
f/3 Newtonian telescope, an InSb detector, and an on-line minicomputer.
4. 3. 2 Raman Scattering
The only quantitative measurement of stack emissions using the
Raman method appears to be that by Nakahara et al. (1972). They used
a frequency doubled Nd: YAG laser at 0. 532 um to obtain a Raman signal
from SC>2 in a smokestack plume at a range of 228 m at nighttime. Using
4 mJ pulses, they measured 1850 ppm whereas the actual concentration
was 1000 ppm. They attributed the discrepancy to uncertainties in the
Raman cross-section for SC>2, and in the system's parameters. (It could
possibly be due to NO2 fluorescence interfering with the SO2 Raman signal
as reported by three of the authors in the later paper (Nakahara 1973)).
57
-------
TABLE 4-3. Differential Absorption
CJl
X
Gas
so2
NO,
co2
N02
NO2
so2
N02
f*\]
CH4
co2
N02
N02
CO
N02
so2
°3
so2
Sensitivity
70 ppm
5-25 ppm
1000 ppm
20 ppm
1 . 1 ppm
)
. 23 ppm
}•
1, 000-
20, 000 ppm
.4 ppm
1 ppm
. 14 ppm )
[
. 35 ppm )
0. 6 ppm )
172 ppm |
. 09 ppm
System Parameters
Laser I.itegr.
Energy pp Time
4-8.mJ 8 1 pulse
ImJ
1 mJ 1 800 s
l.mW
ImJ
1 mJ 1 2000 s
100 mJ 1 pulse
. 5 tnJ 10 8 pulses
. 1 mj 15 100 pulses
Range
200m
200 m
2CQ m
365 rn
300 m
(1.73-
Rcsolutio:
not
given
100m
100m
3. 74 km) 2 km
2. 75 km
1km
750m
1km
300 ni
1. 9 km
100m
100m
15m
100 m
300 n
Comments
] Being
> Raman Diff. Abs. assembled in
) cr.e system.
Does not need
Raman tunable laser
Tunable dye laser - nighttime
- controlled sample chamber
- claim 10s better possible with 2 i
intorf. filter(also same scr.s. in day)
No further information.
Tunable dye laser (nighttime) - •
actual urban ambient measure-
ments.
Lab test of using backscatter from
droplets in plume. Done in IK
3. 4 jim and 4. 2 um
Feasibility study
Measurements of stack emission
300 m from exit of stack, plus
arcal contours over factory
Feasibility study (daytime)
Nighttime - controlled sample
chamber - 2900 A - eye safe.
Nighttime measurements of stack
emission 190 m from exit
Rcf.
(a)
(b)
(c)
(d)
»**/
(e)
(0
fe)
(h)
0)
|(J>
References: (a) Zaromb, 1974; (b) Grant et al. 1974; (c) Igarashi, 1973; (d) Rothe et al. 1974a; (e) Granatstein et al. 1973;
(f) Ahmed, 1973; (g) Rothe et al. 1974b; (h) Byer et al. 1973; (i) Grant and Hake 1975; (j) Hoeii et ai. 1975.
-------
They calculate that 80 ppm of SO2 can be detected at. nighttime with a 10 mJ
laser, at a range of 283 m, with a resolution element of 9 m, using an
integration time of about 4 minutes. In addition to measuring SO2, they
use the Raman signal from N2 to measure the scattering optical thickness
(opacity) of the plume, by observations from the same range through and
off the plume.
Melfi et al. (1973) made a qualitative nighttime observation of SC>2
in a plume 5 m above the top of the stack using a ruby laser at 6943 A.
Using 1. 5 J pulses at a repetition rate of 1 pps averaged over 100 seconds,
they detected SO2 at a range of 210 m with a 3. 7 m resolution element.
They found good qualitative agreement between the SC>2 Raman intensity
and the plant's electrical power output.
Although the two Raman systems discussed above were designed
for SO2, Inaba and Kobayasi (1972) report on a N2 laser (3371 A) demon-
stration system with a monochromator rather than a filter system. Using
this system at a range of 30 m and a resolution of 3 m they analyzed the
Raman spectrum from oil smoke and automobile exhaust gas. Using
0. 2 mJ pulses with a repetition of 50 pps and a 5 sec integration time,
they identified signals from SC>2, C2H4, H2CO, NO, CO, H2S, CH4, in
addition to the usual CO2, O2, N2 and
Poultney et al. (1975) made measurements of SO2 in a controlled
sample chamber at a distance of 300 m at night. Ten percent measure-
ments of 1. 2 x 10^ ppm SO2 were made in 15 minutes using a 1. 5 J ruby
laser at 30 pulses/min, interference filters, photon counting detection,
and an 8 -inch receiver. Certainty of measurement accuracy was checked,
by measuring known concentrations of SO2 in the tank, by tuning the
filters on and off the specific Raman line, by varying the SO2 concen-
tration to very high levels while in SO2 operation, and by viewing the
tank through aerosol veils typical of plumes from stacks with efficient
precipitators.
The only other field measurements using the Raman method appear
to be those by groups from Block Engineering and Edgewood Arsenal
(DeLong 1974). The system uses a frequency doubled ruby laser (0. 347 fim)
operating with 80 to 160 mJ per pulse at two pulses per second. Daytime
field tests have measured controlled amounts of SO2 (30 ppm), kerosene
59
-------
(1. 7 ppm), HNC>3 (7 ppm) and organo phosphate (. 04 ppm) at a range of
200 m with a resolution length of 10 m, using an integration time of
about 1 minute. Based on these measurements and:relative Raman
cross-sections, DeLong(1974) projected the sensitivity of the present
system to other gases, and compared them with calculated values, as
shown in Table 4-4. Note that the nighttime sensitivities are about a
factor of two better than those in daytime.
TABLE 4-4. Projected and Calculated Sensitivities
and System Parameters (DeLong 1974)
Projected
Mil; nil
Nj
Oi
CO
CO,
COj
NO
NjO
H,0
SO,
CH«
H,S
NHj
Clj
C,HS
Hi
Speclial
Shift (cm-l)
2331
1557
2145
1388
1288
1877
1290
3652
1151
2914
2611
3340
S40
2650
530
Spectral
Position (A)
3777
3670
3751
3647
3634
3713
3634
3976
3616
3S62
3818
3927
3538
3823
3536
Day Sensitivity
(PPfn) Rcf.(21)
313
262
317
210
330
682
108
125
58
41
57
79
91
57
SO
8G
62
19
5
9
12
Calculated Seniitivity
Day (ppm) i Night (ppm)
215
179
217 .
144
227
468
74
. 86
39
28
39
54
62
39
34
124
103
125
83
130
269
43
49
23
16
22
31
36
22
20
Present Instrument Parameters
Collector Diameter, cm 91.4
Transmitter Efficiency 0.9
Receiver Efficiency 0.075
Detector Quantum Efficiency 0.3
Laser Energy® 3471. 5A\J 0.083
Receiver Bandwidth, u 0.0005
Receiver Field of View, sr 1 x 10'6
Atmospheric Transmission 0.89
Range, cm 2xI04
Range Gate, cm 103
60
-------
Smith (1972), Barrett (1974), and Klainer (19.74) have suggested
a method of enhancing the Raman sensitivity by using a Fabry-Perot
interferometer in conjunction with the rotational Raman lines instead
of the vibrational lines used in the systems discussed above. The
rotational lines are periodically spaced and may be multiplexed by
using the Fabry-Perot as a filter with similar periodic transmission
peaks. Klainer suggests an increased sensitivity factor over vibra-
tional Raman of 50 for HC1 and 150 for CO2-
Gelbwachs and Birnbaum (1973) have suggested a problem with
the Raman technique due to fluorescence of aerosols which they observed
in some in-situ measurements. They estimate that the aerosol fluores-
cence signal can be equivalent to the Raman signal from 600 ppm of SO™,
or 6000 ppm of NO, or 3000 ppm of CO. In light of the results reported
previously in this section, it appears that these estimated aerosol effects
must be much too high. Of course, the discrepancy reported by Nakahara
et al. (1972) might be due to aerosol fluorescence, rather than the NO2
fluorescence suggested by later work (Nakahara et al., 1973).
The properties of the various Raman systems discussed above
are summarized in Table 4-5.
4. 3. 3 Resonance Raman
The resonance Raman method does not appear to have been deve-
loped much beyond the theoretical stage. Inaba and. Kobayasi (1972)
report that the resonance effect has been observed for some gaseous
molecules in the laboratory. However, they point put that fluorescence
scattering should be expected to be superimposed oh the resonance Raman
spectrum, since both phenomena require similar exciting frequencies
and the emitted frequencies are in the same region.; This probable super-
position would make accurate determination of the pollutant concentration
very difficult, if not impossible. Possibly adjacent frequencies on and
off the Raman frequency could be used in a differencing technique to
subtract out the fluorescence signal; this method may not be amenable
to present filter capabilities.
Fowler and Berger (1974), under EPA sponsorship, investigated
the feasibility of measuring SO2 in stack emissions using fluorescence
and resonance Raman methods. In laboratory measurements they found
a resonance Raman signal 1. 8 times greater than the fluorescence signal
around 3000 A; this ratio could be improved with a narrower spectral
81
-------
TABLE 4-5. Raman
Gas
so2
so2
Kerosene
HCl
co2
so2
so2
SO,
C2H4
H2CO
NO )
CO
H2S
CH4 ,
so2
Sensitivity
Qualitative
30 ppm \
1. 7 ppm )
x50 \
x!50 )
80 ppm
100 ppm
qualitative
70 ppm
System Parameters
i'aser PPS
Energy ™
1.5 J 1
160 mJ 2
Integr. Range
Time
100 s 210 m
200 m
50 s
200 m
compared with vibrational Raman
10 mJ 40
50 mJ 1
0. 2 m J 50
1. 5 J ] 0. 5
250 s 283
100 s 100
5s 30
900 s | 300 m
Resolution
3.7 m
10 m \
10 m )
9 m
10
3
not
given
Comments
Nighttime - smokestack
Daylight - controlled source in field.
Theory of Fabry Perot interferometer
to multiplex rotational Raman lines
Theoretical (nighttime)
Power plant nighttime measurements:
1850 ppm - Raman (1000 ppm - actual
Calculated (daytime)
Experimental
Nighttime - controlled sample
chamber
Ref.
(a)
(b)
(O
(d)
(e)
(e)
(f)
References: (a) Melfi et al. 1973; (b) DeLong, 1974; (c) Klatner, 1974; (d) Nakahara et al. 1972; (e) Inaba et al. 1972; (f) Poultney et al. 1975.
-------
bandpass (they used 24 A). The authors also confirm the problems of
fluorescence from other species and particles making quantitative mea-
surements unlikely.
Laboratory measurements of the resonance Raman cross-sections
have have been reported for SO2 (Rosen et al. 1975).
4.3.4 Fluorescence
Although there have been several theoretical discussions of the
usefulness of fluorescence in the remote sensing of pollutants, there
appears to be only one,report of actual measurements. These were
measurements of NC>2 by Nakahara et al. (1973), who apparently ob-
tained theni as a result of NO2 fluorescence interfering with their Raman
measurement of SO2 in the visible region.
It is clear from this and the preceding discussion on resonance
Raman that there are problems in the interpretation of fluorescence
(and, of course, Raman) signals. In addition, Kildal and Byer (1971)
state that there is no straightforward relationship between the fluores-
cence intensity and the pollutant concentration, making quantitative mea-
surements very difficult. Presumably this latter problem can be over-
come with careful laboratory studies, and, in fact, has been investigated
in the infrared by Robinson and Dake (1973). They found that at atmo-
spheric pressure the IR fluorescence intensity is linearly related to the
laser power, and to the 6/5 power of the concentration, suggesting that
remote monitoring of fluorescence is possible at least in the IR. The
same authors (1974) extrapolate some laboratory measurements of
ethylene fluorescence using a CO2 laser to infer that 0. 1 ppm of ethylene
could be detected at 10 km, or 100 ppm at 1000 km. (Their graphs
actually appear to give 0.2 ppm at 10 km and 1000 ppm at 1000 km.)
They suggest this latter sensitivity would allow satellite monitoring of
stack effluents. However, it appears that they have not considered
problems such as the field-of-view being greater than the stack plume
dimensions, and background noise.
Pikus et al. (1971) have also considered a satellite-borne system
for detecting NO based on fluorescence at 1927 cm-1. The calculations
assume a 5-joule pulse CO laser and compute a threshold NO concentra-
tion sensitivity of 2 x 10llcm~3 (7.4 ppb) at sea level. Even assuming
that a 5-joule laser were available for satellite use, these results may
be optimistic since the calculations neglect detector and system noise
63
-------
sources, 80 cm diameter collecting optics are required, a 50% optical
efficiency is assumed, and interferences due to other species are neg-
lected. (At 1927 cm~l, H2O is a strong absorber over the 0.1 micron
band pass assumed.)
Another estimate (Menzies 1971) of the capability of remote
sensing of NO fluorescence using a 1 mJ pulse CQ laser and sensitive
heterodyne detection indicates that 0.1 ppm in a 300 m path can be mea-
sured at a 1 km range, if the relative humidity is: 50% (quenching by
H2O is faster than that of other constituents for this case).
For fluorescence in the visible/UV region, Kildal and Byer (1971)
have estimated that at nighttime about 0. 5 ppm of NO2 and SQj in a 15 m
path can be detected at a range of 100 m using a 6.1 mJ pulse laser.
They state that this sensitivity drops by a factor of 200 during the day.
Penney et al. (1973) have made laboratory observations of NC>2
and 03 fluorescence using a 0.488 um argon laser, and of SO2 fluores-
cence using a dye laser tuned near 0. 3 Mm. They found SO2 fluores-
cence to be strongest, and estimate that it should be feasible to measure
10~3 pprn Of SQ2 with a resolution element of 100 m at a range of 1 km.
Gelbwachs et al. (1972, 1973) have also made laboratory observa-
tions of NO2 fluorescence using laser excitation at 4416 and 4880 A.
They then developed an in-situ fluorescence technique to measure am-
bient NO2- They do not discuss the possibility of extending the method
to remote sensing, except to point out that they observed aerosol fluores-
cence which may interfere with remote sensing applications.
The sensitivities of the various fluorescence systems discussed
in this section are summarized in Table 4-6.
4. 3. 5 Lidar
Much of the work to date on lidar systems for measuring particu-
late (Mie) elastic scattering has been done by the group at Stanford
Research Institute. Collis and Uthe (1972) discuss the capabilities of
lidar systems.
The backscatter from particles is, of course, dependent on the
shapes, size distribution and refractive index of the particles, so that
return signals will vary from plume to plume. Hence, in order to apply
the technique to enforcement monitoring, it will be necessary to make
calibrations for the different types of particles.
64
-------
TABLE 4-6. Fluorescence
Gas
Sensitivity
In -situ technique shows
NO
S02 1
NO
N02
C2H4
N02
so2
0. 1 ppm
~. 5 ppm
7. 4 ppb
in -situ
fluorescence
.2 ppm
10"3 ppm
Range
that fluorescence
1 km
100 m
Satellite
to
' Surface
10km
1 km
Resolution
from aerosols
300 m
15m
1 m
100m
Comments
600 ppm SO, )
s 6000 ppm NCf > Raman
3000 ppm CO )
1 ppm NO2 - fluorescence
50% R. Humidity. Heterodyne
detector
Theoretical estimate
IR fluorescence
Theoretical. Nighttime UV. Daytime
sensitivity down by 200 x.
Theoretical
|CO2 laser calculations extrapolated
{from lab tests (0. 3 sec time constant!
I
jMeasurements in stack plume -
inot discussed in abstract
Theoretical
Ref.
(a)
(b)
(c)
(d)
(e)
«
(3)
(h)
References: (a) Gelbwachs et al, 1973; (b) Menzies, 1971; (c) Kildal et al. 1971; (d) Pikus et al. 1971;
(e) Gelbwachs, 1972; (f) Robinson et al. 1974; (g) Nakahara et al. 1973; (h) Penney et al. 1973.
-------
Collis and Uthe (1972) summarized the status of lidar systems in
1972 as follows: "Lidar techniques of remote observation of atmospheric
particulate concentrations have obvious value in a wide range of air pol-
lution applications where semi-quantitative spatial and temporal data
are needed, and are already in regular use in research. For routine
operational use, lower cost, eye safe instrumentation must be developed.
The degree to which lidar can be used for making objective, remote mea-
surements of significant parameters (i. e., turbidity, opacity or mass
concentration) has not been fully established. Considerable progress is
being made in this difficult area and already quantitative data can be
derived in research applications. For routine use in emission source
monitoring however, while work in progress appears promising, the
development of a legally acceptable, low cost operational technique is
still some way off. "
Progress has been made in the meantime and an operational
system was fabricated by GE for EPA, based on the original SRI
concept (Evans, 1967). It is shown in its operating configuration at
the Barbados Island Station of the Philadelphia Electric Company in
Figure 4-7.
Figure 4-7.
EPA Mobile Lidar System developed
by GE shown in operating configuration
at the Barbados Island Station at the
Philadelphia Electric Company
(Cook et al., 1972)
-------
Cook et al. (1972) report a method in whicli the plume transmit -
tance is measured by comparing the clear air lidar return from the near
side of the plume with that from the far side. They made field measure-
ments on a power plant smoke plume with a van-mounted ruby laser (1 J)
system at a range of 487 m. A comparison was made with the passive
telephotometer (Conner and Hodkinson 1967), which makes a contrast
measurement and relies on having a target with contrast (e. g. sky and
hillside) behind the plume. The passive technique is somewhat sensitive
to the illumination conditions (e. g. sun position, cloud cover). The lidar
accuracy is limited by the PM tube afterpulsing, but within the error
limits both methods agreed. The lidar system errors are estimated to
be < 12% for an opacity of < 0. 5, and < 2. 5% for opacity < 0. 2.
This system has been recently improved by replacing the PM tube
and hence eliminating the afterpulsing. Conner (1975) considers that in
good atmospheric conditions the method can now measure the opacity
within 2%. Good atmospheric conditions mean that, the atmospheric
aerosol content is the same on both sides of the plume. Under poor
atmospheric conditions, readings are variable and must be averaged
over a period of time.
The technique of Cook et al. is basically the same as that of
Nakahara et al. (1972) who used the Raman signal from N2 in front and
through the plume. Nakahara et al. considers their method to be more
accurate since the N2 concentration and Raman backscatter are better
known than the atmospheric particle concentration and backscatter.
This latter technique appears advantageous since the signal is readily
obtained from a Raman system for gases, so that a separate system
for particle observations is not required. However, due to the low
Raman signals, the range capability would be less, and sky background
noise would be more significant.
It would appear that the remote laser method could provide ob-
servations under certain conditions where the passive observations are
not possible. The laser method can give an absolute transmittance, and
only requires a clear line-of-sight to the plume. In addition, the laser
method is not restricted to daytime observations like the passive one.
However, Belanger (1974) discusses a night vision instrument which uses
an image intensifier to provide a nighttime image of the scene (including
the stack plume) on a phosphor screen. The author considers that the
method would be suitable for enforcement of regulations, but does not
discuss the possible effects of variations of ambient light.
67
-------
Conner (1975) considers that the present pulsed laser system
may be limited for operational use due to eye safety hazards (see also
discussion in Section 5. 2). SRI (Saraf and Jackson, 1975), under EPA
funding, developed a portable, low power CW laser system, to measure
opacity, which would meet safety requirements, and would be smaller
and less expensive. Unfortunately the system was not sensitive enough
for daytime operation, and the development was redirected (Conner 1976)
to a system using a GaAs laser operated at a high, pulse repetition fre-
quency (PRF). It is hoped to demonstrate the feasibility of this high PRF
system by mid-1976. Conner is hopeful that the price for the system
might be less than $20, 000.
It should be noted that optical techniques, as presently known,
cannot separate the scattering by particles and by water droplets. The
remote sensor will have to examine the plume some distance from the
stack exit where visual observation indicates that the droplets have
evaporated. .
