United States
Environmental Protection
Agency
Office of Research and
Development
Washington DC 20460
EPA/600J3-91./018
March 199t
National Dry Deposition
Network: Third Annual
Progress Report (1989)
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EPA/600/3-91/018
March 1991
NATIONAL DRY DEPOSITION NETWORK
THIRD ANNUAL PROGRESS REPORT (1989)
by
Eric S. Edgerton, Thomas F. Lavery,
and Hugh S. Prentice
Environmental Science & Engineering, Inc.
Gainesville, FL 32607
Contract #68-02-4451
Project Officer
Rudolph P. Boksleitner
Exposure Assessment Research Division
Atmospheric Research and Exposure Assessment Laboratory
Research Triangle Park, NC 27711
ATMOSPHERIC RESEARCH AND EXPOSURE ASSESSMENT LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
RESEARCH TRIANGLE PARK, NC 27711
Printed on Recycled Paper
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Notice/Disclaimer
The information in this document has been funded wholly by the
U.S. Environmental Protection Agency (EPA) under Contract No.
68-02-4451 to Environmental Science & Engineering, Inc. (ESE).
It has been subjected to the Agency's peer and administrative review,
and it has been approved for publication as an EPA document.
Any mention of trade names or commercial products does not con-
stitute endorsement or recommendation for use.
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FOREWORD
The Atmospheric Research and Exposure Assessment Laboratory
(AREAL) is committed to performing research and development in the
characterization of air pollutant sources, sinks, transport, and
transformations, in the assessment and prediction of exposure of humans
and ecosystems to environmental pollutants, and in the development of
monitoring systems and other technologies to determine the status and
trends in pollutant concentrations and the condition of the nation's
ecosystems.
As national and international concern over acid deposition has
grown, so has the need for information on spatial patterns and secular
trends. Although wet deposition monitoring has been able to provide
such information, it is realized that total acid deposition has both wet
and dry components; the latter of which has not been well defined.
AREAL has supported the development of a network of dry acid deposition
monitoring sites to demonstrate the feasibility of operating such a
network and to provide a database of atmospheric dynamics and chemical
concentrations that can be transformed to dry acid deposition quantities
through the application of one or more dry deposition algorithms.
Applications for final data may include determination of spatial and
temporal trends, evaluation of deposition models, refinement of mass
balance estimates, and support to effects studies.
This report summarizes the third year's progress of the National
Dry Deposition Network.
Gary J. Foley, Ph.D.
Director
Atmospheric Research and Exposure Assessment Laboratory
Research Triangle Park, NC 27711
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ABSTRACT
The National Dry Deposition Network (NDDN) is designed to provide
long-term estimates of dry acidic deposition across the continental
United States. Fifty NDDN sites were operational during 1989, including
41 sites in the eastern United States and 9 sites in the western United
States. Each site was equipped with sensors for continuous measurements
of ozone and meteorological variables required for estimation of dry
deposition rates. Weekly average atmospheric concentrations of S04",
NOj, NHj, S02, and HN03 were measured (using 3-stage filter packs)
throughout the year, while Na+, K+, Ca2+, and Mg2+ were measured from •
January through September. Separate day/night samples were analyzed
from January through September and around-the-clock samples were
analyzed for the remainder of 1989.
Results showed species-dependent variability in atmospheric
concentrations from site to site, season to season, and day to night.
In general, SO2.", NH^, S02, and HN03 concentrations were much higher
(factor 5-10) at eastern sites than at western sites. On the other
hand, N03, Na+, K+, Ca2+, and Mg2+ concentrations were frequently
,2-
comparable at eastern and western sites. Average SO^", NHJ, and HN03
concentrations were typically highest during summer and lowest during
fall. In contrast, S02 and N03 were highest in winter and lowest in
summer. Day/night variability was low for aerosols, but frequently
pronounced for S02 and HN03, especially during the summer and at sites
located in complex terrain. Comparison of 03 data for 1988 and 1989
showed marked differences between years and a distinct tendency for
higher concentrations in 1988. Ninety-eight exceedances of the NAAQS
were observed at 18 sites in 1988, while only 15 exceedances were
observed at 43 sites in 1989. i Approximations of annual dry deposition
rates for SO2', S02, N03, and HN03 suggest that gaseous deposition
greatly exceeds aerosol deposition and that dry fluxes are similar to
wet deposition at numerous sites in the eastern United States.
Application of site-specific dry deposition models are needed to refine
these estimates.
IV
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CONTENTS
Foreword yj
Abstract jv
Figures vj
Tables xj
Abbreviations and Symbols
Acknowledgement
1. Introduction 1
2. Conclusions 3
3. Network Description and Operations 7
3.1 Network Description 7
3.2 Network Operations 11
3.2.1 Field Operations 13
3.2.2 Laboratory Operations 15
3.2.3 Data Management 19
4. Results and Discussion . 22
4.1 Overall Data Quality 22
4.1.1 Field Data 22
4.1.2 Laboratory Data 25
4.1.3 Collocated Filter Pack Sampling 29
4.2 Filter Pack Measurements 46
4.2.1 SO^ 46
4.2.2 NOg 50
4.2.3 NHj .....' 53
4.2.4 HN03 61
4.2.5 S02 . , '.'.'. 66
4.2.6 Day Versus Night Concentration Data 71
4.2.7 Aerosol Ion Balances 78
4.2.8 1988 Versus 1989 Concentration Data 85
4.3 Ozone 89
4.4 Estimated Dry Deposition 115
References
123
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FIGURES
Number 2a£e
1 Status of NDDN monitoring , sites- -December 1989 ........ 8
2 Filter pack assembly ... ............ ....... 12
3 Recovery of SO2; (A) and NOj (B) in NIST Reference
Sample 2694-11 ..... ............. • ..... 27
4 Relative percent difference from replicate analysis of SO2" in
cellulose filter extracts (A) and NOj in nylon filter extracts
(B) 1989 ........ ........... ........ 28
5 Scattergrams of 1989 collocated SO2" for three eastern sites (A)
and one western site (B) ; ................... 32
6 Scattergrams of 1989 collocated NOg for three eastern
sites (A) and one western site (B) .............. 33
7 Scattergrams of 1989 collocated NH^ for three eastern
sites (A) and one western site (B) .............. 34
8 Scattergrams of 1989 collocated HN03 for three eastern
sites (A) and one western site (B) .......... J ... 35
9 Scattergrams of 1989 collocated S02 for three eastern
sites (A) and one western site (B) . ............. 36
10 Scattergrams of 1989 collocated Na+ for three eastern
sites (A) and one western site (B) .............. 37
11 Scattergrams of 1989 collocated K+ for three eastern
sites (A) and one western site (B) ........ ..... .38
12 Scattergrams of 1989 collocated Ca2+ for three eastern
sites (A) and one western site (B) .......... .... 39
13 Scattergrams of 1989 collocated Mg2+ for three eastern
sites (A) and one western site (B) .......... .... 40
14 Absolute difference and absolute percent difference versus
SO2' concentrations for eastern (A,B) and western (C,D)
collocated samples in 1989 .................. 41
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FIGURES (continued)
Number
15
Page
Absolute difference and absolute percent difference versus
NOj concentrations for eastern (A,B) and western (C,D)
collocated samples in 1989 42
16 Absolute difference and absolute percent difference versus
NH4 concentrations for eastern (A,B) and western (C,D)
collocated samples in 1989 43
17
18
19
20
21
22
23
24
25
26
27
Absolute difference and absolute percent difference versus
HN03 concentrations for eastern (A,B) and western (G,D)
collocated samples in 1989 44
Absolute difference and absolute percent difference versus
S02 concentrations for eastern (A,B) and western (C,D)
collocated samples in 1989 _ 45
Annual average So|' concentrations for the eastern
United States during 1989 47
Average S0|~ concentrations for tbe first (A) and
second (B) quarter 1989
48
Average S0|' concentrations for the third (A) and
fourth (B) quarter 1989 49
Average SO^" concentrations for western NDDN
sites, combined third and fourth quarters 1989 51
Annual average NOg concentrations for the eastern
United States during 1989 . . 52
Average NOg concentrations for the first (A) and
second (B) quarter 1989 • 54
Average NOg concentrations for the third (A) and
fourth (B) quarter 1989 55
Average NOg concentrations for western NDDN
sites, combined third and fourth quarters 1989 56
Annual average NH^ concentrations for the eastern
United States during 1989 . 57
VII
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FIGURES (continued)
Number
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
Page
Average NH^ concentrations (Mg/m3) for the first
(A) and second (B) quarter 1989 58
Average NHj concentrations (Mg/m3) for the third
(A) and fourth (B) quarter 1989 59
Average NH^ concentrations (/ig/ra3) for western
NDDN sites, combined third and fourth quarters 1989 60
Annual average HN03 concentrations (/Ltg/m3) for the
eastern United States during 1989 62
Average HNOa concentrations (/Xg/m3) for the first
(A) and second (B) quarter 1989 ; ... 63
Average HN03 concentrations (jUg/m3) for the third
(A) and fourth (B) quarter 1989 64
Average HN03 concentrations (jUg/ra3) for western
NDDN sites, combined third and fourth quarters 1989 65
Annual average S02 concentrations (/ig/m3) for the
eastern United States during 1989 67
[
Average S02 concentrations (jLig/m3) for the first
(A) and second (B) quartet 1989 68
Average S02 concentrations (jUg/m3) for the third
(A) and fourth (B) quarter 1989 ...