4. 3. 6 Intercomparison of Measurements of Gases
In order to compare the measurements and predictions for the
various methods for remotely measuring gaseous pollutants, the results
discussed in Sections 4. 3.1 to 4. 3.4 are normalized to a certain extent
and presented in Figure 4-8. The results are normalized to a pollutant
thickness (resolution element) of 10 m. (It should be noted that in an ex-
tended source the fluorescence technique cannot achieve 10 m resolution
due to the long lifetime of the phenomenon; this lifetime restricts re-
solution to 100-300 m depending on the molecule.) On the basis of DeLong's
(1974) estimate, we assume that at nighttime the Raman method is twice
as sensitive as during the day; all other published results are for night-
time. In normalizing for the resolution element, it is assumed that the
sensitivity is inversely proportional to the resolution length (see Equation
4-12)). No normalization has been made for the systems parameters
such as receiver aperture, laser energy and repetition rate, integration
time, wavelength, atmospheric effects, type of detector and assumptions
about the relevant scattering cross-section.
-------
100
10
--
w
0
CO
XS02(6)R
SO
R
o N02<2)D
XNO
XCO
*N20
N°2) (D!F
so2j
xso2
N02
<4)D
1 NO »>F
/
/
/
Nighttime, -10 m Thickness
• Measurements
x Theory
100
1000
Range (m)
10000
References:
(1) Kildal et al. 1971
(2) Rothe et al. 1974a
(3) Rothe et al. 1974b
(4) Grant et al. 1974
(5) Ahmed, 1973
(6) Nakahara et al. 1972
(7) Inaba et al. 1972
(8)DeLong, 1974
(9)Menzies, 1071
Figure 4-8. Sensitivity vs. Range. Normalized to 10 m thickness
at nighttime. The data points are labeled as [species
(reference) technique] where D is differential ab-
sorption, R is Raman and F is fluorescence.
69
-------
The results do show that in general the field measurements are
lagging behind the theoretical predictions, which, of course, are all
based on reasonable systems parameters. It appears that predictions
for the important gases are indicating a lower sensitivity limit shown
by the dashed line in Figure 4-8, ranging from about 0.1 ppm at 100 m
to about 2. 5 ppm at 1000 m.
4. 3. 7 Stack Effluent Velocity Measurements
Since many emission standards for stationary sources are based
on a mass emission rate, a remote measurement of effluent concentra-
tion alone is insufficient for determination of non-compliance. The
perimeter measurement and area surveys can provide the mass emission
rate, but, of course, it will not always be possible to isolate a single
source in that type of measurement. Other remote sensing methods
provide only a concentration measurement, and a measurement of ef-
fluent velocity must be obtained to determine the mass emission rate.
In addition to the two vidicon methods being developed for deter-
mining the velocity, EPA and NASA have been supporting the evaluation
of a laser -doppler velocimeter (LDV) at the Raytheon Company (Herget
et al. 1975, Miller and Sonnenschein, 1975). A comparable LDV system
has been built and tested by Lockheed Aircraft Corporation (Lawrence
et al. 1975). The Raytheon LDV system consists of a 20-watt CO2
laser, solid invar interferometer, 30 cm diameter f/8 transmitting and
receiving telescope, and a Ge:Cu detector. An elliptically-shaped flat
tracking mirror and a bore-sighted telescope are used to aim the laser
beam at the stack plume. The difference frequency at the detector pro-
duced by the local oscillator (the laser) and the energy backscattered
and doppler-shifted by the particles in the plume is monitored visually
on a Hewlett-Packard spectrum analyzer. The detector output is re-
corded on separate magnetic tape channels after processing by the
spectrum analyzer and also by a specially built frequency tracker.
The system was tested on a power plant stack (Miller and
Sonnenschein, 1975) at a range of 400 m, and showed good agreement
with in-situ measurements of the velocity as shown in Table 4-7. The
in-situ data were averaged over a 30-minute period, and the LDV
velocity is obtained by analyzing a series of 50 msec intervals over a
period of 2 seconds. Such data show wide fluctuations in velocity and
amplitude. For each velocity determination, the resultant spectrum
is a Lorentzian-shaped curve of amplitude versus velocity extending
from about zero to 2V. The plume velocity V is taken as the peak
amplitude of the spectrum.
70
-------
These tests also showed that a linear relationship exists between
the intensity of the laser return signal and the in-stack measured opacity
of. the plume. Since there is probably a relationship (determined empi-
rically by in-situ methods) between opacity and the mass loading for each
type of source (Conner 1974), the LDV system has the potential to pro-
vide a mass emission rate for particles from a source.
TABLE 4-7.
In-Stack and Remote Velocity Data
(Miller and Sonnenschein 1975)
Load
MW
83
83
as
92
96
88
87
87
109
109
111
111
122
122
124
124
130
130
130
135
135
137
137
Pilot
Velocity
R/Sec
87.3
87.3
100
110
107
107
102
102
118
118
118
118
129
129
136
136
136
142
142
146
148
146
146
Doppler
Velocity
Ft/Sec
80
83
80
95
83
90
90
92
100
107
100
103
113
119
112
' 118
125
117
125
120
130
125
135
4.4 REVIEW OF PASSIVE SYSTEMS
There seems to be less work on passive methods for source
monitoring than on active systems, no doubt due to the data interpre-
tation problems of the passive techniques. However, due to their
generally lower cost, it would appear that passive methods would be
more attractive for routine monitoring, even though, with their complex
data interpretation, they might be more difficult to be introduced in
court as admissible evidence. On the other hand, passive methods may
be best suited for surveillance monitoring.
71
-------
Passive methods may be used in either direct observation of
the plume, as discussed in this section, or indirect observation by the
perimeter monitoring technique, as discussed in Section 4. 5.
Two instruments so far developed, the matched-filter spectro-
meter and the gas filter correlation radiometer, are small, portable
and relatively inexpensive, and are thus appealing for enforcement
monitoring, whereas other passive systems are larger, more expensive
and often require a computer for data processing.
4.4.1 Passive Opacity Techniques
The main passive technique used at the present time is that of
the trained visual observer. The observers are trained at EPA smoke
schools, using controlled smokestacks, to make the measurements
according to Ref. Method 9, taking into account the prevailing illumina-
tion conditions. The training method is such that it is generally aereed
the observer in the field will underestimate, rather than overestimate,
the opacity.
Instrumental techniques for passive opacity measurements have
been discussed by Conner and Hodkinson (1967) and include telephoto-
meters, photography, Volz sun photometers, and observers with com-
parators. In addition, there is the night vision image intensifier for
night observations of opacity.
The Volz photometer method requires a direct line-of-sight to
the sun through the plume and alongside it; these conditions will not
always be available, and so the technique is limited.
The telephotometer and photography method make an observation
of a contrast target (e.g. sky and hillside) through,and alongside the
plume and use the theory of contrast reduction to estimate the plume
transmittance; clearly, a suitable target will not always be available.
Comparators may be used by an observer to estimate the plume
opacity. The comparator, which contains scattering particles of varying
number densities, is viewed in comparison with the plume under the
same illumination conditions. Comparators have been developed for
both black and white plumes.
The night vision image intensifier is being evaluated by personnel
in EPA Region III (Smith 1974), and they consider that it should be suit-
able for use by observers who take a training course. Of course, it will
have the same limitations due to illumination variations.
72
-------
EPA Region III uses photography, both on the ground and from
the air, in their surveillance program. They find that showing a photo-
graph of emissions to the offender usually brings about compliance.
Smith (1974) suggests that the contrast reduction of ground features
might be measured by densitometry of the aerial photographs. Due to
illumination effects and the difficulty of finding constant extensive ground
contrast targets, it is doubtful whether this method can be useful.
However, Pressman (1976) is working on another aerial photography
technique for measuring opacity, illustrated in Figure 4-9. Measurements
Opacity = 1 - T = 1 -
Figure 4-9. Opacity measurement from aircraft
of radiance are made in and alongside the plume shadow on a homo-
geneous reflecting ground surface. It is clear from Figure 4-9 that
the ratio of the radiances gives the transmittance of the plume, assuming
that the diffuse radiance over each area is the same, and that there are
no other sources of radiation, such as reflected sunlight, illuminating
the surface. An analysis of this technique was conducted at EPA-
Las Vegas, taking into consideration the film processing requirements,
spectral variations of ground features, interference due to sunlight
scattered from other structures on to the target surface, and the fre-
quency of suitable targets in photographs of industrial areas. Some pre-
liminary flight tests have been made, indicating that the method works,
although no ground-truth measurements have been made yet. Pressman
thinks that this method can be more accurate than the visual observer.
It can also, of course, check many more stacks per day than the visual
observer.
73
-------
This technique is, of course, limited to daytime operation, and
is best used early morning and late afternoon to obtain clear shadows.
The wind must also be in the right direction so that the shadow lies over
a homogeneous target area. However, within these limitations the
technique does appear promising.
4.4.2 Matched-Filter Spectrometer
This technique, in which the spectrum of the incoming radiation
is automatically compared with a stored (in the spectrometer) replica of
the spectrum of the gas being measured, was pioneered for remote sensing
of air pollution by Barringer (1966), and appears to be the first electro-
optical instrument for air pollution. It uses scattered sunlight and is
restricted to daytime use, and has been developed only for SC-j (at 3100 A)
and NO2 (at 4400 A). The instrument has been refined over the years and
is presently marketed as COSPEC II, which is small and portable. The
instrument, shown in Figure 4-10, consists of two telescopes to collect
light from a distant source (scattered solar radiation), a two-grating
Figure 4-10. The Barringer COSPEC II.
74
-------
monochromator for light dispersion, a disc-shaped multislit mask, and
an electronics system for signal processing. The disc is composed of
circular slits photo-etched in aluminum on quartz, and provides a high
contrast reference spectrum for matching against the incoming ab-
sorption spectra. The arrangement is designed to correlate succes-
sively in a positive and a negative sense with the 862 absorption bands
via disc rotation in the exit plane on the monochromator. The detector
used is the XP 1118 photomultiplier tube. Signal processing results in
a voltage output proportional to the optical depth in ppm-m of gas being
observed. A schematic of one telescope and associated optics is shown
in Figure 4-11.
Ml, M2.M3 MIRRORS
INCIDENT I LIGHT
PHOTOMULTIPLIER
TUBE
EXIT MASK
EXIT APERTURE
t
i ^ALIBR|ATION CELLS
T / cp -—-
i / t
/ FIELD I LENS
Figure 4-11.
Schematic layout of the Barringer remote sensor
(Newcomb and Millan, 1970)
The dynamic range of the instrument is 100-10, 000 ppm-m.
Calibration is effected using four fused-silica cells containing known
concentrations of SO2 that are incorporated into the instrument. The
instrument is 50 x 25 x 37 cm in size, weighs approximately 18 kg and
can be operated by either a 12V DC battery or conventional 115V 60 Hz.
The COSPEC II was recently tested by EPA (Barnes et al. 1974)
in comparison with in-stack measurements for SC*;- The results of the
tests are shown in Table 4-8.
75
-------
TABLE 4-8. Comparison of In-Stack and Remote
Concentration Measurements
Range
(meters)
20
20
200
300
300
300
In-Stack
(ppm)
500
585
700
460
450
700
He mote
(ppm)
465
520
240
224
205
275
(IS) - (REM)
(IS)
7
10
G5
52
54
60
x 100
From the table it is seen that agreement with the in-stack readings
at the closest range is quite good. However, at greater distances cor-
relation becomes quite poor. The value at 200 m is suspect since it re-
presents only one hour of data taken on one day. On that day the plant
experienced an outage terminating further work during the study. At
300 m the results are low, but reasonably consistent (54% to 60%). The
authors consider that this consistency could possibly be applicable in
using the instrument at extended ranges if the range correction factor is
constant under different meteorological conditions, such as rain, haze,
and smog, but that much more work would be needed before a firm con-
clusion could be reached on this matter.
A more serious problem was discussed by Barnes et al. (1974).
In their tests they varied the plume opacity by sequentially inactivating
the electrostatic precipitators in the stack. When the second bank of
precipitators was turned off, resulting in a 20% opacity reading (by in-
stack transmissometer), the COSPEC II signal level dropped to an un-
measurable level although the in-stack SC«2 monitors showed no change
in SC<2 readings. Since this level (20% opacity) is within federal regula-
tions, it would appear that the COSPEC instrument would not be suitable
for enforcement monitoring.
One other problem encountered during the field tests was that of
the plume not remaining in the field-of-view of the COSPEC H. This
occurred even on moderately windy days. This point should, of course,
be noted in the design of any remote sensor. The COSPEC II FOV is
3 mr x 10 mr. Reducing that to 3 mr x 3 mr or perhaps 3 mr x 5 mr
should allow operation during moderately windy days when the plume is
not reasonably vertical for more than one or two stack diameters.
76
-------
Barnes et al. (1974), in their summary, state "In conclusion,
the COSPEC II in its present design is inadequate for EPA use for
routine monitoring purposes. The main limitation, ;as has been noted,
is inadequate AGC compensation for scattering effects by particulates
both in the plume and in the intervening atmosphere: The less serious
problem of field-of-view is perhaps a more readily rectified one. It
should be noted here that EMI, Inc. recommends usage of the COSPEC
in two other modes, namely as a mobile perimeter monitor (looking
vertically into the sky) and from helicopters (sighting horizontally
through plumes). " (See Section 4. 5).
4.4.3 Gas Filter Correlation
Another small portable remote sensor is the Gas Filter Corre-
lation (GFC) instrument, for measuring SC<2, under development for
EPA (Bartle 1974a). It operates in the infrared and can be used both day
and night.
GFC is a modification of Non-Dispersive Infra-Red (NDIR) tech-
nique that has been known for some time. In contrast to pure radiometry
or dispersive spectroscopy, a GFC (non-dispersive) device uses the gas
itself to obtain the ultimate high-spectral resolution filter (provided by
the line-width of the gas). High spectral resolution is the most im-
portant parameter in obtaining specificity and accuracy in pollutant
analysis.
GFC combines the high energy throughput feature of radiometers
and the high-resolution features of dispersive instruments. It makes
use of the contributions of all spectral lines of a band system of a parti-
cular species to obtain sensitivity. Specificity is obtained by making use
of the correlation between spectra arising from the particular species
in the source and in the instrument gas cell. Discrimination for many
pollutant species against many interfering species occurring naturally
and in polluted atmospheres has been studied previously (Bartle 1972).
In addition, a ratioing technique may be employed that minimizes effects
of changes in source intensity, background radiation, and continuum
absorption due to aerosols, water vapor, or other molecular species.
The incoming radiation at the sensor is chopped so that it alternately
passes through two optical paths, one through a transparent cell and
one containing the specific gas. Thus, the radiation is modulated only
at the wavelengths at which the pollutant absorbs and high specificity
results.
77
-------
The JRB instrument operates in the 4 jun combination band of
SC>2 and is designed to minimize the effects of not knowing the plume
temperature, and uses a dual split design, shown in Figure 4-12a. It
has been shown (Bartle 1972) that the signal generated by chopping
between two cells is a non-linear function depending upon the 862 in the
plume and fixed instrument parameters and the difference between the
radiance emitted by the plume and background atmosphere. By ratioing
two GFC signals obtained using different amounts of SC>2 in the specifying
cells, the effects of plume and atmospheric radiance are greatly mini-
mized, so that the plume temperature does not have to be measured.
Separate tuning fork choppers are used for each cell pair. The cells
are 10 cm in length. The image of the stack plume is focused by an
f/2.46 (at 4 urn) lens on a 1 x 1 mm detector which defines the field-of-
view to be 8 milliradians (8 m at 1 km). An ambient temperature oper-
ation (ATO) PbSe detector is used; but a single-stage thermoelectric
cooler is used to provide temperature control.
Section A-A of Figure 4-12a shows the configuration of the dual
split cell. The two gas cells contain different partial pressures of SC>2
and are pressurized to 1 atm with pure N2 to pressure-broaden the SO2
lines. The reference cells contain relatively low pressures of SO2 and
are also pressurized to 1 atm with pure N2- The cell areas are the
maximum allowed by the tuning fork choppers, about 8 mm wide x 1. 9 mm
long.
Section B-B of Figure 4-12a shows the dual tuning fork configuration.
Fork-1 is shown closed, allowing radiation to pass through the V-l re-
ference cells; Fork-2 is shown open, allowing radiation to pass through the
AV-2 gas cell. Fork-1 operates a frequency of 40 Hz and Fork-2 at a
frequency of 100 Hz. Because a single lens serves both AV-1 and AV-2
systems have exactly the same field-of-view at the stack plume, as is
essential for proper cancellation of the stack effluent temperature factor.
The superimposed image signals of the two AV systems are electronically
separated by the signal processing. A third tuning fork chopper, operating
at 800 Hz, is located immediately ahead of the detector aperture to elim-
inate the low frequency 1/f detector noise from the PbSe detector. A photo-
graph of the instrument is shown in Figure 4-12b. __
The JRB instrument was used in a field test conducted by EPA
(Bartle 1974b), in which remote measurements were compared with in-situ
extractive techniques (DuPont Analyzer and EPA Method 6). The results
shown in Table 4-9 show fairly good agreement within the experimental
error between the methods. It appears that the errors in Table 4-9 are
greater than would be expected from the previously quoted instrument sen-
sitivities, but this may be due to instrument balancing problems experienced
7«
-------
rB r
Dual Split Cell
Windows
Objective I.IMIS
Plane of AV Tuning
Fork Choppers
T. E. Cooler
PbSe Detector
4.0 urn Filter
800 Hz Tuning Fork
a
B
\ r
r~
u
n
AV-l
AV-J
Section B-B
Section A-A
Figure 4-12a. Optical System schematic of the JRB remote SO« monitor
Weight: 22 pounds
Size: 38 cm length x 18 cm width x 23 cm height
Field-of-view: 8. 0 m rad x 8. 0 m rad
Power requirements: 3.3 watts
Power source: 8 Eveready E91 NiCd (1.2 arap-hr, 6. 25 V);
(Batteries) 2 Eveready R6 NiCd (6 amp-hr, 1.25 V)
Battery operating lifetime: > 8 hours
Warm-up time: 3 minutes
Time constant (to 1 - 1/e signal level): 1, 3, 10, 30, and
90 seconds
Attenuation: 1, 5, 10, 50 and 100
Gain selection: 1, 5, 10, 50 and 100
Recording: 1% panel meter; recorder jacks for
AVj, AV2 and AVg/AVj signals
Figure 4-12b. Photograph and physical specif Ic at I emu of JRB SO2 munote
79
-------
TABLE 4-9. Summary of Average Values of
Remote GFC and Extractive Data
Range
(m)
160
390
390
390
390
390
160
Date
8-12-74
8-12-74
8-13-74
8-13-74
8-14-74
8-14-74
8-15-74
Time
10:00-11:45
13:45-16:50
09:00-12:00
18:45-21:00
09:15-12:45
14:30-16:30
09:30-11:45
[SO2], ppm
Remote
T,t
150
15)
155
1EJ
123
150
110
Cone.
540 ± 200
630 ± 170
530 ± 120
550 + 120
490 ± 160
440+110
750 ± 70
Extractive*
DuPont
Analyzer
—
—
...
412 ±18
422 ± 28
472 ± 32
519 ±21
EPA
Method 6
...
525 ± 25
398 ± 23
—
401 ± 17
475 ± 7
499 ± 11
%
Donation
-17
-25
-25
-16
+ 7
-32
* All extractive data taken from plume with a temperature of 150 ± 5 C.
in the field. There does not appear to be any effect of range. The
data on 8-14-74 were apparently taken in very hazy conditions, but
there was no significant effect due to the haze. On this same day,
the stack precipitators were sequentially turned off giving a maximum
opacity of about 37%, but there was apparently no effect on the remote
SO2 data. However, these tests and subsequent observations of dif-
ferent smokestacks (Bartle, 1975) and controlled wind tunnel tests
(Herget, 1976) revealed that the sensitivity of the instrument is limited.
This limitation may be overcome by increasing the sensitivity of the
detector through cooling. Bartle (1975) has established the relation-
ship between noise-equivalent optical thickness (in ppm-m) and tem-
perature of the present instrument. His results are shown in Figure
4-13.
800
700
ul
E
2
Q
Z
a.'
UJ
I-
2
fc
600
500
400
300
200
150
1 1 i r
\
| — i — i |
. OPERATIONAL RANGE
CONCENTRATION IN PPM -
L - OPTICAL THICKNESS •
D - EXIT OIA. OF STACK
a - ELEVATION ANGLE
1 — I |'
10
100 600 1000
SO2 CONCENTRATION IN PPM METERS
6000
Figure 4-13. Noise-equivalent optical thickness (rms)
as a function of source temperature
80
-------
4.4.4 Medium Resolution Dispersive Spectrometer System
The dispersive spectrometer system, which bears the acronym
ROSE (Remote Optical Sensing of Emissions), was designed and fabri-
cated for EPA by General Dynamics/Convair. The basic purpose of
the ROSE system is to study the spectra of various stationary source
emissions so as to determine which spectral regions are most suitable
for measurement of particular species and to verify which species are
actually present in the effluent. This information can then be used to
assist in the design of simple instruments for specific pollutants. The
instrument is generally not considered by EPA for routine monitoring
due to its bulk and cost, although it is mounted in a truck like the laser
systems. The contractor's final report (Streiff and Claysmith 1972)
describes the ROSE system in detail. Figure 4-14 shows the principal
parts of the receiver section: Dall-Kirkham telescope optics with a
60-cm diameter primary mirror, Perkin-Elmer Model 210 linear wave
number drive monochromator, and detector housing with a portion of a
Cryogenic Technology closed-cycle helium refrigeration system. The
monochromator contains two gratings that are each used in first order
with long-wave pass filters to cover the spectral ranges 2. 8-6. 5 and
6. 5-14 microns. The spectral resolution is variable, depending on the
Figure 4-14. The ROSE system receiver section
with electronics during field tests
81
-------
the slit opening, but 1 cm"1 at 10 jyim and 4 cm'1 at 5 pm are achievable
in the absorption mode. The Ge:Hg detector is operated at 28°K. The
source unit (not shown) contains identical telescopic optics, and a black-
body radiation source is located at the telescope focus. The blackbody
is operable over the range 1100-1800°K and is optically chopped for long-
path (^ 2 km) absorption measurements. In the remote emission mode,
optical chopping occurs just in front of the monochromator entrance slit.
For intensity calibration purposes the source unit is placed just in front
of the receiver unit so that the blackbody radiation (unchopped) fills the
field-of-view of the receiver unit. Both source and receiver units con-
tain bore-sighting attachments to allow visual alignment of the telescopes.
An example of the type of spectra obtained with the ROSE system
by EPA in a field test (Barnes et al. 1974) is shown in Figure 4-15. SO2
emission bands are readily distinguished, and, if the plume temperature
is known, the SC>2 concentration in the plume may be calculated. The authors
calculated 650 ppm for a 400CK plume, which compared to 710 ppm using
an extractive method, and to 680 ppm measured with an in-stack optical
method. They show that an uncertainty of 6% in the plume temperature
at 400°K produces an error of about 12% in the concentration.
HUM II-01-.10
PIJJHE.
4 C«" t * 3 MM
U • I = •' I I . .! !
-- "!•:•-!-hI-:! :
H- • I • : --!--i--t- i - -
Figure 4-15. Plume and sky spectra from 7-13 microns
(Barnes et al. 1974)
82
-------
The problem with the dispersive technique is the determination of
the plume temperature. Barnes et al. (1974) state that spectral reso-
lution better than 0. 5 cm~l and a computerized data reduction program
would be needed to obtain the plume temperature, but they do not elabo-
rate on the method.
Chen et al. (1974) have suggested a method which utilizes the fact
that the spectral location of the peak of a vibrational-rotational band of a
gas depends on its temperature; the peak of the P or R branch shifts away
from its band center as the temperature increases. Assuming that there
is a region of no spectral overlap of gases in the plume, they estimate
that in the 8. 6 (im SO2 band, the F and R branches separation changes by
0. 5 cm~* per 10°K change in temperature (over the range 375°K to 575°K).
They consider that an interferometer-spectrometer can provide spectra
so that the plume temperature can be measured within 10°K. They also
estimate that at 400°K, a 2% error in temperature results in a 10% error
in S(>2 concentration. They point out that the temperature uncertainty
increases in the presence of particles in the plume.
Another method bf estimating plume temperatures relies on the
use of an interferometer-spectrometer and is discussed in the next section.
4.4.5 Interferometer-Spectrometer
Interferometer-spectrometers have an advantage over conventional
scanning spectrometers with their high optical throughput and rapid ac-
quisition of the spectrum (assuming an on-line special purpose computer
is available). They have been used to remotely sense stack'emissions by
Low and Clancy (1967), the General Dynamics/Pomona group (Streiff
and Ludwig 1973; Tanabe 1975) and by Prengle et aL (1973).
The GD/Pomona system, which is installed in a van for mobility,
was used in comparison with the ROSE system described above. Two
Michelson-type interferometer-spectrometers were used to obtain spectral
data over the 2 to 14 micron region. One interferometer using an LN2
cooled InSb (PV) detector covered the region from 2-5.4 microns while
the second interferometer equipped with an LN2 cooled HgCdTe detector
obtained useful data from 3. 5-14 microns. The characteristics of both
interferometers are tabulated in Table 4-10.
83
-------
TABLE 4-10. Principal Interferometer Characteristics
Item
Model Designation
(Drive Mechanism)
Wavenumber Region
of Operation
Optical Resolution
Spectrum Recording
Rates
Detector
Detector Lens •
Detector Cooling
Foreoptics/Field-
of-View (50%)
Wavelength •
Reference
Interferometer #1
IF-3
1850 to 5000 cm"1
~2.5 cm
1 per 2. 5 sec.
1.0 mm dia. InSb(PV)
1/4" dla. silicon
Liquid Nitrogen
10" dia. /2 m rad.
0. 63282 u
HeNe Laser
Interferometer #2
IF-3
716 to 2500 cm"1
- 1 em"1
1 per 4. 5 sec.
2x2 mm HgCdTe
1 1/4" dia. Germanium
Liquid Nitrogen
12" dia. /6m rad.
0.63282|i
HeNe Laser
The principal element in the interferometer is an optical cube
consisting of a GD/Pomona developed beam splitter, a fixed front sur-
faced mirror and a moving mirror driven by a servo-controlled system
developed by Idealab, Incorporated, in a triangular sweep to obtain
linear mirror movement.
The optical cube is shown in schematic form in Figure 4-16.
As indicated in the schematic, the single cube is used for the informa-
tion channel, a laser reference and a white light synchronization pulse.
The sync pulse is used to locate the exact center of the interferogram
for coherent averaging of successive interferograms (when desired).
The laser provides the digitizing clock and wavelength reference.
Both instruments were equipped with f/8 collecting optics with
the 2-5 micron interferometer having a 10-inch diameter collecting
mirror and the 3. 5-14 micron interferometer having a 12-inch dia-
meter collecting mirror. The design allows for variations in the field-
of-view from a few milliradian to several degrees by means of quick-
change foreoptics elements.
84
-------
FIXED MIRROR —
SCAN
MIRROR
LASER DETECTO"R x
f/1 LENS
TARGET
DETECTOR
f/8
FOREOPTICS
t^-Jr 4
•• WHITE -'--—-.__
LIGHT
SOURCE
V A I "^ WHITE LIGHT DETECTOR
L/ | ' » W SYNC PULSE
' »-~\A/\A/~ INTERFEROGRAM
'WWW1 LASER CLOCK
Figure 4-16. Optical layout of Michelson interferometer
(Streiff and Ludwig, 1973)
Some typical spectra (Streiff and Ludwig, 1973) of a stack plume,
obtained at a range of about 500 m, are shown in Figure 4-17, with the
inset giving the plume concentrations of some gases, measured in-stack.
The plume temperature was not known, and no attempt was made to de-
termine it from the remote spectra, so no estimate of the gas concentra-
tions were made. It is of interest to compare these spectra with those
of the ROSE system, see Figure 4-18, obtained on the same stack. The
ROSE system, with its somewhat lower spectral resolution, detects SO2
in the 8. 6 urn and 7.6 urn regions, whereas these bands are not obvious
in the interferometer spectrum. Tanabe (1975) used the 4. 0 nm region
to estimate the SO2 concentration in the plume, but it appears that know-
ledge of the CO2 concentration in the plume is necessary to determine
the plume temperature from the measured CO2 radiance, and this infor-
mation cannot be obtained remotely. Tanabe found that the method (Chen
et al, 1974) of measuring the separation of the P and R branches to
determine the temperature was not accurate enough.
Prengle et al. (1973) have made interferometer spectrometer
measurements of stack emissions with a mobile system shown in Figure
4-19. The Cassegrain telescope has a 25 cm diameter primary mirror,
and the interferometer is a Block Engineering Co. Model 296, covering
the 2. 8-15 jim range with 0. 5 cm~* resolution.
The authors propose a method to obtain the plume temperature
which is applicable only to the interferometer spectrometer sensor. At
zero retardation of the interferometer mirrors all frequencies have the
same phase and add to give a narrow spike in the interferogram. The
85
-------
.003
r r-i—i r i-n-r| i I-ITTF
GREYBODV RADIAIION
(STACK REMAINED IN FOVI
l/j
.002
I-
a:
,.i
U.I
K.
.001
L.LJ. J .i_l._l_l_UJ.J-l_J..I.J..
o.Qoor_i..i-u.i i.,..
800
...L.J..L.I-
1000
1200 1400
HflVENUMBER f1/CMI
1800
2000
•ri iT~rm~i~| nrrrrrr
— "C02 |-~CO—j
•°02
CO
UJ
t—
2
LU
>
.001 -
i ri rrrn-m i i'i rri i i
0.000 [,., j LJ....JJ .
iV.vryv/
DATE: 7-13-7I
TIME: 1522 - 1542 POST
TEMP: 107°f
REL HUM: 11%
FUEL: COKE BREEZE
(SMALL PARTICLES OF COKE)
ADDITIONAL IMPURITIES:
OIL ft WATER.
STACK EFFLUENT:
SPECIE
-
NOy
(NO • N02\
< ~2 °4 /
CONC (PPM)
14 TO 56
SO2 _ . 148 TO 392
CM, NOT KNOWN
CO NOT KNOWN
A7b738
2000 2200 2400 2600 2800 3000 3200 3400 3600 3800 4000
WRVENUMBER f 1/CM )
Figure 4-17. Emission spectra of the gaseous effluents
from a sinter plant stack measured at
Kaiser Steel Corporation, Fontana
(Streiff and Ludwig, 1973)
86
-------
p :jj4jasd-jr|----^ f^
—J U ' - -. 4_jJ __ ——._'_.—-
. —... ... ,._1 . _.
I • ; \! . -.:•"
=;il^ i ^i
! !-: I': I" ! :-
. (• "I—i"J * —* : •
^ , t/\ ift-' 17 "I '.n ' jf\~3
Figur e 4 -18. Top: Sky Background
Center: Coke Oven Plume Emission
Bottom: Difference: Plume-Sky
(Streiff and Ludwig, 1973)
-------
Figure 4-19. Assembled mobile interferometer -
spectrometer system (Prengle et al. , 1973)
amplitude of this spike represents the total flux incident on the inter-
ferometer, integrated over the 2.8-15 jjm region. This total flux pro-
vides a measure of the plume temperature (this would appear to be true
only if the plume emissivity is constant with time and is known). The
spike amplitude is calibrated against temperature using a blackbody.
Measurements were made of a power plant stack plume at a range
of about 65 m. A single measurement typically consisted of averaging a
total of 20 to 100 scans (mirror scans), each scan taking 3 sec. The
results, compared with in-stack measurements, are given in Table 4-11.
TABLE 4-11. Interferometer Compared with In-Stack
Remote
In-Stack
Tcmp(K)
560
561
CO(ppm)
5550
4130
NO(ppm)
324
376
CH4(ppm)
858
878
C2H4(ppm)
296
292
88
-------
The agreement is very good; details of the data interpretation
are not given, but it appears that some sort of calibration of the system
is required using in-stack measurements, i. e., quantitative measure-
ments cannot be made on a plume without prior access to the stack to
empirically calibrate the interferometer for that particular stack and
its emissions.
It seems that this approach for measuring the temperature of
the plume is not suitable for routine monitoring. Hence the interfero-
meter-spectrometer does not appear to have a significant advantage
over the dispersive spectrometer. Of course, it does have a greater
throughput and makes rapid spectral scans. These .advantages must be
balanced against the disadvantages of the greater expense and complexity
of the interferometer system.
4.4.6 Filter Wheel Sensor
A filter wheel remote sensor was built by Bendix (Prostak and
Dye 1970) for EPA, for 'measuring pollutant spectra in the 7-14 jxm region.
The wavelength is scanned by rotating a continuously variable filter wheel,
which has a resolution of about 1 percent. The radiation is collected by
a 28 cm Ball-Kirkham telescope, and passed through the filter wheel on
to a mercury-doped germanium detector mechanically cooled to 28°K.
An on-line computer is used to analyze the spectra in terms of pollutant
concentration using regression analysis based on previous calibration data.
This device is much simpler, optically (see schematic in Figure
4-20), than the scanning spectrometer or interferometer, but due to the
poor spectral resolution, the data interpretation is difficult even with
help of the on-line computer. The system was also tested in the long-
path active mode at the same time as the ROSE and the GD/Pomona
interferometer systems, but had problems with its computer. The filter
wheel technique does not seem to have been pursued since that time.
4.4.7 Laser Heterodyne Technique
The passive laser heterodyne technique (illustrated in Figure
4-21), discussed by Hinkley (1972), Menzies and Shumate (1974), and
Seals (1974) does not appear to have been applied to field measurements
of stack emissions.
89
-------
TRANSMITTER
SOURCE
. RECEIVER
AUXILIARY
DETECTOR yPASSIVE-MOOE
CHOPPER
MAIN SIGNAL FLOW
COMPUTER.CONTROLLED
COMMAND SIGNALS ,
Figure 4-20.
Block diagram of Bendix filter wheel sensor
(passive mode does not use transmitter)
(Prostak and Dye, 1970)
-BCAM-SPUTTER
Figure 4-21.
Configuration for remote heterodyne
detection of pollutant gases from a
smokestack using a tunable-diode -
laser local oscillator (Hinkley, 1972)
90
-------
The laser heterodyne technique may be used to selectively detect
a pollutant emission line if a local oscillator (laser): frequency is found
to coincide with that emission line. If this line is relatively free from
interference due to other species, the technique can be better than cor-
relation techniques in certain spectral regions where interference is
relatively high, such as in water vapor bands.
The basic detection technique is illustrated in Figure 4-22. The
laser can be at a fixed frequency, or it can be tunable so that it scans
across the emission line of interest, and a beat frequency is produced
whenever the difference frequency is within the bandpass of the electronics.
The sensitive spectral width in this technique can be less than
0. 06 cm'1 (corresponding to an IF bandwidth of 109 Hz), and the NEP
is typically 10'^^W in the 5-12 pm region for a 1 sec integration time.
DETECTOR
.LOCK-IN AMPLIFIER
V
•f TUNED
AM* AMP
LASER LOCAL
OSCILLATOR
f •
I Oi
»e
INT6-
GRATOf
7~1
cj
Figure 4-22. Schematic of heterodyne technique (Hinkley, 1972)
Hinkley (1972) shows that by scanning the laser wavelength through
that of the pollutant emission line, a beat frequency is produced whenever
the difference frequency between the two infra-red signals is within the
bandpass of the detector/amplifier system; the amplitude of the signal is
related to pollutant concentration. The signal-to-noise ratio is given by
SNR =
'LO
[l-exp(-a'cL)ldV
-B L c j
(4-19)
+B
91
-------
where a' is the absorption coefficient of the line per ppm of gas con-
centration (c), L the thickness of the plume, Tg its' temperature,
Cfo the emissivity of the background at temperature T^, v is the infra-
red frequency, h is Planck's constant, k is Boltzmann's constant,
B the system bandwidth, and r the post -detection integration time.
Equation (4-19) is also based on the following assumptions: (i) that the
IF bandwidth is less than the emission linewidth; (ii) that the emission
and absorption due to the pollutant in the ambient atmosphere is not
important because of a low concentration there relative to that in the
plume; (iii) that the background attenuation from the wings of other
molecular absorption lines is negligible; and (iv) that the local oscil-
lator has sufficient power to overcome the other sources of noise.
Equation (4-19) holds, regardless of range, as long as the field-of-view
of the collecting telescope is filled by the plume; in the infrared,
with collection optics only a few centimeters in diameter, it should
be possible to detect pollutants 1 kilometer away. .
Under these conditions the minimum detectable concentrations
for pollutant gases with line strengths of 1 x lO-Scm'iVppm are shown
in Figure 4-23 as a function of wavelength and for different source tem-
peratures and background emissivities. Concentrations of a few ppm
of NO and CO at 400 to 600° C can be detected; and similar sensitivities
are achievable for C2H& and NH3 (at longer wavelengths) at a temperature
of only 50 °C. Since SO2 in the 8. 7 Mm region has a measured absorption
strength almost ten times smaller, the minimum detectable concentra-
tions for this gas should be raised accordingly.
The local oscillator power necessary to satisfy assumption (iv)
above is given by the equation (Keyes and Quist, 1970)
(kTA)(hv)
= - ' (4-20)
where TA is the noise temperature of the amplifier,: G the infra-red
detector gain and TJ its quantum efficiency, RL the load resistance,
and e the electronic charge. For an amplifier with a noise temperature
of 240°K (noise figure of 2. 4 dB), and assuming T? = 0. 5, G = 0. 12,
= 50 ohms, and X = c/v = 10 jjm, we obtain PLQ = 12 mW.
92
-------
Figure 4-23. Theoretical wavelength dependenceifor remote
heterodyne pollutant monitoring for 'various gas
temperatures, and for background emissivities
of 0.2 and 1.0. Useful wavelengths for some
pollutant gases are indicated on the abscissa.
Other parameters are shown in the figure
(Hinkley, 1972).
Using state-of-the-art infra-red detectors and low-noise ampli-
fiers, the local oscillator power of 12 mW is prohibitive, producing not
only excessive heating of the detector, but a substantial liquid helium
boil-off. When wide band (> 500 MHz) photodiodes having nearly unit
quantum efficiency become available, the local'.oscillator power re-
quirement will be less than 100 ^W, a more realistic value.
93
-------
Of course, as with most other passive techniques, knowledge of
the plume temperature is required for quantitative measurements of
the pollutant.
Shumate and Menzies (1974) have developed ai heterodyne radio-
meter, with a CC>2 laser as a local oscillator, and used it to remotely
detect several laboratory samples of gaseous pollutants at ambient tem-
perature. Previous demonstrations of heterodyne radiometer sensiti-
vities to SO2 and CO2 were accomplished by heating the gas samples.
The sensitivities to 03, NH3, and C2H4 were found to be adequate for
detection of ambient concentrations in the parts per billion (ppb) region.
The sensitivities to SO2 are on the border line for detection of ambient
concentrations around 50 ppb, and it may be possible to improve this by
a factor of 4 if detection wavelengths near 8. 8 (zm are being used instead
of their operating region around 9.0 pm. A CO laser, operating near
5.2 nim, has also been used as a local oscillator in the detection of nitric
oxide (NO) at a temperature of 390°K. The passive heterodyne radio-
meter does not appear to be capable of detecting ambient NO at normal
smog concentrations; however, the authors suggest that it can be used
to monitor NO concentrations in stationary source emissions which are
at elevated temperatures.