69
Average S02 concentrations' (jtig/m3) for western
NDDN sites, combined third and fourth quarters 1989 70
Weekly day/night S02 concentrations for sites in complex
terrain (A), mountaintop terrain (B), and rolling terrain
(C), October 1988 through September 1989 .79
Aerosol ion balances for Sites 108, 117, and 133
Aerosol ion balances for Sites 116, 119, and 120
Aerosol ion balances for Sites 135, 156, and 165
81
82
83
vm
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FIGURES (continued)
Number
Page
43 Aerosol cation/anion ratios for eastern (A) and western
(B) NDDN sites 84
44 Total cations versus total anions for a northeastern site (A),
midwestern site (B), and a western site (C)
October 1988 through September 1989 86
45 NH^/SOl' ratios for eastern (A) and western (B)
NDDN sites 87
46 Annual average concentrations (jUg/m3) of SO^" (A)
and N03 (B) for 1988/1989 88
47 Annual average concentrations (/ig/m3) of HN03 (A)
and S02 for 1988/1989 90
48 03 daily averages and maxima for two northeastern
sites: Site 106 (A) and Site 135 (B)--1989 95
49 03 daily averages and maxima for two southeastern
sites: Site 127(A) and Site 150 (B)--1989 96
50 03 daily averages and maxima for two midwestern
sites: Site 122 (A) and Site 134 (B)--1989 97
51 03 daily averages and maxima for two western
sites: Site 165 (A) and Site 168 (B)--1989 98
52 03 frequency distribution for Sites 108 (A) and
119 (B) during 1989 100
53 03 frequency distribution for Sites 116 (A)
and 118 (B) during 1989 101
54 Hourly average 03 concentrations for typical sites in
rolling terrain: Site 108 (A) and Site 129 (B) 102
55 Hourly average 03 concentrations for typical sites in
complex terrain: Site 119 (A) and Site 121 (B) 103
56 Hourly average 03 concentrations for typical
mountaintop sites: Site 118 (A) and Site 126 (B) 104
IX
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FIGURES (continued)
Number
57
58
59
60
61
62
63
64
65
66
Hourly average 03 concentrations for typical urban
or semiurban sites: Site 116 (A) and Site 146 (B)
1988 (A) versus 1989 (B) 03 frequency distribution
for Site 121 .
Page
105
1988 (A) versus 1989 (B) 03 frequency distribution
for Site 129
1988 (A) versus 1989 (B) 03 frequency distribution
for Site 120 :
107
108
109
Number of hourly observations greater than or
equal to 80 ppb, 1988 (A) versus 1989 (B) Ill
7-Hour growing season averages,
1988 (A) versus 1989
Integrated 03 exposure indices (W126) for 1988 (A)
(B)
112
and 1989 (B) 113
Ratios of 1988 versus 1989 growing season solar
radiation (A) and 1988/1989 growing season temperatures (B). 114
Observed wet (A) and estimated dry (B) deposition
(eq/ha-yr) of S0% at selected sites . 121
Observed wet (A) and estimated dry (B) deposition
(eq/ha-yr) of N03 at selected sites 122
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TABLES
Page
1 NDDN site information 9
2 NDDN monitoring equipment 14
3 Precision and accuracy objectives of field measurements .... 16
4 Precision and accuracy objectives for NDDN
laboratory measurements 20
5 Summary of meteorological sensor unadjusted
calibrations during 1989 23
6 Summary of 1989 03 and mass flow controller
unadjusted calibrations ; 24
7 Summary of laboratory accuracy and precision
during 1989 ' 26
8 Results of 1989 collocated filter pack sampling 30
9 Day (D) versus night (N) concentrations of particulate
SOl" for selected sites 72
10 Day (D) versus night (N) concentrations of particulate
NH^ for selected sites 73
11 Day (D) versus night (N) concentrations of particulate
N03 for selected sites 74
12 Day (D) versus night (N) concentrations of HN03
for selected sites 75
13 Day (D) versus night (N) concentrations of total N03
for selected sites 75
14 Day (D) .versus night (N) concentrations of S02
for selected sites 77
15
Summary of NDDN 03 measurements during 1989 91
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TABLES (continued)
Number Page
16 Wet deposition sites used to evaluate wet versus
dry deposition rates 116
17 Estimated deposition velocities for aerosols and
gases H7
18 Estimated dry deposition of S02, SO^", HN03, and NOg
during 1989 119
xn
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LIST OF ABBREVIATIONS AND SYMBOLS
ABBREVIATIONS
cm/sec
CVS
DAS
DMC
EEPROM
EPA
eq/ha
eq/ha-yr
ESE
ft
1C
ICAP
ICP-ES
km
L/mln
m
m3
MAD
MAPD
m/sec
mL
mmHg
MFC
NAAQS
NADP/NTN
NCLAN
neq/m3
NDDN
NIST
NOAA/ATDD
PC
ppb
ppm/hr
QA
QC
ML
Mg
Mg/filter
- centimeters per second
- calibration verification standard
- data acquisition system
- data management center
- electronically erasable programmable read-only memory
•U.S. Environmental Protection Agency
• equivalents per hectare
- equivalents per hectare per year
• Environmental Science & Engineering, Inc.