The sensitivities to the various gases are shown in Table 4-12.
The minimum detectable amounts in the second column (in units of con-
centration times path length) give a signal-to-noise ratio of one when a
time constant, r of 16 seconds is used. For these measurements,
B = 600 MHz. This corresponds to a spectral resolution of 0. 04
TABLE 4-12. Experimental Sensitivities to Pollutant Gases
(Menzies and Shumate 1974). The gases were
at 298 °K, except for NO, which was at 390°K.
The band designations I and II refer to the upper
and lower of the two mixed (lO'O, 02 °0) states.
Gas
• Nitric oxide
Sulfur dioxide
Ozone
Ethylene
Ammonia
Sensitivity
(alni cm)
10-'
10-'
2X 10-4
2 X !0-
5 x 10-'
10-*
Laser line
"C"O: 7-«. POS)
"C"O.: 00°1-II. Jl(40)
"C"0.: OOM-II, /«(40)
'=CstO,: 00°1-IIF /»(14)
"C"O-: 00»l-l, />(I4)
Wavelength
5.19
9.02
9.SO
9.50
10.5}
10.72
94
-------
To date, much of the work in the laboratory on heterodyning has
used helium-cooled detectors, and conventional laser sources, as well
as helium-cooled tunable laser diodes. For field applications, higher
temperatures must be used; developments of both detectors and tunable
laser diodes are progressing along these lines. The HgCdTe detector,
which operates at a higher temperature, has the added advantage of
requiring considerably less local oscillator power than doped germanium
and pyroelectric detectors (Koehler 1974).
4.4.8 Dispersive Hadamard Transform Spectrometer
Hadamard transform spectroscopy (HTS), which appears not to
have been applied to remote monitoring of air pollution, utilizes the
multiplex technique by means of a conventional spectrometer with entrance
and exit masks containing many slits. Spectral scanning is achieved by
moving the slits rather than the grating. The detector output is decoded
by use of Hadamard matrices in a computer. The basic concept is il-
lustrated in Figure 4-24 (Decker 1971). The dispersed radiation is "de-
dispersed" by being returned through the spectrometer on to a detector
adjacent to the entrance slit.
Although spatial information alone, or spectral information alone,
can be Hadamard multiplexed, a combined system (imaging spectrometer)
is possible (Swift and Wattson 1974). The object or field of interest is
focused onto a field stop at the entrance plane of a dispersion spectro-
graph by a conventional lens or telescope (a field stop replaces the usual
entrance slit). A two-dimensional Hadamard mask is then scanned across
the field stop, thus transmitting a time-varying light intensity that con-
tains information about all spatial resolution elements in terms of the
mask pattern. In this way, the spatial information is encoded. The
transmitted radiation is in turn dispersed, and focused as a spectrum
at the exit focal plane of the spectrograph. A one-dimensional Hadamard
mask is placed across the spectrum and stepped one element of its cycle
for each complete scan of the spatial mask. This, £00, produces a tem-
poral modulation which contains encoded spectral information.
A laboratory instrument having a spatial field of 31 x 33 elements
(each one typically 0.1 mm x 3. 5 mm) and a spectral array of 63 elements,
with the detector and gratings covering the 6 tun to 25 jjm region with a
resolution of 0. 02 jim or better has been built (Swift and Wattson 1974).
95
-------
Saj5 - Matrix Mask,
at Exit Pocal Plane
25.5 mm x 1.0
Plaid Stop Silts
150-grooveAun
Crating,Blazed
for 2.0 microns
Hlnuteman Inatrument Company
Model 305 0.5-meter
Corrected Czerny-ftirner
Spectrograph
\ifrarci Detector,
mm Square .Active Area
,to PreampUfler & Data Recorder
ilng-Pork Optical Chopper
Figure 4-24. Schematic of Hadamard transform spectrometer
(Decker 1971)
Theoretical analysis (Larson et al. 1974) shows that HTS is best
suited for spectra which are characterized by a few well-defined and
intense peaks on a low intensity background. Conversely, for spectra
with high backgrounds, for dense spectra, or for weak spectral features,
HTS has no advantage over a conventional scanning spectrometer. On
the basis of this analysis it would appear that, in general, HTS has no
special advantage for remote sensing of pollutants, since their spectra
are generally weak and mixed with interfering spectra.
-------
4.4.9 Passive Vidicon Instrumentation
A UV vidicon system for the measurement of SO2 and plume
velocity is being developed at NASA-Langley (Exton 1974). The vidicon
is sensitive to a bandpass of 50-100 A in the SC>2 absorption band at
3100 A, and detects scattering solar radiation (similar to the COSPEC).
Hence the intensity of the plume image on the vidiqoni screen provides
a measure of the SO2 in the plume. The quoted accuracy is + 20-30 ppm
at a concentration of 100 ppm. However, the instrument has been tested
only with low opacity (< 1%) plumes, and presumably, like the COSPEC,
the accuracy will deteriorate with increasing plume opacity, range, and
increasing atmospheric haze.
It would appear that this device would be most useful as a quali-
tative device for imaging thejnvisible SOj^ plume; it_cou_W_ show_the location
of the plume with respect to the stack"so tEaFother*"remote sensors could
optimize their pointing location. Exton (1974) indicated that on one oc-
casion the device showed downwash of the plume around the stack, which
would change the SC*; path length observed by remote sensors.
This vidicon device has also been used to estimate the effluent
velocity. The plume has eddy inhomogeneities which show as "blobs"
on the SC«2 image of the plume. The plume velocity is obtained by tracking
these eddies as a function of time. However in the tests to date, the
device has consistently underestimated the velocity (Herget 1976), and
is thought to be due to seeing only the outer edge of the plume which is
slower than the inner plume.
Another vidicon device operating in the IR ,(3-4. 5 um) has been
evaluated by Aerospace Corporation under EPA support (Herget 1976).
This too provides an image of SCfcg in the plume due to emission in the
4.0 um SO2 band, and it is hoped to obtain velocity by tracking the plume
inhomogeneities. It does not provide quantitative SO2 data independently,
since the plume temperature is not known, at least not remotely.
4.5 AREA SURVEYS
Area surveys of air pollutants can be useful for episode monitoring,
for selecting plant sites, for evaluating environmental degradation, for
assisting in the design of contact monitoring networks, and for tracking
plumes to study atmospheric dispersion, diffusion and fate of pollutants.
Area surveys can be conducted from airborne platforms such as aircraft,
97
-------
helicopters or blimps; from vehicles driven through and around the
area, and from long-path observation networks traversing the area.
It would appear that space observations of pollutants will not be suit-
able for local purposes since the spatial resolution, both horizontal
and vertical, is too coarse.
From the point-of-view of enforcement, area surveys of ambient
air pollution levels are not relevant except where the area surveyed
includes stationary sources. Since the sources of emissions are mostly
above ground level, ground level long-path measurements of ambient
pollutant amounts will not, in general, provide a measure of the source
emissions. However, if a measure of the pollutant in a vertical column
can be made continuously around the perimeter of an area source, a
knowledge of the mean wind velocity enables the rate of emissions within
that area to be estimated. Measurements of the vertical loading may be
made with ground-based instruments looking up, or airborne instruments
looking down from above the upper altitude limit of the emissions. Con-
siderations of this technique seem to have been limited to gaseous pol-
lutants. Similar measurements for particulate pollutants would be dif-
ficult, although qualitative three-dimensional mapping of particle con-
centrations over an area may be done with an airborne lidar system.
The COSPEC instrument has been used for perimeter monitoring
of NO2 and SO2> in the vertical up-looking mode in which the scattered
(by molecules and aerosols) solar radiation is measured (Langan 197la).
The instrument is periodically calibrated by placing cells containing
known amounts of each gas in the line-of-sight within the instrument.
The problems of multiple scattering, varying sun angle and varying
aerosol amounts and distribution makes quantitative interpretation very
difficult. Some measurements (Langan 1971b) of SOo around a single
stack gave an emission rate of about 24 metric tons/day (averaged over
a 50 minute measurement period), whereas an in-stack chemical method
gave 17. 3 metric tons/day (averaged over a 60 minute measurement
period). The report suggests that some of the discrepancy can be attri-
buted to fluctuating winds. More recent (Sperling 1975) detailed inter -
comparisons showed that the COSPEC could measure the SO2 mass flux
within ± 24% of the in-stack EPA reference method. The errors were
again attributed to inaccuracies in the wind velocity. Herget (1976)
plans to use an LDV system to measure the wind velocity in conjunction
with COSPEC measurements to determine the possible improvement in
accuracy.
98
-------
EPA is currently supporting the development of a GFC instrument
by Science Applications, Inc. (1975) for measuring SQg at 4. 0 urn in the
up-looking mode. Field tests of this instrument will be conducted in the
summer of 1976. Ward and Zwick (1975) describe measurements of CO
and CH4 using a GFC instrument in the up-looking mode.
An up-looking differential absorption laser system installed in a
van would seem to be well suited for perimeter monitoring by measuring
the return signal from an altitude above the emissions to give the vertical
integrated pollutant optical thickness.
This basic technique appears also to be the most suitable for air-
borne measurements. Two instruments are being developed for aircraft
operation, using downward looking laser systems, and utilizing the earth-
reflected laser radiation. EPA, Las Vegas (Melfi 1974) is developing a
system for measuring 03, using two pulsed CO2 lasers, tuned to different
wavelengths. The laser beams must be accurately boresighted so they
are illuminating the same ground patch, and it is assumed that the wave-
lengths are close enough that the ground reflectivity is the same for each.
This basic system could be used for other gases as well, such as SC«2,
C2H4 and NH3, either by tuning or by using isotopic CC<2 laser.
NASA-Langley (Allerio 1976) is also supporting a similar program
at JPL to develop a tunable CC*; laser heterodyne aircraft system for 03.
Another aircraft laser system is being developed at NASA-Langley for the
measurement of CO. This DAS system uses heterodyne detection of the
earth-reflected radiation from a CW tunable diode laser. The shape (due
to pressure broadening) of the pollutant line obtained by the spectral tuning
apparently can be mathematically analyzed to provide coarse information
on the pollutant height distribution.
In all of these differential absorption methods, a careful analysis
must be made of possible errors due to the earth/atmosphere fluctuating
background; to the effects of the aircraft moving Over different targets
between pulses or during the laser tuning; and to variations in the laser
output.
A summary of on-going programs in perimeter and area moni-
toring of different agencies is given in Table 4-13.
99
-------
TABLE 4-13. Area Survey Techniques
NASA/MAPS
(Planned)
NASA/ERTS
(Feasibility Shown)
NASA
(Suggested)
SPACE
IR. Uses GFC method. Not suitable for enforcement—area too large--
gives integrated tropospheric amount. ;
Visible. Radiometer. Not suitable (or enforcement--gives vertical
burden of aerosols over water surfaces.
Visible polarimeter. Not suitable. Tropospheric aerosol amounts—data
interpretation difficult for quantitative information.
AIRCRAFT - HELICOPTER
Airborne lidar system. 3-dlmensional mapping of relative partlculate distributions.
IR. 2 wavelength CO, laser. Earth reflection giving vertical integrated
amount. Looks promising in theory—needs flight demonstration.
IR. Tunable diode laser. Earth reflection. Same comments as for EPA
program above.
Visible—same comments as for spice application.
IR. CFC. 2 flight models tested by NASA—gives vertical Integrated amount —
needs temperature profile—in general, not sensitive to lowest 1 km, so not
suitable for enforcement. '
Visible. COSPEC matched filter—severe data Interpretation problems due
to scattering.
IR. Correlation interferometer—uses reflected sunlight—data problems due
to varying surface reflectance—gives vertical integrated amount, so no good
for enforcement.
EPA
EPA
NASA
POLARIMETER
NASA
BARRINGER RESEARCH
NASA/CIMATS
BARRINGER RESEARCH
BARRINGER RESEARCH
SAI/EPA
BARRINGER RESEARCH
BENDIX
EPA/ROSE
GO/POMONA
SAI/DOT
GE/EPA
LINCOLN LAB/EPA
SAI/EPA
NASA
GROUND - MOBILE
Visible. COSPEC looking upward from van—for area emission monitoring—
quantitative interpretation is a problem duo to scattering.
IR. GFC looking upward from van—for perimeter monitoring.
GROUND - FIXED LONG PATH
Visible. COSPEC. Xenon source. SOj, NO2 only. Aerosol attenuation may
be significant. Present range: 100-930 m.
IR. Filter wheel. Blackbody source—computer problem—not pursued.
Present range: 1.6 km.
IR. Scanning spectrometer. Blackbody source. Used as research tool.
Present range: 400 m - 4 km.
IR. Interferometer spectrometer. Blackbody source. Present range:
2-4 km. :
IR. GFC. Blackbody source—for highway CO emissions. Present range:
50m.
IR. 10.6 |un COj laser source for Oj. Present range: 670m.
IR. 4.7 urn tuned diode laser source for CO. Present range: 610 m - 2 km.
IR. GFC. Blackbody source—HjS, VC - for area surveys. Present range: 1 km.
Visible DAS. SOj and Oy Present range \. 9 km.
100
-------
4.6 ON-GOING AND PLANNED RESEARCH PROGRAMS
In addition to the literature search for remote sensing work rele-
vant to this program, we have learned from various government agencies
and laboratories their relevant on-going and planned programs. A listing
of these programs is given in Tables 4-14 and 4-15.
101
-------
TABLE 4-14. Non-Extractive Electro-Optical Techniques for
Air Pollution Measurements, NERC/RTP
(Courtesy of W. Herget, EPA)
"GASES**
"'" 1'
•TECHNIQUE
Fl uoresca.ice
Vibratlenal Raman
Rotations
DIAL .....
DIAL
1 Raman
IR Imagery. _ .. _.
UV Imagery
" QUANTITY"
MEASURED
so2.
S02, NO..
NO
S02, N02
-
7^ TYPE ,
MODE1
.-.I,R
R
R
R
multi-gas
S02, Velc
city
. S02, Velocity.—
LDV - Reference Beam | Velocity i Mass
Dispersive Correlation ....
:
S02. N02
Dispersive Correlation . S02
Gas-Filter Correlation
Gas-Filter Correli
tion
... ._
Multi-gas
i
so2
Gas-Filter Correlation.... NO. H2S
Diode Laser .. [ .... JCO, NO
Gas Laser
ROSE Sys
Notes:
em
.._.^
. .. .
f\ milt f U
U« 1 l»n
-------
TABLE 4-15. -Review of Other Agency Programs
NASA/LANGLEY
2.
Evaluation of AAFE Instruments (or A/C use
CIMATS - correlation interferometer
MAPS - GFC
HSI • - Interferometer spectrometer
JPL program—03 from A/C looking down at earth reflected signal from tunable CO,
laser, plus heterodyne detection on A/C October 1976.
3. AIL program—passive radiometry using CO2 laser heterodyne for 03 and NH3. On
A/C October' 1976.
4. Tunable diode laser plus reflector on A/C fuselage for CO. On A/C May 1976.
S. Differential absorption for SO, using 3000 X dye laser. Have SOg spectrum with 0.2 A resolution.
Tested downstream of stack emission.
6. SAI program—GFC with retroreflector on A/C wing for. HC1 and CO.
7. Differential absorption in IR (1.4-4.0 pm) with tunable diode lasers. (Joint NASA, EPA,
• AF, Army program).
8. Differential absorption for SOj and Oj in ambient air with double-pulse dye laser.
9. Differential absorption for H,O using 7000 X tunable dye laser. Used in up-looking mode
for humidity profiles.
10. UV camera for SC>2 and velocity In plumes.
EDGEWOOD
ARSENAL
1. Passive LOPAIR, advanced development. 8-12 urn, 16-channel filter Instrument. Looks at sky
at low elevation angle. Uses on-line computer using selected channels and weighting functions for
Dimethylmethylphosphonate (DMMP).
2. Isotoplc CO, laser using topographic reflector. Exploratory programs.
3. Differential absorption program (with EPA)—Ahmed of New York. Visible laser (. 09 ppm NO,
at 1 km). ^
4. Raman. Currently planning on using Ruby laser without doubling to get increased S/N.
NSF 1. SRI. SO., O3, NO2— Differential absorption laser system.
EPA-LAS VEGAS 1.
2.
In-house Oj 2X CO,, laser on A/C—area survey.
Aerial photography for opacity.
USAF 1. Brooks AFB-JRB — GFC in-situ for HC1 and HF designed for ready adaptation to long-path measurements.
2. RADC-Block Engineering — Remote detection, tracking and quantitative measurement of HC1 in the
residual effluent plumes of solid booster launches.
DOT
SAL GFC long path (50 m) with retroreflector across highway to monitor CO (car emissions)
and SFg (released tracer). ..
NCAR
2.
Transportable system: Lidar for aerosols
Doppler velocimcter
Can also do Raman scattering.
Acoustic soundings for inversion height.
103
-------
PRACTICAL CONSIDERATIONS
5.1 OVERVIEW
In this section we consider the practical systems constraints
due to safety considerations and to instrument and source parameters.
One of the first constraints to consider is the eye safety re-
quirements. Thus, an analysis of the performance standards for laser
products as proposed by the DREW is made and the maximum permis-
sible exposure and minimum safe range are determined. The resulting
instrument parameters are then used as an input into the error analysis
of the different laser systems. It is obvious that the errors of single-
ended pulsed laser systems vary due to the method of data interpre-
tation, the scattering cross sections, and the assumptions made about
the pollutant and the atmosphere. A detailed analysis to intercompare
techniques is very much dependent on the system design, and on the
assumptions made about the atmospheric properties (temporal, spatial
and spectral). A true comparison will ultimately be made only by
simultaneous field tests on a plume with coincident in-situ measure-
ments of the pollutant. However, when the instrument and atmospheric
parameters are made equal for the most promising systems (DAS,
Raman and Lidar), operating at 4500 A, with a minimum eye safe range
(MSR) of 50 m, which is probably acceptable for operational use, the
calculations show that the Raman system probably does not have an
acceptable range and accuracy capability during nighttime use, and is r
not at all useful for daytime operation. The lidar system and the DAS
system appear to have useful accuracies up to at least 1000 m range
for the integration times permitted by the existing Federal Regulations
(15 seconds for opacity and 15 minutes for gas concentration). The day-
time use of DAS may be limited to smaller ranges at large gas concen-
trations.
105
-------
In the case of passive remote monitors, a. different set of re-
quirements must be met. The source radiation is dependent on pol-
lutant and interfering species concentration, as well as gas temperature
for infrared methods, and on the plume and atmospheric particle con-
centration for UV/visible methods. Hence, knowledge about, or a
technique to eliminate, these latter parameters is required in the data
interpretation. The degree of accuracy with which the temperature
and particle effects can be measured/eliminated is directly reflected
in the ability to determine the pollutant concentration. It is found that
instruments which utilize optical correlation and.ratio techniques are
better suited than other spectroscopic instruments. However, the
sensitivity of the infrared method is still dependent on the source
strength, i. e., blackbody temperature, so that cool exhaust plumes
near ambient temperatures will be very difficult to observe. The UV/
visible method is considerably hampered by variations in the plume
opacity and atmospheric particle amounts. Active systems are not
constrained by these limitations.
5. 2 EYE SAFETY HAZARDS FROM LASER SYSTEMS
A consideration in evaluating remote sensing laser systems for
air pollution is the potential safety problem. The most obvious hazard
with the laser beam is direct axial viewing, but specular reflections
are also a risk, and considerable care must be exercised in the use of
laser systems. The skin can be burned by some laser beams, but the
main hazard is damage to the eye. The danger varies with wavelength
of the laser radiation due to the wavelength dependent optical properties
of the eye. •
0
The most sensitive region is the visible region, 4000 to 7000 A
since the cornea, aqueous humor and the lens readily transmit this
radiation. Thus, the full laser beam energy is focused on the retina
in an image H)-20 jjm in diameter resulting in high power densities of
kilowatts/cm2 on the retina even for milliwatt powers incident on the
pupil of the eye. These high power densities can-cause severe damage
to the retina, and the fovea, resulting in loss of visual acuity.
In the UV region, the radiation below about 4000 A is absorbed
by the lens, and below about 3150 1 it is absorbed by the cornea, so
that UV hazards are mostly limited to these parts of the eye. The ra-
diation is absorbed over a larger area than is the focused visible light
106
-------
on the retina, so the power densities are not so high. Exposure to ex-
cessive UV results in inflammation of the cornea and conjunctiva at the
shorter wavelengths, and cataract formation at the longer wavelengths.
In the DR region, up to 1.4 pm, the ocular: transmission is less
than that of the visible, and varies with wavelength. From 1.4 to 1.9
essentially all radiation is absorbed by the cornea and the aqueous humor
Beyond 1.9 ym all the radiation is absorbed by the cornea alone. The
absorbed energy can be conducted to the interior of the eye, raising the
temperature of the lens, and resulting in the formation of cataracts.
5. 2.1 The Maximum Permissible Exposure and Minimum Safety Range
The Department of Health, Education, and Welfare proposed (1974)
Performance Standards, which were subsequently promulgated (1975), for
Laser Products to minimize the hazards of laser systems. The standards
are somewhat complex, but may be summarized in terms of a Maximum
Permissible Exposure (MPE) which varies with the laser emission duration
The following values are for pulse lengths of about 0.1 usec. In the visible
band from 4010 to 7000 A the MPE is 5.2 x 10-7J/cm2. In the IR, this
value increases to 2.6 x 10~6J/cm2 at 1. 06 urn and remains constant out
to 1.4 urn. From 1. 5 to 13.0 urn the MPE is 2.1 x 10'4 J/cm2 In the UV,
the MPE at 4000 A is 2.1 x 10-2J/cm2 decreasing to 6.2 x lQ-5J/cm2 at
3020 A, and remaining constant to 2500 A. It should be noted that there
are discontinuities (or, at least, a very rapid change in MPE with wavelength)
in the MPE values near 4000 land near 1. 5 urn. ; For CW lasers the
exposure time must be considered, and the DREW Performance Standards
(1975) should be consulted for the details. For exposure times (tsec) be-
tween 2 x 10~5 and 10 seconds, which should cover most situations, the
MPE, in the visible band (4010 to 7000 A) is given by 1.8 x 10-4t3/4J/cm2,
and in the IR (1. 5 to 13 urn) is given by 1. 1 x 10-2t1/4J/cm2. In the UV
(2500 to 4000 A) the MPE is independent of t * 3 x 104 seconds.
These standards will have an impact on the choice of laser systems
for operational use in the remote sensing of air pollution. The minimum
safe range (MSR) beyond which the beam irradiance is less than the MPE
may be calculated approximately, without consideration of atmospheric
effects (attenuation, scintillation), by assuming that the energy is uni-
formly distributed across the beam, and by assuming the laser to be a
point source (a typical beam might be 1 cm diameter unexpanded, or 10 cm
107
-------
when a beam expander is used to reduce its divergence). If the laser beam
has a divergence solid angle Q and an energy of P joules, then the radiant
exposure (irradiance), E, at a distance R cm is given by
2
E = -f- J/cn/ (5-1)
When E is the MPE, then R is the MSR, i.e.,
(MSR)2 = p
When the MSR is computed for the promising laser techniques,
Raman, DAS, Lidar and LDV, it appears that the existing Raman and
lidar systems cannot safely be used operationally in the visible region,
since the beam would present a hazard at great distances. The existing
DAS system is possibly acceptable in the visible,; and all systems appear
acceptable in the near UV and IR. The MSR could be reduced by in-
creasing the laser beam divergence. The present typical system di-
vergence of 0. 3 m rad results in a 0.1 m beam width at 300 m. A larger
divergence of 3 m rad, (1 m at 300 m) would seem still small enough
for use with most stacks.
It is theoretically possible to further reduce the MSR by con-
sidering the basic laser system equation, given by Equation (4-12). From
this equation it is seen that, for a fixed return power, P(R), the trans-
mitted signal, Pt, can be reduced by increasing the product L x N(R);
this product is proportional to the optical thickness which is the mea-
sured parameter in the DAS and lidar techniques: This means the mini-
mum detectable concentration is increased, since increasing L does
not help in the case of a stack plume where L> plume width. In addition,
the integration time of the system may be increased, so that n pulses
are integrated rather than the single pulse represented by Equation (4-12).
This enables Pt to be reduced in proportion to n1/*.
5.2.2 Required Modifications
•
Sensitivity Change: The minimum detectable optical thickness
should not be increased for the Raman system since it is already quite
high (~ 300 ppm-m for SC>2, see Table 4-5).
108
-------
The DAS sensitivity is about 1 ppm in a plume at 750 m range
isee Table 4-3), and could be relaxed to at least 10 ppm for enforce-
ment monitoring; this would reduce the DAS visible MSR, by a factor
of 3. 2 to 0. 05 km (including the divergence modification).
The accuracy of the present GE lidar system is 2. 5% at 0. 2
opacity, and could reasonably be relaxed to say 10%, giving a reduction
by a factor of 4 in Pj., resulting in a lidar MSR of about 2.8 km (in-
cluding the divergence modification). ;
Integration Time Modification: In evaluating the possibility of
increasing the integration time to decrease the MSR, the Proposed
Performance Specifications of Continuous Monitoring Systems must be
considered. These specifications for opacity monitoring with an in-
stack transmissometer call for a 10 second maximum response time,
although the visual determination of opacity (Method 9) calls for readings
at 15 second intervals. Thus, presumably the integration time for a
lidar system should not exceed 15 seconds. For in-stack monitoring
of SC>2 and NOx, a 15 minute maximum response time is required.
Hence, the integration time for the DAS and Raman systems should
not exceed 15 minutes.
The GE lidar system is a single pulse device and is capable of
only 3 pulses per minute, so it can obtain a reading every 20 seconds,
slightly exceeding the 15 seconds suggested above. It is clear that the
integration time of this system cannot be increased.
The DAS integration time is already long (10-30 min.) so that
within the specifications constraints discussed above, no significant
improvement of MSR is possible, and, in fact, it would worsen for the
system which must reduce its integration time to 15 minutes.
i
The Raman integration time is typically about 1 min., so
lengthening this to the permitted 15 min. allows about a fourfold de-
crease in Pt, resulting in a Raman visible MSR of about 1. 0 km (in-
cluding the divergence modification).
5.2. 3 Final System Parameters
In summary, the present systems might be considered safe for
near UV and IR operational use, but some modifications must be made
for their use in the visible as discussed above. The suggested simple
modifications result in MSR's, for all methods, of 2.5 km and less as
shown in Table 5-1.
109
-------
TABLE 5-1. Proposed System Parameters
and Achievable MSR's
Ca fa
Divcrcencc p n ^c°" Sensitivity MSR Beam
_______ Diameter
DAS (Visible) 3 x 10"3 ra-1 10"4J 15 min. lOppm! 50m 15cm
Raman (Visible) 3 x 10 rail 40 x 10"3J 15 min. 300 ppm-m 1000 m 300 cm
Lidar (Visible) 3 x 10"3 rad 250x 10"3J 20 sec. 10% accuracy 8500 in^ .?50 pm
LDV (IR) 3 x 10"3 rart 20W 2 sec. 20% accuracy 150 m* . 45 cm*
* One second exposure
In order to make these systems eye safe at the exit of the trans-
mitter, the beam diameter at the transmitter must be equal to the beam
diameter at the MSR. These values are shown in Table 5-1. The values
for DAS and LDV are not unreasonable for a practical system, but for
the existing Raman and Lidar systems, the beam diameters are im-
practicable.
It is seen that both the MSR and the safe beam diameter of the
Lidar are probably excessive using the present GE approach even with
the proposed modifications. However, it would appear that the DAS
system, which has reasonable values of MSR and! safe beam diameter
could be used for opacity measurements since, like the Lidar, it mea-
sures the transmittance of a plume. In terms of system sensitivity,
it does not matter whether gas or particle transmittance is measured.
In fact, due to the four signal measurements required for DAS com-
pared to the two required for the Lidar, the Lidar system is more
accurate for a given transmittance, as shown in Section 5. 3.4.
It should be noted the MPE is much larger, and hence the MSR
and safe beam diameter are much smaller, in the near UV (< 4000 A)
and in the IR (> 1.4 >jm). Thus, systems should preferably operate in
these regions; they have the additional advantage that the solar radia-
tion background is greatly reduced or is negligible. The use of these
regions appears possible for the detection of gases using Raman (in
the UV) and DAS (in the UV and IR). However, opacity measurements
are traditionally based on observations in the visible, and the Federal
Regulations are based on visible measurements, so that opacity mea-
surements in the UV may not be readily accepted. Lidar opacity
110
-------
measurements below 4000 A could probably be related to conventional
visible measurements on a theoretical or empirical basis for all types
of particles.
5. 3 ERROR ANALYSIS OF ACTIVE SYSTEMS
The errors involved in the various remote sensing techniques
can be estimated by consideration of the equations describing the signal
and noise of the system, although very little error analysis of systems
has appeared in the literature. Of course, the final estimate of the
accuracy of a system will be made in controlled tests in comparison
with in-situ standard methods.
5. 3.1 DAS Systems
In DAS the laser radiation is transmitted (and received) at the
line center (wavelength 1) and at the wing (wavelength 2). The values
of N(R) and 0 here refer to the Rayleigh and Mie scatters behind the
plume and are assumed constant during the measurements.
TA(R) may also be assumed constant so that Equation (4-12)
may be rewritten:
In P(R) = In C -2 In R + In rr(R) (5-3)
where C = ePj.LN(R)j8ATA(R) and is assumed to be constant with time
and wavelength. Now
-2/R(p(R)k+o)dR
= e
where p(R) is the pollutant density at range R and k is the absorp-
tion coefficient of the pollutant, a is the attenuation coefficient of
particles in the atmosphere or plume, so we have
rR
In P(R) = In C - 2 In R - 2J (p(R)k +
-------
At range (R + AR)
/.R+AR
lnP(R +AR) = lnC-21n(R>AR) - 21 (D(R)k«r)dR (5-5)
Jo
Differencing (5-4) and (5-5) for wavelengths 1 and 2:
InP.fR -TAR) = 2 lri(-^^-) + 2p(R)k1/!R+2a1AR (5-6)
1 i\ 11
In P2(R) - In P2(R + AR) = 2 In ** + *" + 2 p(R) kgAR^AR (5-7)
where p(R) is the mean pollutant density over the distance AR.
Assuming aj = v^i tne difference of (5-6) and (5-7) gives
p(R)AR = * 1
In =—TKX - In
P2(R + AR)
In Q " rs\ni ^2*
TTT where Q = P /R\P /P^AP\ (5-8)
In deriving Equation (5-8) it was assumed'that the atmospheric
and pollutant properties are constant while the data are taken at the
two wavelengths. This is true if they are taken simultaneously, but if
it takes more than 1 msec to tune the laser between wavelengths, then
atmospheric scintillation and air motion adds an error term to Equation
(5-8). Schotland (1974) discussed these errors but did not provide an
estimate of their magnitudes. Ku et al. (1975) experimentally varied
the pulse repetition rate in a diode laser system operating in the CO
4. 6 urn band, and found that 1. 5 msec intervals did indeed "freeze" the
atmosphere over a 610 m path using a retroreflector at a range of 305 m.
At longer intervals the signals varied significantly. Of course, the
effects may be different at shorter wavelengths, and it is clear that
simultaneous measurements at the two wavelengths are preferred, if
possible. If separate lasers are used for each wavelength, then care
must be taken to match the beam divergences and the distribution of
energy across the beam. Northam (1976) is investigating the significance
of these requirements.
112
-------
In Equation (5-8) the absorption coefficients may be uncertain
by as much as ± 20% (Schdtland 1974), but this would be merely a sy-
stematic error which could be eliminated by careful calibration. Random
errors in the absorption coefficient can arise by variation of pressure
and temperature of the pollutant, and by fluctuations in the laser wave-
length. The other errors in using Equation (5-8) to estimate p(R)AR
occur in the measurement of the received power as a function of range
and wavelength. The received power can possibly be measured to
within 1% (implying a signal-to-noise ratio of 100), but since four power
outputs must be measured in Equation (5-8), 'Q has an error of 4%, and
when Q is close to unity, then the error in In Q, and hence, in
p(R)AR is large.
This latter conclusion is verified by Inomata and Igarashi (1975),
who have developed a DAS system with simultaneous wavelength oscil-
lation of a dye laser. Two linearly polarized components of a dye laser
cavity were independently tuned and used to measure 85 ppm-m of NO2
in a test cell. The laser power fluctuates by more than 10%, and it was
necessary to monitor the output power at each wavelength. Their results
are shown in Figure 5-1 which shows the standard deviation of a series
of measurements. (V/P)0 and (V/P) correspond to P2(R)/Pi(R) and
?2(R + AR)/Pi(R +AR) respectively, in Equation (5-8). These errors
also include those due to wavelength instabilities of the laser, although
the authors measured them and found them to be negligible.
The advantage of simultaneous wavelength measurements is
demonstrated by the reduction in the fluctuations in (c) and (f) compared
with (a), (b), (d) and (e). The error in Q [Equation (5-8)] for this set
of data is only 4% [(c) + (d) errors], but translates strikingly into a
47. 5% error in the measurement of pollutant optical thickness. This
error will be even larger if the SNR is less than the apparent value 100
for each power measurement, as will occur for increasing range.
Of course, for larger optical thicknesses this error is reduced.
For 850 ppm-m, which might be more representative of a plume, and
4% error in Q, the error in the optical thickness is about 25%. For
increasingly larger optical thicknesses, the transmission of the plume
will decrease so that the resulting return signal decreases and the mea-
surement errors increase, and finally cannot be measured due to
system or background noise.
It should be noted that these measurement errors could be re-
duced by averaging more pulses [SNR oc (number of pulses)1/2], but a
long averaging time may not always be practicable, particularly if the
source and atmosphere are fluctuating.
113
-------
_l
_l
u
7.
3
U
<
?•
CJ
E
•fc
c.
in'
a
ui
^
*"E
E o.
2
1.5—
1.3 —
1 g_ A
»-***i^v «*^\. /^^v y
1.5- XX^V *v^
1.4-
1.0-
OJ-
1.5 —
1.3- «N/^— *"X/
1.6 —
5~* .^^
I.A — •^'"^ ^""'i/
1.0-
03 —
200-
150-
100- . . '
50 —
•
0 -
isay
TJetP"
fft
Tiirty
Slg.P
RetP
_JL
1.38!
C2.71«
1340
(2.MO
O.B97
(1.8'i)
1.342
1.429
(2.6V.)
0540
(U%)
91.8 ppm-m
(47.5V
a]
b)
c)
d)
(e)
If)
(g)
Figure 5-1. DAS experimental errors (after
Inomata and Igarashi 1975)
5. 3. 2 Raman Systems
In the Raman technique, the transmitted laser radiation is shifted
in wavelength when it is scattered by the pollutant, so that in Equation (4-12)
we can consider TA(R) to be the atmospheric attenuation of the out-
going laser beam, and TQ(R) to be the atmospheric attenuation of the
return Raman radiation. If it is assumed that these atmospheric at-
tenuations are constant then
P(R) = K N(R)
(5-9)
where
K = cPtL/3AR
-2
114
-------
i. e., the pollutant number density is directly proportional to the mea-
sured return signal, so that the precision of the pollutant measurement
is the same as that of the power measurement: if SNR = 10, N(R) is
determined within
There are uncertainties in the value of K due to instrument un-
certainties, due to the value of |3 , which is uncertain by up to + 20%
(DeLong 1974) and due to the atmospheric attenuation uncertainties. All
of these may be considered as systematic errors, although the atmo-
spheric attenuation may fluctuate during a series of pulses being averaged.
A common method to obtain the pollutant concentration in ambient
air directly, and to minimize the uncertainties, is to ratio the return
signals from the pollutant and from nitrogen, the concentration of which
is assumed known and constant in the atmosphere:
This method must be used with caution for plumes, since they may be
oxygen enriched, and. at varying exit pressures and temperatures. If
the Raman returns of the pollutant and nitrogen are close together so
that atmospheric scattering losses can be assumed the same, and do
not exhibit different atmospheric absorption losses, then the only random
uncertainties in obtaining the pollutant concentration occur in the power
measurements, and may be reduced by pulse averaging without concern
for atmospheric fluctuations. If SNR = 10, then the pollutant concen-
tration error is 20% since two power measurements are needed.
An error source in Raman scattering, not described in the above
equations, is that due to overlapping of Raman spectra of different species
contained in a pollutant source. The Raman system measures the Q-
branch of the vibrational -rotational Raman spectrum, but, as shown in
Figure 5-2, the S- and O-branches of strongly scattering, high concen-
tration species may overlap and mask the weaker Q-branch of another
species. To retain specificity, narrow band interference filters with
high rejection ratios must be used.
Typical Raman signals are small and the SNR is low so that
nighttime operation provides greater sensitivity and range. Daytime
115
-------
.3371
Ul
o:
in
10*
3500
V/AVELENGTH
35CO 3700
(A)
2SOO 3900
\ 'cO(iov)'
- i/PURE- \ ~~~
•XROTATIOW
: S02
- \ 1
j ^
-
|
|
: t
2
i
^
;
i
1
!
i
i
1
02 (iv.) CO (iv.)
1 y
1 f
i ;!
; |
NO
:\
:
i
:
_.,
/:
I ':
r ;
/ |
*
\
\
1 1
N2(80V.) H20
H2S C«4
,. *""""
\
:
t
,
/
•
•
J
•'
."""•
\
• •
"
f:-\
*
•
•
~. ,
t —
(T/.J
_..._
'
'
"
i
1000
2030
FREQUENCY
3000
4000
(cm-)
Figure 5-2. Theoretical distribution of Raman volume back-
scattering coefficient due to a molecular mixture
contained in a typical oil smoke as a function of
Raman-shifted frequency. The solid line is the
Q-branch and the dotted lines are the O- and S-
branches (Inaba and Kobayasi 1972).
operation would be improved if designed for the solar-blind region below
about 3000 A, but at shorter wavelengths the Raman lines are closer,
so that the filtering requirements discussed above are greater, and the
errors due to interfering gases increase.
5. 3. 3 Lidar Systems
!
The lidar system for measuring plume opacity makes a measure-
ment of the return signals from in front of the plume and from behind it,
so that it is similar to DAS, but uses only one wavelength. Hence, in
Equation (4-12), TQ(R) may be written as the scattering losses due to
passage of the radiation twice through the plume, rJ*, where (1-Tp)
is the plume opacity. It is assumed that there is no absorption by the
plume at the laser wavelength. If it is also assumed that the backscatter
from in front and behind the plume is constant, i. e., N(R)0 is constant,
then the signal from in front of the plume is given by
116
-------
P(R1) = DRj^r^Rj) (5-11)
and the signal from behind the plume is given by
P(R2) = p R2-2 TA(R2) rp2 (5-12)
where
D = ePtLN(R)jSA
Hence
TP =
1/2
(5-13)
Since Rj is not greatly different from R2, it may.be assumed that
TA(RI) and TA(R2) are equal. Hence, the error in Tp is determined
by the errors in measuring the powers. Since the square root of the
power ratio is involved, the percentage error in T« is the same as in
the power measurement, i. e., if SNR = 100, then the uncertainty in
Tp is 1%. It must be noted that the .error in opacity is greater than
that in Tp, especially for low opacities. For example, a 1% error in
Tp results in 4% and 9% opacity errors when the opacity is 0.2 and 0.1
respectively.
The assumption of the backscatter being the same on both sides
of the plume may not be a good one, particularly in an industrial'.atmo-
sphere where measurements would be made. This error is difficult to
estimate, but could be eliminated by using the Raman scatter of nitrogen
as the scattering source, but of course, the SNR would decrease and
maybe result in uncertainties larger than those eliminated.