• foot
• ion chromatography
• inductively coupled argon plasma
• inductively coupled plasma emission spectroscopy
• kilometer
• liters per minute
meter
cubic meter
median absolute difference
median absolute percent difference
meters per second
milliliters
millimeters of mercury
mass flow controller
National Ambient Air Quality Standard
National Atmospheric Deposition Program/National Trends
Network
National Crop Loss Assessment Network
nanoequivalents per cubic meter
National Dry Deposition Network
National Institute of Standards and Technology
National Oceanic and Atmospheric
Administration/Atmospheric Turbulence and
Diffusion Division
personal computer
parts per billion
parts per million per hour
quality assurance
quality control
microliter
microgram
micrograms per filter
xm
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LIST OF ABBREVIATIONS AND SYMBOLS
ABBREVIATIONS, continued
/Ltm
MS
UV
W/m2
-- micrograms per liter
- - micrometer ;
-- micrograms per cubic meter
-- ultraviolet
- - watts per square meter
SYMBOLS
Br'
"C
Ca2+
HN03
K+
K,C03
Ms2*3
Na+
NH4N03
NO
N03
°3
SO,
o/^2-
-- bromide ion
-- degrees Celsius
-- particulate calcium
-- nitric acid
-- particulate potassium
-- potassium carbonate
-- particulate magnesium
-- particulate sodium
-- particulate ammonium
-- ammonium nitrate
-- nitric oxide
-- particulate nitrate
- - ozone
- - sulfur dioxide
-- particulate sulfate
XIV
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ACKNOWLEDGEMENTS
The authors gratefully acknowledge the following site operators-
without whose dedicated efforts NDDN could not succeed: S. Lumpkin
(101); S. Scott (104); R. Prins and R. Hopkins (105); D. DeCapria (106)-
F. Wood (107); G. Brooks (108); S. Nolan (109); T. Butler (110)-
Russell (111); D. Dorn (112); D. Croskey (113); S. Hammond (114)-
Chilcote (115); V. Miller (116); J. Hufman (117); R. Gubler (118)-
Jenkins (119); S. Long (120); M. Brotzge (121); T. Chatfield (122)-
Stineman (123); F. Matt and J. Matt (124); P. Hughes (126)- M Hale
(127);-S. Scamack (128); M. Yewell (129); M. Snider (130); D. Conrad
(133); F. Emstrom (134); D. Olberding (135); R. McCollum (137)- M Lang
and T. Mouzin (140); H. Burnett (144); P. Hess (146); W. Dunn (149)-
D. Honnell (150); B. Scobey; Jr. (151); P. Ruf (152); J. Melin (153)-
J. Bishop (156); W. Steiner and B. Steiner (157); B. Barr (161)-
C. Jenson (162); J. Moubray (163); S. Kiracofe (164); C. Laster'(165)•
B. Smith, D. Anderson, and A. King (167); R. Ljung (168); D. Lukens
(169); and P. Hays (174).
XV
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SECTION 1.0
INTRODUCTION
Atmospheric deposition takes place via two pathways: wet
deposition and dry deposition. Wet deposition is the result of
precipitation events (rain, snow, etc.) which remove particles and gases
from the atmosphere. Dry deposition is the transfer of particles and
gases to the landscape in the absence of precipitation. Wet deposition
rates of acidic species across the United States have been well
documented over the last 5 to 10 years; however, due to measurement
difficulties, comparable information is unavailable for dry deposition
rates.
The direct measurement of dry deposition can be extremely
difficult, but a number of investigations have recently shown that it
can be reasonably inferred by coupling air quality data with routine
meteorological measurements (Shieh et al.. 1979; Hicks et al.. 1985;
Meyers and Yuen, 1987; Wesely and Lesht, 1988). Using analogies with
heat and momentum flux, Shieh et al. (1979) computed submicron particle
and sulfur dioxide (S02) deposition velocities as a function of land
use, season, windspeed, and meteorological stability class. Results of
this calculation for the eastern United States showed that deposition
rates for both species were strongly dependent on windspeed, solar
radiation, and the condition and type of ground cover. For example,
rapidly growing vegetation was found to experience higher deposition
rates than senescent vegetation, and forests generally experienced
higher rates than short grass or snow. Wesely (1988) has expanded this
approach to calculate deposition rates for various additional species
[including nitric acid (HN03) ] and to use site-specific meteorological
data. Seminal work on the subject of dry deposition has been performed
by Hicks et al. (1985) and Hosker and Womack (1986), who developed,
tested, and deployed the first field system for inferential dry
deposition measurements.
In 1986, the U.S. Environmental Protection Agency (EPA) contracted
with Environmental Science & Engineering, Inc. (ESE) to establish and
operate the National Dry Deposition Network (NDDN). The objective of
NDDN is to obtain field data at 50 to 100 sites throughout the United
States to establish patterns and trends of dry deposition. Ultimately, •
dry deposition fluxes will be calculated using measured air pollutant
concentrations and inferred deposition velocities estimated from
meteorological, land use, and site characteristic data. One or more of
the inferential approaches developed by Hicks et al. (1985) and Wesely
(1988) will be used to estimate dry deposition velocities.
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This report describes progress on the NDDN during calendar year
1989. The purpose of this report is to familiarize the reader with the
general approach of NDDN and the various types of data which are being
produced and reported to EPA. This report is not intended to provide a
definitive analysis of the 1989 database. It describes the network
configuration and deployment schedule as well as procedures developed
for field operations, laboratory operations, database management, and
quality control (QC). An overview of air quality data for 1989,
including QC results, is also presented. Finally, dry deposition rates
are estimated for selected sites and compared with wet deposition rates.
These estimates were prepared using literature values for deposition
velocities (not the inferential approaches cited previously) and,
therefore, are intended only to illustrate likely ranges of deposition.
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SECTION 2.0
CONCLUSIONS
Deployment of all currently planned NDDN sites was completed during
1989. Forty-one primarily rural monitoring sites were operated
throughout the year in the eastern United States. These sites form a
relatively dense sampling array, especially in the midwest and
northeast, and should provide useful data for evaluation of spatial
patterns east of the Mississippi River. In addition, nine sites in the
western United States were established and operational by midyear. Due
to the size and diversity of the western region, the sparse distribution
of sites was designed to assist in the quantification of total
atmospheric deposition at specific receptors, rather than to evaluate
spatial patterns of dry deposition.
Each NDDN site was equipped with sensors and sampling apparatus for
continuous measurement of ozone (03) and meteorological variables
required to estimate dry deposition. Weekly average atmospheric
concentrations of particulate sulfate (SO^"), particulate nitrate (NO3) ,
particulate-ammonium (NH^), S02, and HN03 were determined throughout the
year so weekly dry deposition loadings could be calculated. Particulate
sodium (Na+), particulate potassium (K+), particulate calcium (Ca2+),
and particulate magnesium (Mg ) were determined for part of the year to
evaluate the presence of atmospheric base cations. EPA is currently ,
working jointly with the National Oceanic and Atmospheric
Administration/Atmospheric Turbulence and Diffusion Division (NOAA/ATDD)
to finalize the algorithms for dry deposition calculations.
NDDN sites were also equipped with a sophisticated data acquisition
system (DAS), which permitted automated data retrieval and real-time
access and review of data from a central data management center (DMC).
Overall, this system has worked well; the networkwide data capture rate
for 1989 was 93 percent.
The following conclusions summarize results of a preliminary
analysis of the 1989 database:
1. In general, the data produced by NDDN meet quality assurance
(QA) objectives established at the onset of the program.
Results of equipment calibrations and audits show that NDDN
meteorological and chemical sensors produce data of sufficient
quality for dry deposition estimates most of the time. For
example, the network-wide average accuracy for 03 monitors was
in the range of 98 to 102 percent for each calendar quarter.