Another error source in this lidar technique is that of thermal
lensing (Cook et al 1972) by the hot plume gases, which causes some
117
-------
of the radiation from behind the plume to refract in and out of the re-
ceiver field-of-view, and hence causes the return signal to fluctuate;
the magnitude of this effect is uncertain. This thermal lensing effect
(which is similar to scintillation) does not affect the DAS technique if
simultaneous measurements at two wavelengths are made.
5. 3.4 Intercomparison of Laser Techniques
A comparison of the promising laser techniques has been made
on the basis of the error considerations discussed above. Let us con-
sider the signal-to-noise relationship (Equation 4-16b):
1/2
SNR = " P * n ur (5-14)
P(R) + TJ PB)
where the signal P(R) = ePtLN(R)/3AR-2TA(R)TG(R) photons,* and the
background noise is
fBAOAXt
PB - HIT—1- <5
where tg is the gate width in seconds, and n is the number of pulses
integrated.
For this intercomparison we have assumed system parameters
common to all techniques; only the parameters N(R), 0, and TQ vary
with the method. The system parameters in Table 5-2 were assumed
as reasonable, based on existing or planned real systems. The wave-
length of 4500 A is chosen since it is one used for NO2 DAS measure-
ments, it can be used for Raman, and it is not far removed from lidar
wavelengths.
* Note that p(R) JQules = P(R) photons.
hi/
118
-------
Table 5-2. System Parameters
x = 4500 A
AX = 5|
L = 10 m
IT
t = 10 sec (> pulse width)
'A = 500cm2
« = 0.1
P. = lO^J (based on DAS MPE eye safety
requirements)
n '= 0.2
n " 900 pulses (based on 1 pps and 15 minute
integration time permitted by
Federal Regulation;; for gas
measurements)
B = lO'^Sv cm'^sr"1!"1
(daytime) andjs negligible at night.
Using these values:
1"
P(R) = 1.14 x 1013N(R)0-£Tr photons (R in m) (5-16)
R u
Pfi = 5.66x10 photons during the day (5-17)
and
SNR = 13.4 -. P(R) ^-ry during the day (5-18)
SNR = 13.4 (p(R))1/2 during the night (5-19)
119
-------
The values of N(R) and |3 assumed for this comparison are
given in Table 5-3.
Table 5-3. Backscatter Parameters
DAS and Lidar
Raman
^(cnAr"1)
N(R)(cm'3)
Raleigh
ID'27
1019
Mie
ID'8
102
1C'28
1015(100ppm;
10
-13
The Mie parameters, given here for a clear day (25 km visibility), will,
of course, vary with the particle type and size distribution and with the
visibility. The Raman parameters vary with the gas and with the con-
centration. All of the values of J9 depend on wavelength.
These values Of 6N(R) are consistent with independent estimates
by Collis and Uthe (1972) for Rayleigh scattering:
where ap is the Rayleigh attenuation coefficient (cm-1) and by Measures
and Pilon (1972) for Mie scattering:
N(R) = 4 x 10~2 a.
M
where aM is the Mie attenuation coefficient (cm'1). The value of
TA(R) is based on the model atmosphere (Elterman 1968) (25 km visi-
bility), and ranges from 1.0 at 10 m to . 12 at 5000 m for the 2-way
transmission. The parameter TQ does not appear in the Raman equa-
tion, and in the lidar equation it represents the 2-way particulate trans-
mittance of the plume. In the DAS equation 2 In TQ is used, so that
the difference in the absorption coefficient on and off an absorption line,
is used directly. We have assumed that kj = 16 (atm-cm) and
120
-------
k2 = 8 (aim-cm)'1 based on the work of Grant et al (1974) in using DAS
for NO2 measurements. Thus, the effective value of TQ for 100 ppm
with aim stack exit diameter is given by
e-(16-8) ID'2
for passage once through the plume. This same transmittance for part-
icles in the lidar system represents an opacity of 0.08.
Using the above values of the systems parameters, and Equations
(5-18) and (5-19), calculations were made of the SNR of the backscattered
signal for DAS, Raman and lidar, both day and night, for 100 ppm of the
gas in a 1 m exit diameter (or 0. 08 opacity). It is assumed in these cal-
culations that the transmitter and receiver fields-of-view completely
overlap at all ranges, and that the effective backscattering cross-sec-
tions and system efficiency are constant at all ranges.
Figure 5-3 shows the results for DAS and lidar at a single wave-
length. These values are computed for the backscattered signal in front
of the plume, and are the same for each DAS wavelength. The SNR is
slightly smaller from behind the plume and different for each DAS wave-
length. The SNR for the Raman system is not shown in Figure 5-3 since
the values are very low: SNR = 13.4 at 10 m and 1. 31 at 100 m during
the night, and SNR < 1 during the day.
In order to compute the concentration (opacity) measurement
errors, the SNR values were used in conjunction with the data analysis
procedures developed in Section 5. 3. It is assumed that SNR = 100
represents 1% signal error, SNR = 40 represents 2. 5% signal error,
etc. These error estimates must be considered as optimistic since
(a) the value of N(R)j8 is likely to vary during the measurement period,
(b) the value of N(R)j8 may not be the same on each side of the plume,
(c) signal processing errors are not considered, (d) laser wavelength
and intensity fluctuations are not considered, (e) thermal lensing
effects by the plume are not considered, and (f) spectral interference
effects of other species are not considered. There is also a systematic
error due to lack of precise knowledge of the absorption coefficient and
its variation with temperature, but this can be eliminated by careful
laboratory and field calibration.
121
-------
loom
at,—>\
Figure 5-3.
SNR calculations for DAS and Lidar
(900 pulses at one wavelength) for a
1 m plume (100 ppm) at range R
The calculated measurement errors, shown in Figure 5-4,
indicate that DAS and lidar are probably useful (20% error) out to 1 to
3 km, but that Raman does not have a useful range capability. It should
be noted that the lidar system, to comply with the present Federal
Regulations for opacity, cannot use a 15 minute integration time, but
must use 15 seconds. Figure 5-4 is strictly for intercomparison of
the techniques and does not represent optimized systems, although it
does indicate that a Raman system designed for eye safe operation in
the visible does not have a useful range capability. Figure 5-5 shows
the effect on the lidar accuracy of reducing the integration time to 15
seconds and indicates a useful (20% error) range of 650 m during the
day and 840 m at night. The calculations shown in Figures 5-4 and 5-5
are for 100 ppm in a I'm stack'exit diameter, and for 0.08 opacity in
the case of lidar. The 100 ppm-m of gas is probably a lower limit of
interest in enforcement monitoring. For opacity, 0. 08 is less than
the 0.10 and 0. 20 limits prescribed for most sources in the Federal
Regulations; presumably enforcement personnel will be most concerned
with monitoring larger opacities which are exceeding the limits.
122
-------
Night
Night
100 ppm-m
100 ppm
0.08 opacity
1000
10000
Range (m)
Figure 5-4. Intercomparison of measurement errors for plume
at various ranges. These results are not optimized
for each system, but are strictly for intercom-
parison of techniques.
0.08 Opacity
ma
Integration 15 sec
Time:
ISmln
Day Night Day Night
Range (m)
Figure 5-5. Lidar errors as a function of integration time
123
-------
As the gas concentration in a stack plume increases, so the
Raman return signal will increase, and the errors will decrease.
However, for DAS and lidar, the transmittance of the plume decreases
with increasing gas (particle) amounts, and the errors will decrease at
first, and then increase. The signal-to-noise of the measurement
behind the plume, of course, decreases continuously with increasing
gas (particle) amounts, but this decrease is initially outweighed by
the increase in the values of Q and opacity, so that initially the per-
centage errors in concentration (opacity) decrease. However, at
larger amounts the signal-to-noise decrease outweighs the increase in
Q and opacity, and the errors increase again.
Figure 5-6 shows the increasing range capability of the Raman
technique with increasing gas concentration. However, even at very
high concentrations (10, 000 ppm) the system does not have a useful
daytime range capability. Figure 5-7 shows the variation of lidar errors
with increasing opacity up to an opacity of 0. 8. It is seen that 0. 8 opa-
city is not large enough to increase the errors during nighttime operation,
but during the day the errors start to increase between 0.4 and 0. 8 opa-
city values. Figure 5-8 shows that at nighttime the DAS errors decrease
10000 100 1000
ppm ppm ppm
Day Night Night
10000
ppna
Night
100
Range (m)
1000
Figure 5-6. Raman errors as a function of gas concentration
124
-------
10
1000
10000
Range (m)
Figure 5-7. Lidar errors as a function of opacity
80
60
fflt
:;&:'-"
&
40
20
tffflt
PT!"
Ui-.rii
Sh
Hi
TTTT
iLlt.
II
•! .«.
|h'|"::
Ij..!<-(.
a
I
EJTfrP
JSJta
S£rfc
3wp
*rifc
10
Dl = 100 ppm-m Day
Ml - 100 ppm-m Wight \.\":
"THT
D2 = 1000 ppm-m Day ]'<[•••
N2 = 1000 ppm-m Night
D3 = 2500 ppm-m Day ';,
N3 • 2500 ppm-m Night jijj
Tmrrnrnn
;;
!i;
±:
:,.
i
a
100
Range (m)
1000
Figure 5-8. DAS errors as a function of gas optical thickness
125
-------
up to 1000 ppm-m, but increase between 1000 and 2500 ppm-m. During
the day the errors decrease slightly going from 100 ppm-m to 1000
ppm-m; however, increasing the gas optical thickness to 2500 ppm-m,
dramatically increases the errors and greatly reduces the daytime range
capability of this DAS system.
5. 4 ERROR ANALYSIS OF PASSIVE INFRARED SYSTEMS
In the following, results of calculations are presented which show
that the influence of source and atmospheric parameters is significant
in the data interpretation of passive systems. Because of the importance
of the pollutant SO2, the calculations were made for this species. Since
SO2 has several infrared bands, some calculations were made for more
than one band in order to show that the proper selection of the infrared
spectral region is important, and also that the device is dependent upon
many competing parameters and requirements.
5.4. 1 Source Strength
In order to assess the plume radiation and the required noise-
equivalent-radiance, the radiated energy of SO2 in the 4 jim spectral
region was calculated in a line-by-line model. The line parameters
(position, shape, strength and lower energy level) were developed pre-
viously (Ludwig et al. 1974). The results are shown in Figure 5-9 in which
a normalized filter function with a bandwidth of 60 cm'l at half-height was
used (2465-2525 cm'1).
The influence of other species in the plume and of the atmo-
sphere between the observer and the plume and beyond was found to
be negligible. (For a pathlength of 130 m between observer and plume,
the atmospheric absorption transmission is 0.998 for 50% relative
humidity, 0. 3 ppm N2O, 320 ppm CO2, 1.4 ppm Cftj, and 0.01 ppm
SO2- The radiance of the clear sky was calculated to be less than 2
percent than that of the plume having 500 ppm-m SO2 at 650°K). The
contribution of scattered sunlight was not considered in these calculations.
From the results shown in Figure 5-9, it is clearly seen that
the emitted radiance is a sensitive function of the plume temperature.
(An uncertainty of + 10% in temperature at 500°K results in an uncer-
tainty of over + 100% in SO2 concentration). Thus, in order to determine
126
-------
Figure 5-9. Emitted radiance as a function of 863 optical thickness
the SO2 concentration in the plume, a method must be found to either
eliminate the temperature dependency or to determine the plume tem-
perature accurately by an independent measurement. (One such method
is the SAI-GFC analyzer that was described in Section 4.4. 3 and whose
field test results indicated an uncertainty of about ± 25% in the deter-
mination of SO2 concentration.)
For the case of perimter monitoring in the up-looking mode,
which was schematically shown in Figure 2-1 , line-by-line calculations
were made for measuring SO2 at 4 ym and 8.6 ym. The results for
the received radiance at 4 am as a function of SC>2 loading (ppm-m) are
shown in Figure 5-10 with a given filter function. The atmosphere has
no significant influence in this spectral region and different amounts
of water vapor would not give significantly different radiance values.
However, the influence of the atmospheric temperature is significant,
as shown in Figure 5-11, where the mean atmospheric temperature
was varied between 270 and 300°K. It is easily seen that a pure radio-
meter measurement would not be very sensitive to the S(>2 vertical
loading, but much more to the atmospheric temperature.
127
-------
2.0
£
V
2
z
10
too
ppm-m
1000
Figure 5-10. Radiance versus SOo vertical loading in 4 nm band
"
1.0
10
00
_il
in
ppm-m
1000
3DO
Figure 5-11. Radiance as a function of SO2 vertical loading in the
4 jim band for three different average atmospheric
temperatures
128
-------
This situation is reversed for the 8.6 ym region, where SC>2 has
a fundamental band. Here, the influence of the mean atmospheric tem-
perature is somewhat reduced, as shown in the comparison between the
Figures 5-1.1 and 5-12, even though an accurate radiometer measure-
ment of the SO2 vertical loading remains impossible. The influence
of three different water vapor concentrations (0. 5, 1. 0 and 1. 5 normal
Gutnick corresponding to about 30, 60 and 90 percent relative humidity)
on the effect of the observed radiance is shown in Figure 5-13.
5.4. 2 The Measurements of Signal Differences:
The influence of the atmospheric temperature and the water
vapor can be reduced by using the signal differencing method, as is
being done in the gas filter correlation instruments. In this case, the
instrument measures the difference of the incoming energy as it passes
through the gas channel, containing SC>2, and the reference channel,
containing N2.
Figure 5-12. Radiance versus vertical SO2 loading in the 8. 6 urn
region for three different mean atmospheric temperatures
129
-------
I 1 4 1 i t i 9100 i 4 s t ; i 1000 1 till! 10000
Figure 5-13. Radiance versus vertical SQg loading in the 8. 6 urn
band for three different water concentrations
We have calculated the correlation term AV for several atmo-
spheric conditions with different SO2 vertical loadings in the two infra-
red rotation-vibration bands at 4 jjm and 8.6 ^m. The 4 urn band is
essentially free of interfering species, whereas the 8. 6 um band has
many interfering water vapor lines. It is apparent from the following
analysis that although the signal in the 8. 6 ^m is stronger, its uncer-
tainty, due to water vapor interference, is significantly greater than
in the 4 jim band. The GFC signal in the 4 urn band for SO2 amounts
from 10 ppm-m to 5000 ppm-m for the effective atmospheric tempera-
tures of 275, 286 and 300°K is shown in Figure 5-14.
The results show that the AV signal is a strong function of
the SO2 concentration, but that its dependence on the atmospheric tem-
perature is relatively small; an uncertainty in temperature of + 2°K
results in an uncertainty of only about + 10 ppm-m at 100 ppm-m. The
influence of the normal atmosphere was also considered. It was found
130
-------
lO"8!
1 w*.
E ••
!
10
I- 'I!
rifi
7
itl
Uit
iilii
I
ii
Mi
i
ZUl.
ill:
/ \
z
\
§,..!
j
Iffi
\\:
II:
Tii
SO2 Optical Thickness, ppm-m
tobo
HU
!!
Figure 5-14. GFC signal as a function of SO2 loading at 4 urn
for three temperatures
that about 1000 very weak lines of ^O, 10 lines of CO2, 53 lines of
CH4 and the water vapor continuum contribute in this spectral interval.
The influence of these species was found to be negligible. The effect
of clouds in the field-of-view was also investigated. It was found that
stratus clouds decrease the AV signal. High clouds will, of course,
have less influence.
The results of the calculations as a function of the vertical
loading of SO2 at 8.6 jwn are shown in Figure 5-15. The results show
that the AV signal is stronger than in the case for the 4 itm due to the
higher radiance values in the blackbody function and the somewhat
larger band strength. Because the slope of the Planck function is much
less in this spectral region than in the 4 nm regibn, the error due to
+ 2°C in atmospheric temperature is only ± 2 ppm-m at 100 ppm-m.
However, the influence of the other atmospheric species, especially that
of water vapor, is considerably greater than in the 4 jun region. There
131
-------
10".
10
SO, Optlcil ThlckntM,
Figure 5-15. GFC signal as a function of SC>2 loading at 8. 6 urn
for three temperatures without interfering atmosphere
are 262 lines of CO2, 2518 lines of O3, 1078 lines of N2O, 89 lines of
CH4 and 497 lines of water vapor plus its continuum radiation in this
spectral region. A change in concentration of any of these species will
influence the determination of SO2 vertical loading.
The influence of water vapor variation was investigated in
greater detail. The effect manifests itself both in terms of a "cor-
relation signal" (water lines on SO2 lines) as well as an "uncorrelated
emission signal". Representative results are shown in Figure 5-16
for the AV signal as a function of SC>2 loading at three different water
vapor concentrations. The water vapor concentrations chosen cor-
respond to the 0. 5, 1.0 and 1. 5 normal Gutnick distribution. They
correspond to about 30%, 60% and 90% relative humidity at 286°K.
132
-------
10
io
p;: nr -m
'ooo
Figure 5-16. GFC signal as a function of SO2 loading for three
water vapor concentration (8.6 jjm)
If the water vapor profile were known, it would be possible to
correct the AV signal for the interference. However, it can be shown
that a single measurement of the relative humidity near the surface is
not representative of the total water vapor content in the atmosphere.
A single surface measurement can result in great errors and the un-
certainty is estimated as much as + 50% in total H2O loading in the
atmosphere, which results in an uncertainty of almost 100% in SO2
vertical loading at 100 ppm-m (see Figure 5-16). This error becomes
somewhat smaller at higher levels of SO2 loading, but is significantly
larger at lower levels. In order to reduce these errors, it would be
necessary to include additional channels into the instrument which
would permit a better estimate of the water vapor content. However,
one single measurement, giving the total amount in precipitable centi-
meters would not be sufficient, since the radiance signal is both a
function of the water vapor vertical distribution and the atmospheric
temperature profile.
133
-------
5.4. 3 Perimeter Monitoring Data Analysis (Infrared)
The radiative transfer equation shows that the downwelling
radiance depends on the atmospheric temperature profile and on the
vertical distribution of the pollutant and other species. However, in
perimeter monitoring the pollutant is confined to the lowest layers
of the atmosphere, probably below 500 m. Since the temperature lapse
rate is typically about 6°C km"*, the mean temperature of the 500 m
layer is only 1. 5°C different from the values at the top and bottom.
Hence, assuming the pollutant temperature has equilibrated with that of
the atmosphere, the radiance from the layer may be approximated
closely by a single slab model, i. e.,
N(X,T)
Of course, the lapse rate will vary with the meteorological conditions,
but it is believed that the mean temperature of the 500 m layer can be
estimated within + 2°C by measuring the surface temperature, and by
knowing the typical diurnal meteorological variations for the location.
Of course, a measured temperature at altitude may be available by
other means, such as a radiosonde, a tower or tethered balloon, in
which case the mean layer temperature would be known more accurately.
In using the perimeter monitoring technique; the pollutant may
be dispersed uniformly (or non-uniformly) through the lowest layers, or
it may be confined to a relatively thin layer, depending on the meteoro-
logical conditions and the distance from the source. These variations
of distribution mean that the pollutant mean temperature and pressure
varies, so that the GFC signal is expected to vary. However, since
the vertical variation of temperature and pressure is small within the
anticipated vertical extent of the pollutant, the effect of the pollutant
vertical distribution is small.
In our computer simulations for the GFC instrument, we found that
the single slab model AV signal at 8. 6 jun, in the range 100 ppm-m to
10, 000 ppm-m SOg, varies from the model using a profile by less than 1%,
and that the maximum effect of the variations in the SO2 vertical distribution
is an additional 1%. These uncertainties are small compared with the un-
certainties in the 8. 6 urn water vapor effects. These single slab effects are
approximately twice as great in the 4 fim SO2 band due to the greater change
of blackbody radiance with temperature, so that at 4 urn there is a 4% un-
certainty in addition to the approximately ±10% uncertainty due to the +2°C
temperature uncertainty.
134
-------
6
ADVANTAGES AND DISADVANTAGES OF
REMOTE MONITORING
TECHNIQUES
6.1 OVERVIEW
In this section we summarize the advantages and limitations of
remote monitoring techniques in EPA's enforcement and R&D pro-
grams.
In general, one of the advantages of "remote sensing" from
elevated platforms, namely the coverage of large areas, has been re-
cognized for-some time since the advent of Landsat 1 (previously known
as ERTS 1). Many interesting features of land and water areas could
be recognized, which would otherwise have remained undetected. Some
Landsat programs were even concerned with measurements of air pol-
lution (Griggs 1974), and others provided evidence in legal cases. For
example, spectral data taken by the Multi-Spectral-Scanner (MSS) on-
board the spacecraft were used to determine that a major city water
intake pipe had been placed directly into a large pool of pollutants in
Lake Superior. The image was used in a court suit by the city against
a firm that was paid about $8 million, before the Landsat program, to
position the intake pipe in a clean water area. The suit was settled
out of court, according to the NASA official (AWST 1975). In the
same article it was stated that Landsat 1 has been used in cases
involving the violation of pollution laws. In addition, the spacecraft
has been actively involved in enforcing water impoundment laws
governing the safety of damming facilities at about 50, 000 sites.
Inadequate damming was involved in at least one major fatal accident
in recent years involving the water containment areas now under
Landsat observation.
-------
Besides the advantage of wide area coverage from elevated plat-
forms, there are several other advantages of either stationary or
mobile ground-based as well as airborne remote monitors, which can
be realized in enforcement and regulatory programs. These include
cost effectiveness, direct measurement of mass emission rates, and
rapid response in air pollution episodes.
There are, of course, certain disadvantages to remote monitoring,
which include the greater initial capital outlay of the instruments, the
effect of atmospheric interference on the measurements, the increased
requirements in calibration procedures of the instruments, and lastly,
the possible eye safety hazard of the more powerful lasers operating in
the visible spectrum.
In balance, however, we found that the advantages outweigh the
disadvantages.
6. 2 COST EFFECTIVENESS
We have attempted to estimate the costs involved in remote
monitoring and to compare them with present in-stack costs. We have
chosen the monitoring of 100 stacks as a basis. Basically, there are
three types of costs: research and development, purchase price of the
operational field instrument, and operating costs,; which include the man
hours needed for maintenance, calibration and operation. Obviously,
cost estimates for the remote monitors must be regarded as very ten-
tative at this stage. EPA has published cost figures for the continuous
monitors (40FR46254, Oct. 6, 1975). For opacity monitoring alone,
investment costs including data reduction equipment and performance
tests are approximately $20, 000, and annual operating costs are approxi-
mately $8, 500. For power plants that are required to install opacity,
nitrogen oxides, sulfur dioxide, and diluent (03 or CO2) monitoring
systems, the investment cost is approximately $55, 000, and the operating
cost is approximately $30, 000.
136
-------
The cost estimates given in the following are based on evi-
dentiary monitoring for case development. It is obvious that sur-
veillance monitoring requires less effort, and that greater effort is
required for compliance monitoring. This is true for both point
sampling as well as remote monitoring. In fact, ,the development
of portable in-stack monitors will make it possible to spend relatively
short time for surveillance, and possibly even for enforcement at the
stack(s) in question (Nader 1975). Nevertheless, indications are that
significantly more time will be required for case development because
of the number of samples and the extent of documentation needed. In
any case, our studies show that more time will be needed using the
in-situ and extractive sampling techniques than the remote ones.
6.2.1 Opacity
In this case, we consider as the present methods, both the visual
observations of trained observers and the in-stack monitoring by trans-
missometers. An overview is given in Table 6-1. For the visual ob-
servations, we have assumed that approximately one-half day is used
for observing each stack and one-half day to record and analyze the data.
For 100 stacks this amounts to about 0.4 man year. The initial cost of
$1K is assumed to be needed for the smoke school.
TABLE 6-1. Initial and Operational Costs for
Monitoring the Opacity of 100 Stacks
Man Years
Monitoring Mode
Present (visual)
Present (ln*stack)
Initial Cost
IK
5-10 K
Per 100 Stacks Remarks '
0. 4 Initial Cost: Smoke School
2. 5 Initial Cost is per instrument
Remote Monitor 20-80 K 0.4
137
-------
The cost figures are drastically different for inrstack transmis-
someters. New stationary sources are required to have continuous
monitors installed and the results have to be reported to the appropriate
agencies. Thus, it is difficult to assign any additional cost figures,
because the analysis of the company reports demand an uncertain
amount of man hours. However, if a state agency is compelled to
conduct in-stack monitoring for evidentiary purposes, the cost for
100 stacks increases significantly. The cost of an in-situ transmis-
someter is estimated to 5-10 K, and the additional effort to monitor
is estimated to be about 5 man days per stack (Smith 1974). Thus, for
100 stacks, 4000 man hours or about 2.0 man years will be needed. In
contrast, the remote Lidar instrument is estimated to be in the price
range from about $20 K to $80 K, which is several times higher than
the in-stack instrumentation. However, the operational costs are lower,
and are judged to be the same as for the visual observations.
The estimate of the cost for enforcement monitoring with a tele-
photometer shows a low initial purchase price, but comparable opera-
tional costs.
The cost estimates for aerial surveillance from aircraft are
relatively low because of the large area coverage, assuming automatic
data analysis. Of course, the cost of operating the aircraft must be
added.
6.2.2 Gas Concentration
An overview for gas instrumentation to measure gases is given
in Table 6-2. The present monitoring is conducted by analyzing the
company reports, required for new stationary sources. If in-stack
monitoring is required, for case development, the reference method
costs about $1. 5 K, and takes about 10 man days to collect the data.
Thus, for 100 stacks, 8000 man hours or 4 man years would be required.
However, if portable in-stack monitors were to be used, the operational
time would probably be reduced.
As in the case for opacity monitoring, the initial price of the
remote sensors is higher than the in-stack monitor, but the operational
costs are considerably less. We have assumed that the operational
laser systems will be about $20-$80 K; the passive instruments are
less expensive and will be in the range from about $10-$40 K. The
operational costs are assumed to be similar to the ones used for the
remote opacity measurements.
138
-------
TABLE 6-2. Initial and Operational; Costs for
Monitoring Gases in 100 Stacks
Man Years
Monitoring Mode
Present (in-stack)
Active Remote Sensor
Passive Remote Sensor
Initial Cost
1.5K
20-80 K
10-40 K
Per 100 Stacks
8.0
0.4
0.4
Remarks
Initial Cost is per Instrument
Limited in concentration and
temperature range
* Veloclmeter (50 K) needed for mass now.
The price for other spectroscopic instruments such as multi-
spectral photometers, interferometer-spectrometers, and monochrom-
ators can vary widely, depending on the complexity.
6. 3 MEASUREMENTS OF MASS EMISSION RATES
The signals of the laser Doppler velocimeter (LDV) are not only
related to the particle velocity, but also to the opacity of the plume.
When the relationship between the opacity and mass loading for each
type of source has been established, the LDV will provide the particulate
mass emission rate from a source.
6.4 UNANNOUNCED AND NON-INTERFERING MONITORING
The requirements to install continuous monitors in new stacks
keeping the records and making them available to*enforcement personnel
does not appear to alleviate the need for "unannounced" enforcement
monitoring, including night observations. The remote techniques dis-
cussed in this report are the possible tools for this kind of monitoring.
In addition, remote techniques do not interfere with the normal operation
of the plant, by the requirement of installing temporary in-stack monitors
by the enforcement personnel.
139
-------
6. 5 SURVEY OF WIDE GEOGRAPHICAL AREAS
Surveys of wide geographical areas are required for the design
of contact monitoring networks, assessment of site selection and in the
evaluation of environmental degradation.
For the design of contact monitoring networks are based upon
extensive measurements of "dispersion meteorology" data. The term
"dispersion meteorology" used here refers to the air mass transport,
speed and wind direction, frequency distribution, and to the vertical
dimension available for dilution of contaminants, j In addition, stream-
flow or streamline maps are useful in understanding how the air moves
from one area to another, and is influenced by the topography. The
complexity in the streamline flow patterns is reflected in the conver-
gence and divergence of the streamlines and in the deflection by ele-
vated terrains. As a general rule, experience has shown that complex
flow patterns have more turbulent mixing associated with them than
do less complex flow patterns.
Based on these results the location of contact monitoring for
area sources should be concentrated in areas of convergence. In areas
where similar data are not available, remote monitors will be of great
assistance in setting up contact networks.
Also the knowledge of point sources in relationship to the "dis-
persion meteorology" will influence the location of a contact monitor
network. The assessment of the impact to be encountered on the site
selection of a new industrial/power plant involves a great number of
aspects. Beside the aspects of the water and land requirements, the
most difficult ones are the requirements for minimum impact on air
quality. This involves a preliminary analysis of available meteorological
and background air quality data; plume simulation utilizing aerial smoke
techniques under a variety of adverse pollution weather conditions; in-
stallation of a meteorological and background air quality monitoring
system to obtain routinely ambient data and using tracer systems. The
releases of tracers are made in various combinations of time periods
depending upon the prevailing weather conditions. To follow the trace
gases, an extensive sampling network must be established of radial
lines and concentric arcs in the downwind sector on ground level and
elevated terrain consisting of numerous individual bag samples and
sequential samplers. It is obvious that remote monitors mounted in
vans/aircraft would be of great assistance in these programs.
140
-------
6. 6 RAPED RESPONSE IN POLLUTION EPISODES
The mobility and near real-time analysis of remote sensed data
make it possible to monitor the sources and ambient air in pollution
episodes, where it is vital to determine early, whether or not the
emergency measures are effective.
For example, many local Air Pollution Control Districts have
defined alert stages. The ones given in the Los Angeles APCD have
been listed in Table 3-5. When any one of these alert stages is readied,
the LA APCD prescribes certain actions that are to be undertaken to
prevent further increase of pollutant levels and to effect a decrease in
the pollutant levels. The public at large is to be informed about the
alert stages by public announcements, and through announcements of
the L. A. County Sheriff to its sub-stations, all city police departments
and California Highway Patrol as well as local public safety personnel,
air polluting industrial plants and processes which require "alert" data
in order to effect pre-arranged plans, are designed to reduce the output
of air contaminants.
In the LAAPCD, the pollution levels are monitored by twelve
(12) widely separated local sampling stations (see Figure 6-1). In
addition, inspectors in radio-equipped cars are utilized to spot "ex-
cessive visible" emissions and odors. However,.they are not equipped
to measure the ambient air in real time, and cannot determine, the
air pollution levels in alert episodes, when it is of vital importance.
Again, remote sensors mounted either in vans or on airborne plat-
forms would fulfill this aspect of monitoring tasks.
141
-------
Figure 6-1. Los Angeles County Air Pollution Control
District Air Monitoring Network
142
-------
6. 7 LIMITATION UNDER CERTAIN ATMOSPHERIC CONDITIONS
Remotely sensed data are influenced by the intervening atmo-
sphere, and extreme conditions such as fog and rain will limit the use-
fulness of those data. Although in many cases the direct influence is
accounted for (for example, by using multiple wavelength detection), the
transmission is, in general, reduced, thus reducing the signal levels.
In the limit of opaque atmospheres, no signals from the remote source
are obtained while extractive and in-situ samples continue to function.
6. 8 INCREASED REQUIREMENTS IN CALIBRATION
As pointed out in Section 3.4, the calibration procedures for
remote monitors are quite different than for point samplers because
of the significant influence of the intervening atmosphere and background.
Large calibration cells to simulate long atmospheric path must be
provided. Test ranges with a calibrated stack emission generator
must be provided.
6.9 SAFETY HAZARD
The potential health hazard for eye safety has been discussed in
Section 5.2. It was pointed out that the most obvious hazard with the
laser beam is direct axial viewing, but specular reflections are also a
risk, and considerable care must be exercised in the use of laser systems.
The most sensitive region is the visible region, since the cornea,
aqueous humor and the lens readily transmit this radiation. Thus, the
full laser beam energy is focused on the retina in;an image 10-20 pm
in diameter resulting in high power densities of kilowatts/cm^ on the
retina even for milliwatt powers incident on the pupil of the eye.
In the spectral regions outside the visible one, the dangers are
reduced since the radiation is absorbed by the lens, cornea, and/or
aqueous humor. However, inflammation of the cornea and conjunctive
can result and cataracts may be formed.
143
-------
Thus, performance standards for laser products have been pro-
mulgated to minimize these hazards. Any operational laser system
utilized in remote monitoring must conform to these ^standards. The
analysis made in Section 5.2 showed that the Minimum Safe Range and
safe beam diameter for existing laser systems vary widely. While the
present laser systems used in the differential absorption by scattering
method, in the visible, and the laser Doppler velocimeter method, in
the infrared, appear safe, with certain slight modifications, the existing
Raman and Lidar systems do not appear safe for operational use in the
visible regions.
144
-------
CONCLUSIONS AND RECOMMENDATIONS
7. 1 GENERAL CONCLUSIONS
Based upon the material presented in the body of this report,
the following conclusions are drawn:
Certain remote monitoring techniques can be successfully
applied to
• Surveillance and compliance monitoring of smoke-
stacks and extended sources;
• Monitoring to support EPA's research studies for
the evaluation of control equipment and development
of performance standards;
• Surveys of emissions for validation of dispersion
models;
• Surveys to determine the representativeness of
ambient air point measurements and to assist in
developing optimum contact monitoring networks;
and
• Quick response in air pollution episodes.
The results of the present survey of the published literature
on remote instruments/techniques are summarized in Tables 742 and
7-3 for the active and passive systems, respectively. In these tables,
only the generic terms of the techniques and/or instruments are j[iyen
and no specific instruments by manufacturer or model are identified,
except for two instruments that are commercially available. The
column entitled "Development Status" distinguishes between "com-
mercially available", "advanced prototypes" and "prototypes". The
145
-------
TABLE 7-1. Pollutant Amenable to Remote Monitoring
IMMEDIATE/NEAR TERM LONG TERM
Particles (Opacity)*
SO2* Heavy HC ;
NO2* Oxides of Sulfur
CO' Certain Specific Elements
Light HC Chlorinated HC
HF*
HC1
NH3
NOx*
H2S-
HMO,
°3
Vinyl chloride
°2-
co2
N2
* Standards of Performance for New Stationary Sources '
Proposed or Promulgated
** Species used for determining excess air flow
latter two designations refer to the fact that more "product engineering"
has yet to be done before the instruments could be marketed; the
"advanced" ones less so that "non-advanced prototypes". Most of
them have been field tested. The last column contains also the section
or table number(s) where more details are given.
In general, we believe that the active systems appear to have
a larger applicability in terms of range in plume temperature and
pollutant concentrations. This is because measurements made with
passive systems are more influenced by source and background para-
meters than the active systems. The measurements made by infrared
passive systems are particularly dependent upon the plume tempera-
ture. Thus, either multiwavelength and/or ratio techniques are re-
quired to minimize the temperature dependence. •• In addition, relatively
cool plumes will have lower signal strength. Thus, pollutant measure-
ments in these cool plumes will be more difficult to achieve with
passive systems. On the other hand, passive systems tend to be less
expensive than active systems, and appear therefore more attractive
for the application of surveillance monitoring, where the data are
not used in case development. The laser systems that are most
146
-------
TABLE 7-2. Active Systems
Technique/
Instrument
Differential
Absorption
Lidar
Laser Doppler
Velocimeter
Long -Path
Raman
Resonance
Raman
Fluorescence
Fabry-Perot
Raman
Spectral
Region
Vis/UV
IR
ViB
IR
Vis/UV
/IR
Vl»/UV
Vls/UV
Vis/UV
Vie/UV
Species/
Parameter
SO^NO,
SJ.NO,
Many
Cases
Opacity
Particles
Velocity
Mass Flow
Many
Gases
SOj
Many
Gases
Many
Gases
Many
Gases
Some
Gases
Mode
Stack/Area
Perimeter/'
Area
Slack
Perimeter/
Area
Stack
Area
Stack/
Perimeter
Stack
Area
Stack
Stack/
Area
Slack/
Area
Stack/
Area
Stack/
Area
Development Status
Prototype
Field Tested
Prototype
Field Tested
Prototype; Under
Development
Under Development
for Aircrnit
Application
Prototype
Flold Tented
Prototype on Air-
craft Field Tested
Advanced Prototype
Field Tested
Demonstrated in
Field Tests
Several Techniques
Field Tested, Some
Prototypes Developed;
COSPEC Commer-
cially Available
Field Tests Not
Encouraging
Theoretical
Laboratory Study
Laboratory Study '
Laboratory Study
Remarks
Ground -based - Present instrumentation
as used not eye-safe (Section 4. 3. 1)
lias been done for SO? (Section 4. S. 1)
Development Is primarily for ambient
air (see Table 4 -IS)
See Table 4-15, at present for ozone
Not eye cafe yet, (Section 4. 3. 5) eye-
onfa system being rtrvrlupcd
Gives 3-dimcnslonal mapping of relative
concentrations (Section 4. 5)
Necessary for some emission standards
(Section 4. 3. 7 and Table 3-1)
Relates opacity and mass concentration
(Section 4. 3. 7)
Using remote transmitter or retro-
reflector; can be laser, dispersive or
non-dispersive systems (Table 3-16):
useful mainly for ambient air monitoring
Limited In range, especially during day
(Sections 4. 3. 2 and S. 3. 2)
Usefulness limited (Section 4. 3. 2 and
5.3.2)
Needs to be .demonstrated In field:
poiialble interference duo to fluorescence
by gases and other species (Section 4. S. 3)
Looks doubtful in terms of sensitivity
and specificity; (Section 4. 3. 4)
Provides Increased sensitivity over
vibratlonal Raman; still limited In range,
especially during day (Section 4. 3. 2)
147
-------
TABLE 7-3. Passive Systems
Technique/
Instrument
Spectral Species/
Region Parameter
Mode
Development Status
Remarks
Matched Filter
Correlation
Gas Filter
Correlation
UV/Vis SOj,, N02
Photography
Vidlcon
IR
VlB
UV
IR
Heterodyne IR
Kadiumeter
Dispersive IR
Spectrometer
CO
Opacity
Stack/ COSPEC Commercially
Perimeter Available, Field Tested,
Not Encouraging for
Stacks; Main Emphasis
on Perimeter Monitoring
Stack JRB Sensor Commercially
Available, Being Field
Tested
Perimeter Prototype Field Tested
Many
Gases
Many
Cases
Perimeter Prototype Under
Development
Stack Being Field Tested on
Ground and From
Aircraft
Stack Prototype Field Tested
Stack prototype Field Tested
Stark/ . Laboratory Study and
Perimeter/ Aircraft Based Proto-
Area' type Under Develop-
ment
Stack Several Techniques
Field Teated, Some
Prototypes Developed
Quantitative Interpretation difficult due
to varying aerosols (Section 4.4.2 and
4.5)
Limited in concentration and tempera-
ture range (Section 4.4.3), but tem-
perature effect reduced
<
No quantitative data reported as yet
(Section 4. 5)
See Table 4-14
Needs further development for quantita-
tive analysis; nighttime observations
feasible with image Intenslfler (Section
4.4.1)
Quantitative Interpretation difficult due
to varying aerosols; has potential as a
velocimeter (Section 4.4.9)
Independent knowledge of plume tempera-
ture required for quantitative analysis;
has potential as a velocimeter (Section
4.4.9)
Achieves hlRh nprrlflrlty; had yrt lo be
demonstrated In field (Section 4.4.7)
Includes scanning spectrometer and In-
terferometer-spectrometer: requires
high spectral resolution for specificity
and requires knowledge of plume tem-
perature (Section 4.4.4 and 4.4.5)
148
-------
promising for near-term operational use are differential absorption,
Lidar and laser Doppler velocimeter. The most promising passive
systems are correlation instruments, vidicons and photographic
techniques. Of course, for area surveys, both active and passive
systems (ground-based and airborne) have application.