Individual 03 monitors were invariably in the range of 90 to
110 percent for the entire year.
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Operation of dual, side-by-side air samplers at four sites
indicates that filter pack measurements can be very precise .
For three eastern sites, analyses of 864", S02, and NH^
exhibited precision estimates of 5 percent, or better; N03 and
HN03 exhibited precision of 10 percent, or better. For a
single site in the western United States (i.e., 167),
measurements of the above species uniformly exhibited precision
within 5 percent. Median absolute differences between paired
samples from site 167 were within 0.03 microgram per cubic
meter (/ig/m3) for S0|", N03, NH^, S02, and HN03. Although the
accuracy of filter p£ck measurement is currently unknown, this
degree of precision will prove useful in the eventual
determination of secular trends and spatial patterns .
Results of filter pack analyses throughout the year at 41
eastern sites show species-dependent variability from site to
site, season to seaspn, and day to night. Annual average
concentrations of atmospheric SO^" exhibited peak values of
approximately 7.8 JUg/m3 in western Ohio and central Kentucky
and minimum values around 2 to 4 /ig/m3 on the periphery of the
network (i.e., Maine, Wisconsin, and Florida). Spatial
variability for SO^" was relatively low as compared to other
species.
Data for nine western sites operated over the last half of 1989
showed appreciably lower concentrations than eastern sites
(i.e., 0.7 to 1.7 Jtig/m3) . Among western sites, the highest SO^"
concentrations were consistently observed in northern and
southern Arizona.
4. Annual SO, concentrations for eastern sites showed a maximum of
23.2 /!ig/m in western Pennsylvania and an ellipse of values
above 10 Mg/m3 extending eastward from Illinois and Kentucky to
the eastern seaboard. As for SO^", the lowest concentrations
of S02 among eastern [sites (i.e., 2.4 to 3.0 /Ltg/m3) were
observed in Maine, Wisconsin, and Florida. Western sites
exhibit dramatically|lower S02 than eastern sites (i.e., 0.4 to
1.3 Jig/m3) and highest concentrations in southern Arizona.
5. Annual average HN03 ranges from 3.6
Pennsylvania to 0.7 Mg/m3 in Maine.
in southeastern
Concentrations above 2.0
/ig/m3 cover a broad region (excluding a few isolated sites)
from the Great Lakes t to northern Alabama and Georgia. Average
HN03 for the western sites is typically about 50 percent of the
lowest values observed in the east. Arizona sites, however,
exhibit HN03 concentrations similar to those reported for
Florida, Wisconsin, and Maine.
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Annual N03" concentrations show marked regional character, with
maxima above 2.0 /zg/m3 throughout the midwest and minima below
0.5 pg/m3 at scattered locations from New England to Florida.
Examination of land use characteristics suggests a link between
N03 and land use. The highest overall concentrations correlate
strongly with agricultural areas (in any region) , and the
lowest concentrations correlate with forested areas. Results
for the western sites indicate comparable concentrations to
forested areas of the eastern United States.
Annual average NH$ data range from 3 . 2 ^ig/m3 in northern
Indiana to 0.7 fig/m3 in Maine and exhibit spatial variability
similar to N03. Concentrations above 2.0 pg/m3 are observed
throughout the midwest and at sites near agricultural activity
in the southeast and northeast. Only sites in extreme northern
New York, New Hampshire, Maine, and Florida exhibit annual
averages below 1.0 ftg/m3 . Data for western sites show the
majority of NH^ concentrations in the range of 0.2 to
0.4 /zg/m3. Consistent with other measurements, the highest .
concentration among western sites occurs in southern Arizona.
Dry deposition rates for SCf, S02, N03, and HN03 for 28 sites in
the eastern United States were estimated based on annual
average concentration data and literature values for deposition
velocity. Results have an uncertainty of at least 50 percent.
Estimated dry deposition of sulfur species (i.e., SO^" and S02)
range from about 100 equivalents per hectare (eq/ha) in Maine
to about 750 eq/ha in western Pennsylvania.
Estimated dry deposition of nitrogen species (i.e., HN03 and
N03) ranges from about 55 eq/ha in Maine to about 290 eq/ha in
eastern Pennsylvania. Due to faster deposition velocities, the
gaseous species HN03 and S02 represent a large fraction of dry
nitrogen and dry sulfur deposition, respectively. Comparison
of wet and dry deposition of sulfur and nitrogen at 28 eastern
sites suggests that the two are of similar magnitude over large
areas. However, wet deposition appears to be dominant around
the periphery of the network, where atmospheric concentrations
of S02 and HN03 are low.
Inspection of 03 data for 1989 shows that there were relatively
few episodes of elevated concentrations. Comparison of 03 data
for 1988 and 1989 shows that these two years differ
significantly by virtually any measure. For example, the 18
sites operational during 1988 reported 98 exceedances of the
National Ambient Air Quality Standard (NAAQS) for 03 of
120 parts per billion (ppb) , while the 43 sites operational
during 1989 reported only 15. Similarly, calculations of
various exposure indices show that growing season exposures
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r
10.
were about 25 to 100 percent higher in 1988 than in 1989,
depending on the index selected. Results suggest that 1988 and
1989 represent years!of extremely high and low to moderate 03
exposure, respectively. Examination of meteorological data for
the 2 years also shows that 1988 was substantially hotter and
sunnier than 1989. |
Calculations of aerosol ion balances for selected sites
indicate general differences between regions and land use
categories. Forested northeastern and southeastern sites
exhibit an excess of measured anions (SOl" and NO^) over
measured cations
Na"1", K"1
Ca , andMgz+); agricultural
midwestern sites exhibit a slight excess of cations over
anions; and western sites exhibit a substantial excess of
cations over anions. The anion/cation imbalance increases
(both on relative and absolute bases) with increasing SO^" plus
N03 at northeastern and western sites. Thus, the importance of
an unmeasured ion (e^g., H+ in the northeast, HCO^ in the west)
appears to increase during periods of high concentration.
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SECTION 3.0
NETWORK DESCRIPTION AND OPERATIONS
3.1 NETWORK DESCRIPTION
The status of the NDDN, as of December 1989, is shown in Figure 1.
Forty-one eastern sites and two western sites were operational
throughout the year. Eight additional sites (one eastern, seven
western) were brought up to full operational status by July 1, 1989.
The single eastern site (Site 111--Speedwell, TN) replaced another site
in Tennessee (Site 102--0ak Ridge), which was demobilized due to the
proximity of large point sources of S02 and NOX.
The names, locations, reporting dates, elevation, terrain, and
land-use classifications of all NDDN sites are listed in Table 1.
Terrain and land-use information refers to a 10-kilometer (km) radius
around the site and is presented to convey a sense of the setting within
which each site operates. Note that site numbers are used for
identification purposes only and do not correlate with order of
installation or operation.
For purposes of discussion, the eastern United States sites can be
divided into three regions: midwestern, northeastern, and southeastern.
The midwest includes Illinois, Indiana, Kentucky, Michigan, Ohio, and
Wisconsin and contains 14 NDDN sites. The southeast includes Arkansas,
Alabama, Georgia, Florida, Mississippi, North Carolina, and Tennessee
and contains 10 sites. The northeast includes the remaining eastern
states, from Virginia to Maine, and contains 17 sites. Inspection of
land-use information in Table 1 shows marked regional differences in the
character of sites. Midwestern sites are predominantly agricultural,
while those in the northeast and southeast are mostly forested. Terrain
characteristics also differ systematically between regions. The
majority of midwestern sites are located on flat countryside and only
that in eastern Kentucky (Site 121--Lilley Cornett Woods) is in complex
terrain. Northeastern sites, in contrast, are almost evenly divided
between rolling and complex terrain, and only one site (Site 116--
Beltsville, MD) is in flat terrain. Three northeastern sites are
located above 1,000 meters (m) and form a line extending southward from
northern Virginia to North Carolina. Site 118 (Big Meadows, VA) is
situated at the crest of the Blue Ridge; Site 120 (Morton Station, VA)
and Site 126 (Cranberry, NC) occupy the spine of the Appalachian
Mountains. Due to the unique, exposure of these sites, they have been
placed in a separate terrain category (i.e., mountaintop).
-------
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TABLE 1. NDDN SITE INFORMATION
Site
No.
Initial
Reporting
Site Name Date
Elevation
Latitude Longitude (m)
Land
Use
Terrain
101 Research Triangle
Park, NC
102* Oak Ridge, TN
103** West Point, NY
104 West Point, NY
105 Whiteface
Mountain, NY
106 PSU, PA
107 Parsons, W
108 Prince Edward, VA
109 Woodstock, NH
110 Connecticut Hill,
NY
111 Speedwell, TN
112 Kane Experimental
Forest, PA
113 M.K. Goddard, PA
114 Deer Creek State
Park, CH
115 Ann Arbor, MI
116 Beltsville, MD
117 Laurel Hill State
Park, PA
118 Big Meadows, VA
119 Cedar Creek State
Park, WV
120 Horton Station, VA
121 Illley Comett
Woods, Kf
122 Oxford, OH
123 Lykens, OH
124 Unionville, MI
126 Cranberry, NC
127 Edgar Evins State
Park, TN
128 Arendtsville, PA
129 Perryville, KY
130 Bondville, LL
01/06/87
35.91
78.88
94 Forested-Urban Rolling
01/06/87
01/06/87
01/06/87
01/06/87
01/06/87
01/14/88
11/01/87
12/31/88
09/14/87
07/01/89
12/31/88
01/08/88
09/30/88
06/30/88
12/31/88
12/10/87
06/30/88
11/09/87
06/03/87
01/19/88
08/18/87
09/30/88
06/30/88
12/31/88
03/22/88
06/30/88
08/11/87
02/09/88
35.96
41.35
41.35
44.39
40.73
39.09
37.17
43.94
42.40
36.47
41.60
41.43
39.63
42.42
39.03
40.00
38.52
38.88
37.33
37.08
39.53
40.92
43.61
36.11
36.04
39.92
37.68
40.05
84.29
74.05
74.05
73.86
77.95
79.66
78.31
71.70
76.65
83.83
78.77
80.15
83.26
83.90
76.82
79.25
78.44
80.85
80.55
82.99
84.72
83.00
83.36
82.04
85.73
77.30
84.97
88.37
341
203
203
570
378
505
146
258
515
372
618
384
265
267
46
616
1,073
234
972
335
284
296
198
1,219
302
269
279
212
Forested
Forested
Forested
Forested
Agricultural
Forested
Forested
Forested
Forested
Agricultural
Forested
Forested
Agricultural
Forested
Urban- Agric.
Forested
Forested
Forested
Forested
Forested
Agricultural
Agricultural
Agricultural
Forested
Forested
Agricultural
Agricultural
Agricultural
Complex
Complex
Complex
Complex
Rolling
Complex
Rolling
Complex
Rolling
Rolling
Rolling
Rolling
Rolling
Flat
Flat
Conplex
Mountaintop
Conplex
Mountaintop
Complex
Rolling
Flat
Flat
Mountaintop
Rolling
Rolling
Rolling
Flat
-------
TABLE I.1 (continued)
Site
No.
133
134
135
137
140
144
146
149
150
151
152
153
156
157
161
162
163
164
165
167
168
169
174
Initial
Reporting
Site Name Date
Salamonie, IN
Perkinstown, WI
Ashland, ME
Coweeta, NG
Vincenres, IN
Washington's
Crossing, NJ
Argonne National
Laboratory, IL
Wellston, ML
Caddo Valley, AR
Coffeeville, MS
Sand Mountain, AL
Georgia Station,
GA.
Sumatra, EL
Alhanbra, IL
Gothic, 00
Uinta, UT
Reynolds Creek, ID
Saval Ranch, NV
Pinedale, WY
Chiricachua, AZ
Glacier National
Park, MT
Centennial, WY
Grand Canyon, AZ
06/30/88
09/30/88
12/31/88
11/03/87
08/05/87
12/31/88
07/01/87
06/30/88
09/30/88
12/31/88
12/31/88
06/30/88
12/31/88
06/30/88
07/01/89
07/01/89
07/01/89
07/01/89
12/31/88
07/01/89
12/31/88
07/01/89
07/01/89
Latitude
40.82
45.21
46.61
35.06
38.74
40.30
41.70
44.22
34.18
34.00
34.29
33.18,
30.11
38.87
38.96
40.55
43.21
41.29
42.93
32.01
48.51
41.31
36.06
Longitude
85.66
90.60
68.41
83.43
87.49
74.87
87.99
85.82
93.10
89.80
85.97
84.41
84.99
89.62
106.99
110.32
116.75
115.86
109.79
109.39
114.00
106.15
112.18
Elevation Land
(m) Use
249
472
235
686
134
58
229
295
71
134
352
266
14
164
2,926
2,377
1,198
1,873
2,388
1,570
963
2,579
2,073
Agricultural
Agricultural
Agricultural
Forested
Agricultural
Agric. -Urban
Urban- Agric.
Forested
Forested
Forested
Agricultural
Agricultural
Forested
Agricultural
Range
Range
Range
Range
Range
Range
Forested
Range
Forested
Terrain
Flat
Rolling
Flat
Complex
Rolling
Rolling
Flat
Flat
Rolling
Rolling
.Rolling
Rolling
Flat
Flat •
Complex
Complex
Rolling
Rolling
Rolling
Complex
Complex
Complex
Complex
Note: m - meter.
^Operation terminated 12/31/88.
**0peration terminated 01/30/88.
Source: ESE, 1990.
10
-------
Regional differences in terrain and land use, as reflected by the
network, appear to faithfully represent actual differences between the
midwest, northeast, and southeast. The midwest is largely agricultural
flatland, while the northeast is largely forested. By definition, the
Appalachian Mountains and foothills are almost entirely contained within
the northeastern and southeastern regions. Many of the sites classified
as complex are located along the eastern and western flanks of the
Appalachians. As will be discussed later, these regional differences in
terrain and land use appear to exert significant influences over the air
quality observed across the network.
Despite apparent similarities in land use and terrain, the western
sites are by no means homogeneous in character. In fact, nearly every
site is located in a distinct subregion of the west. Site 161 (Gothic,
CO) occupies a mountain valley within the central Rocky Mountains.
Site 162 is located on the foothills of the High Uintas, the most
prominent east-west mountain range in North America. Sites 163 and 164
are located in similar surroundings near the northern extreme of the
Great Basin. Sites 165 and 169 represent the transition from the
western Great Plains to the Rocky Mountains. Sites 167 and 174 are
located in the arid southwest; however, Site 167 is in the Sonoran
Desert, while Site 174 is on the extensive and forested Kaibab Plateau.