7.2 RECOMMENDATIONS
Based upon the general conclusions and, in particular, upon
Table 7-2 and 7-3, we recommend that EPA continues, initiates and/
or participates in the development of several instruments discussed
below. Many agencies, in particular NASA, have been involved in
the development of remote sensing techniques for air pollutants. It
would appear cost-effective for EFA to coordinate (as is already
done in some developments), its research programs with these
agencies.
Laser Doppler Velocimeter
o Stack Emission Monitoring
--Make system operational in a van, together with the
DAS system
--Make it available to field enforcement personnel
--Continue the development of the LDV system to relate
its output to opacity, and to mass flow.
Lidar
o Stack Emission Monitoring
—Make the lidar system operational in a van for the
measurement of opacity;
—Make it available to field enforcement personnel.
• Perimeter and Area Monitoring
—Adapt lidar to airborne operations for the measurement
of mixing height, dispersion of plumes and air quality
assessment over wide geographical areas.
149
-------
Differential Absorption
• Stack Emission Monitoring
—Make the DAS system operational in a van for the mea-
surement of SO2 and NO,;
—Make it available for field enforcement personnel;
—Develop the DAS system for the measurement of other
species, using the infrared spectrum.
• Perimeter and Area Monitoring
—Continue to develop the DAS system to airborne operation
to measure 03 between the aircraft and the ground for air
quality assessment over wide areas;
—Initiate the development of the DAS system for van and
aircraft based operations to measure all gases for
perimeter and area monitoring.
Correlation Techniques
• Stack Emission Monitoring
—Continue the field testing of the passive GFC instrument
for the measurement of SQg and make it available to
field enforcement personnel;
—Extend the techniques to other species, such as
CO, HC1, and light hydrocarbons.
• Perimeter and Area Monitoring
—Make available to field enforcement personnel the pas-
sive MFC instrument, van based, for SO« and NO«;
--Develop the correlation techniques in the IR for SO2
and other pollutants, van based.
150
-------
Vidicons
• Stack Emission Monitoring
--Make vidicon system for SO2 operational;
—Make it available to field enforcement personnel;
—Develop system to NO2 in the UV;
—Investigate possibility further to use vidicon as
velocimeter.
Photographic Techniques
• Stack Emission Monitoring
--Develop aircraft based photographic techniques for
daytime opacity measurements.
Long-Path Systems
• Perimeter/Area/Ambient Air Monitors
—Continue to develop laser systems;
—Continue to develop non-dispersive techniques.
Resonance Raman
--Conduct field tests to verify feasibility of resonance
Raman technique to measure SOg in stack emissions.
Heterodyne Radiometry
--Continue the development of passive/active heterodyne
radiometry for measuring gaseous pollutants in smoke-
stack plumes.
Based on the specifications which EPA have proposed or pro-
mulgated for continuous in-stack and ambient air monitors, we recom-
mend that performance specifications for remote ^monitors be developed
as more experience is gained in the field with existing monitors.
151
-------
The important instrument parameters specified by EPA for
continuous in-stack and ambient air monitors, which will also apply
to remote techniques when used in enforcement monitoring, include:
• Span (lower detectable limit to upper level which must be
than the emission standard by a given factor)
• Noise (usually 0. 5 of the lower detectable limits)
• Accuracy (percentage difference between values measured
by the sensor and the applicable reference method)
• Calibration error (absolute mean value plus 95 percent
confidence interval)
• Zero and calibration drift (absolute mean value plus 95
percent confidence interval)
• Response time (rise and fall time to 95 percent)
• Interference equivalent (influence on pollutant signal by
interfering gases in plume)
Additional parameters that will have to be specified for
remote techniques will include, but may not be limited to the
following:
Range (minimum and maximum distance between source and
observer, in which the instrument must give data with the
specified accuracy)
Field-of-view (should be small enough for the source plume
to fill the fov)
Interference equivalent (influence on pollutant signal by the
interviewing atmosphere as well as sky radiation beyond the
plume)
Supporting plume data (velocity, temperature, etc.)
152
-------
The information'recorded during field observations should
include those given in ,Method 9 for the visual observation:
• Time
• Observer location (distance, direction, height)
• Background description
• Weather conditions (wind direction and speed,
ambient temperature)
• Sky conditions
• Plume description (if visible)
153
-------
8
REFERENCES
Ahmed, S. A. (1973) Appl. Opt. 12, 901.
Allerio, F. (1976) private communication.
Acton, L.L. etal. (1973) AIAA J. 11,899
Asai, K. and Igarashi, T. (1974) Preprint, private communication.
AWST (Aviation Week and Space Technology) (1975) February 3.
Barnes, H. M., Herget, W. F. and Robbins, R.. (1974) "Analytical
Methods Applied to Air Pollution Measurements", p. 245, Ann
Arbor Science Publishers, Inc., August.
Barrett, J. J. (1974) "Laser Raman Gas Diagnostics", Ed. M. Lapp
and C. M. Penney, Plenum Press, New York.
Barringer, A. R. et al. (1966) 4th Symposium on Remote Sensing,
Ann Arbor.
Bartle, E. R. et al., (1972) J. Spacecraft and Rockets 11, 836.
Bartle, E. R. (1974a) EPA Contract 68-02-1208, JRB Assoc. Inc.,
La Jolla, CA, April.
Bartle, E. R. (1974b) EPA Contract 68-02-1481, Final Report, October.
Bartle, E. R. (1975) private communication. :
Baumgartner, R., Warshaw, S., Wolfe, D. and Byer, R. L. (1976)
Mtg. Abstract, J. Opt. Soc. Am. 66, 386.
Byer, R. L. (1975) Optical and Quantum Electr. 7, 147.
Byer, R. L. and Garbuny, M. (1973) Appl. Opt. 12, 1497.
154
-------
Chen, S. H., Lin, C. C. and Low, M. J. D. (1974) Environ, Sci. and
Techn. 7_, 424.
Collis, R. T. H. and Uthe, E. E. (1972) Opto-electronics 4, 87.
Conner, W. (1974) EPA-650/2-74-128.
Conner, W. (1975) EPA, private communication.
Conner, W. (1976) EPA, private communication.
Conner, W. D. and Hodkinson, J. R. (1967) P. H. S. Publ. 999-AP-30.
Cook, C. S., Bethke, G. W. and Conner, W. D. (1972) Appl. Opt. 11,
1742. * ~~
Decker, J. A., Jr. (1971) Appl. Opt. 10, 510.
Derr, V. E., and Little, C. G. (1970) Appl. Opt. 9, 1976.
DHEW (1974) 39 FR 32094.
DHEW (1975) 40 FR 32257.
DeLong, H. P. (1974) Opt. Eng. 13, 5.
Ellison, A. H. (1974) EPA-650/2-74-015.
Elterman, L. (1966) Appl. Opt. 5, 1769.
Elterman, L. (1968) AFCRL-68-0153, April.
Emission Monitoring Requirements and Performance Testing Methods
(1974) 39 FR 32852.
Exton, R. (1974) NASA-Langley, private communication.
Evans, W. E. (1967) "Development of Lidar for Stack Effluents Opacity
Measurements" SRI Project No. 6529 for Edison Electric Inst.
(NTISPBNo. 233-135/AS)
Fowler, M. C. and Berger, P. J. (1974) EPA Report No. EPA-650/
2-74-020, January.
155
-------
Gelbwachs, J. A., Birnbarm, M., Tucker, A. W. and Finder, C. L.
(1972) Opto-electronics 4, 155.
Gelbwachs, J. A. and Birnbarm, M. (1973) Appl. Opt. 12, 2442.
Granatstein, V. L., Rhinewine, M. and Fitch, A. H. (1973) Appl.
Opt. 12, 1511.
Grant, W. B., Hake, R. D., Jr., Listen, E. M., Robbins, R. C. and
Proctor, E. K., Jr. (1974) Appl. Phys. Lett. 24, 550.
Grant, W. B. and Hake, R. D., Jr. (1975) J. Appl. Phys. 46, 3019.
Griggs, M. (1975) "Measurements of Atmospheric Aerosol Opticsl
Thickness over Water Using ERTS-1 Data, J. Air Poll. Control
Assoc. 25, 622.
Herget, W. F. (1976) EPA, private communication
Herget, W. F., Miller, C. R., and Sonnenscheiri, C. M. (1975) Paper
WA7, Spring Conf. OSA, Anaheim, CA, March 19-21.
Hess, R. V. and Seals, R. K., Jr. (1974) "Applications of Tunable
High Energy/Pressure Pulsed Lasers to Atmospheric Trans-
mission and Remote Sensing", NASA TM X-72010.
Hinkley, E. D. (1972) Opto-electronics 4, 69.
Hinkley, E. D. and Calawa, A. R. (1974) "Analytical Methods Applied
to Air Pollution Measurements", Ed. R. K. Stevens and W. F.
Herget, Ann Arbor Science Publishers, Inc.
Hirschfeld, T. B. (1974) Opt. Eng.. 13, 15.
Hoell, J. M., Jr., Wade, W. R. and Thompson, R. T., Jr. (1975)
Int. Conf. on Environ. Sensing and Assessment, Las Vegas,
Nevada, Sept. 14-19.
Hoffman, A. J., Curran, T. C., McMullen, T. B., Cox, W. M. and
Hunt, W. F., Jr. (1975) "EPA's Role in Ambient Air Quality
Monitoring", Science 190, 243.
Hulburt, E. O. (1946) J. Opt. Soc. Am. 27, 377.
156
-------
Igarashi, T. (1973) 5th Conference on Laser Radar Studies, Williams-
burg, Virginia, June.
Inaba, H. and Kobayasi, T. (1972) Opto-electronics 4, 101.
Inomata, H. and Igarashi, T. (1974) Radio Research Laboratories,
Tokyo, Preprint, private communication.
Keyes, R. J. and Quist, T. M. (1970) "Semiconductors and Semimetals",
Eds. R. K. Willardson and A. C. Beer (Academic Press, New York,
Chapter 8).
Kildal, H. and Byer, R. L. (1971) Proc. IEEE 59, 1644.
Klainer, S. M. (1974) SPIE Meeting, San Diego, August.
:
Kobayasi, T. and Inaba, H. (1975) Optical and Quantum Electronics 7.
319.
Koehler, T. (1974) Electro-Opt. Systems Design, page 24, October.
Ku, R. T., Hinkley, E. D. and Sample, J. O. (1975) Appl. Opt. 14,
854.
Kuhl, J. and Schmidt, W. (1974) Appl. Phys. 3, 251.
Langan, L. (197la) AIAA Meeting, Palo Alto, November.
Langan, L. (197Ib) Environmental Measurement Inc., San Francisco,
Report EPA Contract No. 6H-02-0124.
Larson, N. M., Crosmun, R., and Talmi, Y. (1974) Appl. Opt. 13,
2662.
Lawrence, T. R., Krause, M. C., Craven, C. E. and Herget, W. F.
(1975) AIAA Mtg. Paper 75-684, May.
Low, M. J. D. and Clancy, F. K. (1967) Environ. Sci. and Techn. L
73.
Ludwig, C. B., Griggs, M., Malkmus, W. and Bartle, E. R. (1974)
Appl. Opt. 13, 1494.
Measures, R. M. and Pilon, G. (1972) Opto-electronics 4, 141.
Menzies, R. T. (1971) Appl. Opt. 10. 1532.
157
-------
Menzies, R. T. and Shumate, M. S. (1974) Science 184, 570.
Melfi, S. H., Brumfield, M. L. and Storey, R. W., Jr., (1973) Appl.
Phys. Lett. 22, 402.
Mclfi, S. H. (1974) EPA, Las Vegas, private communication.
Miller, C. R. and Sonnenschein, C. M. (1975) Contractor Report
EPA-650/2-75-062.
Murray, E. R., Hake, R. D., Jr., van der Laah, J. E. and Hawley,
J. G. (1976) Appl. Phys. Lett., May.
Nader, J. (1975) EPA-Research Triangle Park, private communication.
Nakahara, S. et al. (1972), Opto-electr. 4^ 169-177
Nakahara, S., Ito, K., and Ito, S. (1973) Abstract from 5th Conference
on Laser Radar Studies, Williams burg, Virginia, June.
Newcomb, G. S. and Millan, M. M. (1970) IEEE Trans. Geosci.
Electronics GE-8, 149.
Northam, G. B. (1976) NASA, Langley Research Center, private
communication.
Pikus, I. M., Goldstein, H. W. and Reithof, T. R. (1971) AIAA Meeting,
Palo Alto, November.
Penney, C. M., Morey, W. W., St. Peters, R. L., Silverstein, S. D.,
Lapp, M., and White, D. R. (1973) NASA CR-132363, September.
Poultney, S.K., Brumfield, M. L. and Siviter, J.H. (1975), 7th Intern.
Laser Radar Conf., SRI, Nov. 4-7
Prengle, H. W., Jr., Morgan, C. A., Fang, Cheng-Shen, Huang,
Ling-Kim, Campani, P., and Wu, W. W. (1973) Environ. Sci.
and Techn. 7, 417.
Pressman, A. (1976) EPA-Las Vegas, private communication.
Prostak, A. and Dye, R. H. (1970) Bendix Final Report, Contract
CPA 22-69-55, October.
150
-------
Robinson, J. W. and Dake, J. D. (1973) Spectr. Lett. 6, 685.
Robinson, J. W. and Dake, J. D. (1974) Anal. Chem. Acta. 71, 277.
Rosen, H., Robrish, P. and Chamberlain, O. (1975) Appl. Opt. 14,
2703. ~~
Ross, R. D. (1972) Air Pollution and Industry, Van Norstrand Reinhold
Co., New York.
Rothe, K. W., Brinkmann, U. and Walther, H. (1974a) Appl. Phys. 3,
115.
Rothe, K. W., Brinkmann, U. and Walther, H. (1974b) Appl. Phys. 4,
181.
Saraf, D. G. and Jackson, D. W. (1975) 7th Int. Laser Radar Conf.,
Menlo Park.
Schotland, R. M. (1974) J. Appl. Meteorol. 13, 71.
Science Applications, Inc. (1975) EPA Contract 68-02-1798.
Seals, R. K., Jr., "Analysis of Tunable Laser Heterodyne Radiometry:
Remote Sensing of Atmospheric Gases", AIAA J. 12, 1118.
Smith, W. (1972) Opto-electronics 4, 161.
Smith, H. (1975) EPA, private communication.
Sperling, R. B. (1975) Contractor Report EPA-600/2-75-077.
Streiff, M. L. and Claysmith, C. L. (1972) Contractor Report EPA-
R2-72-052.
Streiff, M. L. and Ludwig C. B. (1973) Contractor Report EPA-650/
2-73-026.
Swift, R. D. and Wattson, R. B. (1974) 9th Remi Sens. Symp. Abstracts,
Ann Arbor, April. :
Tanabe, L. H. (1975) "Determination of Sp2 Concentrations by Remote
Sensing in the Infrared", General Dynamics/Pomona Report No.
TM/6-125PH-430 on EPA Contract 68-02-1804.
Ward, T. V. and Zwick, H. H. (1975) Appl. Opt. 14, 2896.
Zaromb, S. (1974) Pittsburg Conference, Cleveland, March.
L59
-------
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA 340/1-76-005
2.
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
Application of Remote Techniques in Stationary
Source Air Emission Monitoring
5. REPORT DATE__,.
June 1976
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
C. B. Ludwig and M. Griggs
8. PERFORMING ORGANIZATION REPORT NO.
SAI-76-687-LJ
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Science Applications, Inc.
P. O. Box 2351
La Jolla, California 92038
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
68-03-2137
12. SPONSORING AGENCY NAME AND ADDRESS
13. TYPE OF REPORT AND PERIOD COVERED
Environmental Protection Agency
Stationary Source Enforcement
401 M Street, S. W., Washington, D. C. 20460
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
replaces EPA 340/1 - 75- 009
16. ABSTRACT
The usefulness of remote sensing techniques for monitoring the gaseous and
particulate emissions from stationary sources is analyzed. The present status
of active and passive remote monitoring instruments is evaluated. This study
confirms that the technique of differential absorption has the best sensitivity for
the single-ended measurement of gaseous and particulate pollutants. In general,
data interpretation problems of the passive techniques make them less accurate
than the active methods.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Air Pollution Observations
Remote Monitoring
13B
14D
13. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
178
20 SECURITY CLASS (This page)
Unclassified
22. PRICE
-------
INSTRUCTIONS
1. REPORT NUMBER
Insert the EPA report number as it appears on the cover of the publication.
2. LEAVE BLANK
3. RECIPIENTS ACCESSION NUMBER
Reserved for use by each report recipient.
4. TITLE AND SUBTITLE
Title should indicate clearly and briefly the subject coverage of the report, and be displayed prominently. Set subtitle, if used, in smaller
type or otherwise subordinate it to main title. When a report is prepared in more than one volume, repeat the primary title, add volume
number and include subtitle for the specific title.
5. REPORT DATE
Each report shall carry a date indicating at least month and year. Indicate the basis on which it was selected (e.g., date of issue, date of
approval, date of preparation, etc.).
6. PERFORMING ORGANIZATION CODE
Leave blank.
7. AUTHOR(S)
Give name(s) in conventional order (John R. Doe. J. Robert Doe, etc.). List author's affiliation if it differs from the performing organi-
zation. . = • ' •
8. PERFORMING ORGANIZATION REPORT NUMBER
Insert if performing organization wishes to assign this number.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Give name, street, city, state, and ZIP code. List no more than two levels of an organizational hirearchy.
10. PROGRAM ELEMENT NUMBER
Use the program element number under which the report was prepared. Subordinate numbers may be included in parentheses.
11. CONTRACT/G R ANT NUMBE R
Insert contract or grant number under which report was prepared.
12. SPONSORING AGENCY NAME AND ADDRESS
Include ZIP code.
13. TYPE OF REPORT AND PERIOD COVERED
Indicate interim final, etc., and if applicable, dates covered.
14. SPONSORING AGENCY CODE
Leave blank.
15. SUPPLEMENTARY NOTES
Enter information not included elsewhere but useful, such as: Prepared in cooperation with, Translation of, Presented at conference of,
To be published in, Supersedes, Supplements, etc.
16. ABSTRACT
Include a brief (200 words or less) factual summary of the most significant information contained in the report. If the report contains a
significant bibliography or literature survey, mention it here.
17. KEY WORDS AND DOCUMENT ANALYSIS
(a) DESCRIPTORS - Select from the Thesaurus of Engineering and Scientific Terms the proper authorized terms that identify the major
concept of the research and are sufficiently specific and precise to be used as index entries for cataloging.
(b) IDENTIFIERS AND OPEN-ENDED TERMS - Use identifiers for project names, code names, equipment designators, etc. Use open-
ended terms written in descriptor form for those subjects for which no descriptor exists.
(c) COSATI FIELD GROUP - Field and group assignments are to be taken from the 1965 COSATI Subject Category List. Since the ma-
jority of documents are multidisciplinary in nature, the Primary Field/Group assignment(s) will be specific discipline, area of human
endeavor, or type of physical object. The application(s) will be cross-referenced with secondary Field/Group assignments that will follow
the primary posting(s).
18. DISTRIBUTION STATEMENT
Denote reusability to the public or limitation for reasons other than security for example "Release Unlimited." Cite any availability to
the public, with address and price. .
19. & 20. SECURITY CLASSIFICATION
DO NOT submit classified reports to the National Technical Information service.
21. NUMBER OF PAGES
Insert the total number of pages, including this one and unnumbered pages, but exclude distribution lit), if any.
22. PRICE
Insert I hi; price set by the National Technical Information Service in Hie Go vein men I Printing Office, if known.
CCrt Kerni 22ZO-I (9 73) (Reverie)
------- |