Site 168 (near the Canadian border) alone represents the western boreal
forest. Thus, although these sites are collectively termed the western
part of the network, they represent a wide range of environments.
3.2 NETWORK OPERATIONS
This section provides an overview of the field, laboratory, and DMC
operations for NDDN. Step-by-step protocols and additional details of
these activities can be found in the NDDN Field Operations Manual,
Laboratory Operations Manual, and Data Management Manual (ESE, 1989a
1990a, 1990b).
Ambient measurements for 03, S02, SO^", NOj, HNO
'3.
NHj, windspeed,
wind direction, temperature, relative humidity, solar radiation,
precipitation, and delta temperature were performed throughout the year
at .each NDDN site. In addition, atmospheric Na+, K+, Ca°+, and Mg^+
were measured from October 1988 through September 1989, and surface
wetness sensors were deployed in April 1989. Meteorological parameters
and 03 concentrations were recorded continuously and reported as hourly
averages consisting of a minimum of nine valid 5-minute averages.
Atmospheric sampling for particles and gases (except 03) was integrated
over weekly day and night collection periods from January through
September and weekly around-the-clock collection periods from October
through December, using a 3-stage filter pack (see Figure 2). In this
approach, particles and selected gases are collected by passing air at a
controlled flow rate through a sequence of Teflon®, nylon, and base-
impregnated cellulose filters. Filter packs were prepared and shipped
11
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12
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to the field weekly and exchanged at each site every Tuesday. Day
filter pack samples were collected over the hours of 0800 to 2000 local
standard time, while night samples were collected over the remaining
hours of the day. Blank filter packs were collected monthly to evaluate
passive collection of particles and gases as well as contamination
during shipment and handling. At 14 sites located more than 50 km from
National Atmospheric Deposition Program/National Trends Network
(NADP/NTN) sites, wet deposition samples were collected weekly
(according to NADP/NTN protocols) and shipped to ESE for chemical
analysis beginning in early 1989. Table 2 lists the equipment installed
and operated at NDDN sites.
3.2.1 Field Operations
Each site was equipped with a shelter (complete with telephone and
100-amp electrical service), two 10-m towers, a meteorological system,
an 03 and air-quality monitoring system, and a DAS. One tower was used
for meteorological measurements, using a Climatronics F460 system or
R.M. Young AQ system. Windspeed and wind direction were measured at
10 m, temperature was measured at 9 m and 2 m, and relative humidity was
measured at 9 m. Precipitation and solar radiation were measured on 1-m
platforms located outside the rain and sun shadows of the shelter and
towers. Surface wetness was measured at a height of approximately 3 to
6 inches above the surrounding low-lying vegetation (typically grass).
Filter pack sampling and 03 measurements were performed at 10 m
using a tilt-down aluminum tower (Aluma, Inc.). Day, night, and blank
filter packs were fitted with noninterchangeable quick connects to
prevent confusion and to reduce time for exchange in the field. Filter
pack flow was maintained at 1.50 liters per minute (L/min) at eastern
sites and 3.00 L/min at western sites [for standard conditions of
25 degrees Celsius (°C) and 760 millimeters of mercury (mmHg)] with a
Teledyne-Hastings CST-10K mass flow controller (MFC) and recorded as
hourly averages on the DAS. Switching from the day filter pack to the
night filter pack was performed by a relay-activated solenoid controlled
by the DAS.
03 was measured via ultraviolet (UV) absorbance with a Thermo-
Environmental Model 49-103 analyzer operating on the 0- to 500-ppb
range. Ambient air was drawn through an all-Teflon® sampling line
upstream of the 03 analyzer. A 3/8-inch TFE Teflon® sample line was
used to sample air atop the 10-m air quality tower. Teflon® filters
housed at the tower inlet and the analyzer inlet prevented particle
deposition within the system. Periodic checks indicated that line
losses through the inlet system were consistently less than 3 percent
Zero, precision (60 ppb), and span (400 ppb) checks of the 03 analyzer
were performed every third day using an-internal 03 generator.
13
-------
TABLE 2. NDDN MONITORING EQUIPMENT
Item
Manufacturer
Model Number
Equipment Shelter
[8 feet (ft) by 8 ft by 10 ft with
electricity and telephone]
Ozone (03) Analyzer
Meteorological System
Windspeed
Wind Direction
Temperature
Delta Temperature
Relative Humidity
Solar Radiation
Precipitation (tipping bucket)
Surface Wetness Sensor
10-Meter (m) Tower
Data Acquisition System (DAS)
Primary DAS
Backup DAS
Personal Computer (PC)
' Telecommunications Modem [
Printer
Air Quality Monitoring System
10-m Tower
Filter Pack
Pump
Flow Controller
Wet Deposition Equipment (selected sites')
Precipitation Collector !
Rain Gauge (weighing)
Triple-Beam Balance
EKTO
Thermo-Environmental
Climatronics
or
R.M. Young
Vaisala
Universal Mfg.
Odessa
Odessa
Various
Packard-Bell
Star
Aluma Tower
Savillex
Thomas
Teledyne-Hastings
Andersen
Belfort
Ohaus
8810
49-103
F460
AQ Series
DRD-11
4-30
DSM-3260
DSM-3260L
Various
2424
SD-10
AT-048
0-473-4N
101-CA11
CST-10K
APS
5915R-12
1119-D
Source: ESE, 1990.
14
-------
The onsite DAS consisted of a primary datalogger (Odessa 3260) a
backup datalogger (Odessa 3260L), an IBM-compatible personal computer
(PC), a printer, and a telephone modem. The primary datalogger was used
to acquire, average, store, and communicate readings from all continuous
sensors. It also performed a variety of control functions, such as '
switching the day/night filter pack relay and activating the internal 0,
generator for consistency checks. Data were stored on the primary and
backup dataloggers in electronically erasable programmable read-only
memory (EEPROM) cartridges, both of which were accessible through the
onsite PC, or remotely, through the telephone modem. Printouts of
hourly averages for the previous day were automatically generated by the
onsite printer a few seconds after midnight each day.
Site operators visited each site on Tuesdays and Fridays and in
response to equipment malfunctions or suspected problems. On Tuesdays
filter packs were exchanged, sample lines were leak tested, sensors were
subjected to electronic and reasonableness checks, and data from the
dataloggers were downloaded to diskettes. The site operator telephoned
the NDDN operations center at ESE following site inspections to relay
observations and problems. Data, documentation, and samples were
shipped to ESE the day of collection. On Fridays, the site operator
performed a limited inspection of sensors and equipment.
All field equipment was subjected to inspections and multipoint
calibrations [using standards traceable to the National Institute of
Standards and Technology (NIST)] by ESE personnel on a quarterly basis.
In addition, independent equipment audits were performed semiannually by
ERG Environmental and Energy Services, Inc., and randomly by EPA or its
designee. Results of field calibrations were used to assess sensor
accuracy and flag, adjust, or invalidate field data.
The overall precision of field measurements was evaluated by
operation of collocated sets of equipment at four sites located in the
northeast (Site 107), southeast (Site 153), midwest (Site 157) and
southwest (Site 167). The purpose of this spatial distributioA was to
capture precision data across a broad range of meteorological conditions
and ambient concentrations. Precision and accuracy criteria for NDDN
field measurements are shown in Table 3.
3.2.2 Laboratory Operations'
Filter pack samples were loaded, shipped, received, extracted and
analyzed by ESE personnel at the Gainesville, Florida laboratory.
Filter packs contained three types of filters in sequence: a Teflon®
filter [Gelman, Zefluor, 1.0 micrometer (/im) pore size] for collection
ot aerosols, a nylon filter (Gelman, 1.0 /im) for collection of HNO, and
dual potassium carbonate (K2C03) impregnated cellulose filters
(Whatman 41) for collection of S02. In practice, exposed nylon filter
extracts virtually always contained detectable amounts of SO^, which
15
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was interpreted to represent a fraction of the atmospheric S02. Thus,
for the purpose of this report, SO2" data from the nylon and cellulose
filters were added prior to calculation of S02 concentrations .
Recoveries of SO2" in nylon and cellulose extracts are reported
separately to EPA to accommodate differing interpretations of S02
partitioning in the filter pack, All filters were subjected to
acceptance tests before they were used in the network. Filters showing
detectable levels1 of SO2" or NCJ3 [i.e., greater than or equal to
1.0 microgram (//g) S042 or 0.88 jLig NO^] were either washed and retested
or returned to the manufactured for credit.
Following receipt from the field, exposed filters and blanks were
placed in color-coded bottles land extracted in 25 milliliters (mL) of
deionized water (Teflon®), 25 mL of 0.003 N NaOH (nylon), or 50 mL of
0.05-percent H202 (cellulose). ' Filters and extraction solutions were
shaken for 15 minutes, sonicated for 30 minutes, and shaken again for
15 minutes to ensure complete dissolution of particles and gas reaction
products. Multiple extractions of Teflon®, nylon, and cellulose filters
have repeatedly shown that greater than 95 percent of the available SO2."
and NO, is recovered in the first extract. Extracts were then analyzed
for SOij and NO^ by micromembrane suppressed ion chromatography (1C)
using a Dionex Model 4000i 1C equipped with an Autoion 1000 Controller.
The instrumental configuration included a Dionex Automated Sampler, an
AG4-A guard column, a AS4-A separator column, and a 25- or 50-microliter
(jUL) injection loop. Data acquisition, display, and analysis were
accomplished with a Maxima computer integration system. The 1C was
calibrated prior to each analytical run with five standards plus a blank
covering the typical range of Jsample concentrations: 35 to
9,000 micrograms per liter (Mg/L) for NOj and 40 to 10,000
Extraction and calibration solutions were made up with 1,000
bromide ion (Br~) as a check on 1C column condition and to assist in
detection of spurious sample injections. All 1C analyses were completed
within 72 hours of filter extraction.
Beginning in the third quarter of 1988 and continuing through the
third quarter of 1989, Teflon® filter extracts were analyzed routinely
for water-soluble NH$, Na+, K+ , Mg2+, and Ca2+. Analysis of Na+ , Mg2+ ,
and Ca2+ was performed with a Perkin- Elmer P-2 inductively coupled argon
plasma (ICAP) emission spectrometer. Analysis of NH^ was by the
automated indophenol method using a Technicon II or TRAACS-800
AutoAnalyzer system. Analysis of K+ was via atomic emission on a
Perkin-Elmer 5100 atomic absorption spectrophotometer.
Various QC samples were routinely analyzed to track the accuracy
and precision of laboratory data. NIST SRM No. 2694-11 (simulated
rainwater) was analyzed immediately after instrument calibration and at
the end of each run to monitor accuracy. Recoveries within ±5 percent
of certified values were required for analyses to continue. A midrange
calibration verification standard (CVS) was analyzed after every
10 environmental samples to monitor within- run precision. For the
for SO2" .
18
-------
analytical batch to be accepted, the maximum difference between the
first analysis and each subsequent analysis was limited to ±5 percent
for anions and ±10 percent for cations. Blind replicates (10 percent of
samples) were also analyzed to monitor between-run precision. Due to
the potential for change in analyte concentration between analyses,
replicate samples were not used as a control for acceptance of batches.
Instead, replicate analyses were used to assess the stability of ions in
filter extracts. Finally, one unknown sample provided by EPA was
analyzed with each batch. The unknown consisted of filter media spiked
with salt solutions containing SO2" and NOg. These were carried through
the entire extraction and analysis procedure established for Teflon®,
nylon, and cellulose filters. Precision and accuracy objectives for
NDDN laboratory analyses are listed in Table 4.
Results of all valid analyses were stored in units of micrograms
per filter (jLtg/filter) in the laboratory data management system.
Concentrations of all species were then calculated (based on volume of
air sampled) following validation of hourly flow data. Atmospheric
concentrations of particulate SO2", NOj, NH^, Na+, K+, Mg2+, and Ca2+
were calculated based on the analysis of Teflon® filter extracts; HN03
was calculated based on the NOj found in nylon filter extracts; and S02
was calculated based on the sum of SO2." found in nylon and cellulose
filter extracts. Since Teflon® filters were extracted in deionized
water, it should be noted that reported concentrations of particulate
species refer to the water soluble component only.
3.2.3 Data Management
DMC activities consisted of three major operations: data
acquisition, validation, and transmittal to EPA. The data acquisition
process stressed multiple levels'of redundancy to minimize data loss.
The primary mode of data acquisition from the field was via telephone
modem. Each site was automatically polled between 2:00 a.m. and
4:00 a.m. every day using a PC and software developed by Odessa
Engineering, Inc. The polling software permits recovery of hourly data
and status files, power failure logs, and automated calibration results
from the previous 7 days. The program also maintains synchronization of
the network by checking the clock within each DAS and correcting the
time if it deviates from expectation by more than 2 minutes. If daily
polling resulted in incomplete data capture from any site, then
diskettes of data from the primary and backup DAS were read into the
database management system. If the database was still incomplete, then
missing data were entered manually from site printouts. Each datum was
automatically given a source flag that could be used to trace its mode
of entry into the system (i.e., modem, primary DAS, backup DAS, or
printout).
Data validation consisted of a thorough review of operator logs,
onsite reasonableness checks, results of field calibrations and audits,
19
-------
Acceptance Criteria
Analyte
SHj
Ca?+
*?
Na+
K1"
scf
N3
Sample
type
Filter Extract
Filter Extract
Filter Extract
Filter Extract
Filter Extract
Filter Extract
Filter Extract
Method
Technicon AutoAnalyzer
ICP-ES
ICP-ES
ICP-ES
Atomic emission
1C
1C
Precision*
(percent) !
10
10
10
10
10
5
5
Accuracy
(percent)
90 to 110
90 to 110
90 to 110
90 to 110
90 to 110
95 to 105
95 to 105
Jfote: Ca2"1" - particulate calcium.
K1" - particulate potassium.
Mg?+ — particulate magnesiun.
Na+ — particulate sodium.
NH^ •» particulate amnonium.
NK - particulate nitrate.
SCf - particulate sulfate.
1C - ion chromatography.
ICP-ES - inductively coupled plasma emission spectroscopy.
*Determin3d from midlevel standard and initial calibration curve.
^Determined from NIST SRM 2694-11.
Source: ESE, 1990.
20
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and a variety of parameter-specific range and consistency checks. In
addition, diurnal patterns for numerous parameters and a variety of
interparameter relationships were examined for reasonableness. Solar
radiation data, for example, were used to check for time shift errors in
the database (caused by power failure), while rainfall data verified the
response of relative humidity sensors (should approach 100 percent),
delta temperature sensors (should approach zero), and wetness sensors
(should indicate presence of surface wetness).
Following validation of data for a calendar quarter, flag counts,
and parameter averages were calculated. The database, flag counts, and
averages were then loaded onto a 9-track tape and submitted to the NDDN
Project Officer. To verify the data transfer, flag counts and parameter
averages were independently generated by EPA and repeated to ESE. Data
sets were subjected to a variety of reasonableness and consistency
checks by EPA before final acceptance into the EPA data archive.
21
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: SECTION 4.0
RESULTS AND DISCUSSION
4.1 OVERALL DATA QUALITY
As described in Section 3.0, extensive QC measures were undertaken to
assure and document the overall quality of the NDDN database. This
section summarizes NDDN QC data for 1989. Additional details on field
and laboratory QC checks can be found in quarterly data reports (ESE,
1989b, 1989c, 1989d, 1990c).
4.1.1 Field Data
Results of meteorological equipment calibrations performed during 1988
are summarized by quarter in Table 5. In general, calibration data show
the majority of sensors were pperating within accuracy requirements for
the network. Temperature and delta temperature sensors typically
responded within a few tenths and a few hundredths of a degree Celsius
of NIST-traceable standards. Relative humidity, solar radiation, and
rainfall amount responded within a few percent of calibration standards.
Windspeed and wind direction were generally within ±0.2 meter per second
(m/sec) and ±2 degrees of target values. Inspection of quarter-to-
quarter variability for calibration data shows that all sensors, except
relative humidity, responded consistently throughout the year. Data
quality for relative humidity improved markedly from first quarter
through fourth quarter due to adoption of a new sensor (Rotronics
MP100-MF) and the use of calibration salts in the field for adjustment
of sensors.
Calibration data for continuous 03 analyzers and MFC are presented in
Table 6. 03 and MFC calibrations were performed through the entire
ambient air inlet to compensate for small line losses (less than or
equal to 3 percent) and pressure drops within the system, respectively.
In general, results for 03 show that the instrument employed is highly
stable and that it functioned with acceptable accuracy during the year.
Aggregate errors for the network were within ±2 percent during each
quarter, as represented by sensor versus transfer standard slopes, and
errors for individual analyzers were invariably better than ±8 percent.
Although occasional equipment failures occurred, no 03 data were
invalidated for lack of compliance with accuracy requirements.
MFCs also showed excellent stability, the errors tabulated in Table 6
indicate little instrumental drift between calibrations (i.e., from
quarter to quarter). Thus, the overall impact of uncertainties in flow
on concentration calculations is expected to be minimal.
22
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TABLE 6. SUMMARY OF 1989 03 AND MASS FLOW
Quarter s
03
Intercept Filter pack
lope (ppb) flow (% error)*
First
Mean |0.997 0.35 -1.5
Standard Deviation ±0.012 ±1.01 ±2.6
Second
Mean 0.999 0.28 1.0
Standard Deviation ±0.009 ±0.72 ±3.4
Third > 0
Mean 1-001 0.17 -1.3
Standard Deviation ±0.014 ±1.15 ±4.0
Fourth
Mean 0.996 0.08 2.2
Standard Deviation ±0.024 ±1.33 ±b . 1
Note: Slope and intercept are regression coefficients for calibration
versus a NIST-traceable 03 transfer standard.
*Error is calculated relative to target flows of 1.50 liters per minute
(L/rain) for eastern sites and 3.00 L/min for western sites.
Source: ESE, 1990.
24
-------
4.1.2 Laboratory Data
Laboratory accuracy and precision data from 1989 are summarized in
Table 7. As described previously, an NIST reference (2694-11) is
analyzed once per analytical batch, a calibration verification standard
(CVS) is analyzed after every tenth environmental sample, and 10-percent
blind replicates are analyzed on separate run dates. Replicate analyses
for Teflon® filters were discontinued from third quarter 1988 through
third quarter 1989 due to insufficient volumes after cation analyses.
Replicate analyses were restarted during the fourth quarter of 1989
following removal of Ca2+, Mg2+, Na+, and K+ from the analytical scheme.
Results for the NIST reference material and the CVS samples show that
for each filter type and for each quarter instrumental error and drift
was on the average less than or equal to 2.5 percent. During every
quarter, all mean values were within 2.5 percent of 100 percent, while
all standard deviations were less than 1.7 percent. Figure 3 depicts
NIST reference sample recoveries of SO2" and NOj for all Teflon® filter
batches analyzed in 1989.
Results of replicate analyses show that, on average, the between-run
differences for all analytes is less than 5 percent. However, there is
considerable scatter in results, as reflected in the standard deviations
for each analyte. This scatter is due to a small number of outliers
associated with analytical values near the detection limit.
Less than 2 percent of the replicate samples analyzed in 1989 exhibited
mean percent differences greater than 20 percent (see Figure 4). For
S04" on cellulose filters, 388 samples were replicated, and only
8 samples showed greater than ±20 percent difference. For NOg on nylon
filters, 362 samples were replicated, and only 8 samples showed greater
than a ±20 percent difference. In general, mass recoveries from
replicate analyses differed by 1.0 /Xg/fliter or less for each analyte.
Given a typical sample volume of 7.5 cubic meters (m3), this translates
into a mean laboratory precision of approximately 0.15 /ig/m3 for SO2",
NOj, and HN03 and 0.2 Jig/m3 for S02.
Laboratory accuracy and precision should not be confused with overall
sampling accuracy and precision. The accuracy of the NDDN filter pack
sampling approach is unknown, and its precision should be determined
based on collocated field sampling, rather than laboratory replicates.
Nevertheless, the results in Table 7 show that laboratory operations
conform with accuracy and precision requirements for this project.
25
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27
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Figure 4. Relative percent difference from, replicate analysis of SO^' in cellulose filter
extracts (A) and NOj in nylon filter extracts (B)
28
-------
4.1.3 Collocated Filter Pack Sampling
Results of collocated filter pack sampling during 1989 are shown in
Table 8 and Figures 5 through 13. In general, data from the three
eastern United States sites show that annual average SO2;, S02, NH^,
Na , and Ca2+ differ by less than 5 percent between collocated samplers
Annual average N03, HN03, and Mg2+ differ by approximately 10 percent;
in the case of Site 153, K+ differs by as much as 12 percent.
Inspection of median absolute differences (MADs) suggests that typical
differences between paired samples are 0.2 /ig/m3 (or less) for SO2', NO
S02.
HN03, and NH^, and 0.4 Jig/m3 (or less) for
'3.
Interestingly, the single western collocated site shows uniformly better
precision than the three eastern sites. For example, MAD values for
S04-, N03, HN03, and NH^ are all well below 0.05 Mg/m3 and that for S02
is less than 0.1 /ig/m3. Differences between annual means are
considerably below 5 percent for all species except Ca2+ (6.0 percent)
and K (6.3 percent). This finding is somewhat surprising, given that
observed concentrations at the western site are much lower than at the
eastern sites.
Intuitively, one might expect that the lower the concentration the
poorer the precision (on a relative basis); however, the 1989 results do
not uniformly support this expectation. This is illustrated in
Figures 14 through 18, which show the relationship between mean absolute
,
percent difference (MAPD) (or MAD) and concentrations for SO2; ,
S0
N0
ns or , 3, ,
and HN03. For the eastern sites, only N03 and perhaps SO2; show a
clear tendency for decreasing precision with decreasing concentration.
For the western collocated site, no such tendency is observed, and the
overall precision (absolute or relative) is superior to the eastern
sites, despite lower overall concentrations.
Two potential explanations for those differences in precision between
eastern and western sites involve operating conditions and environmental
conditions. The first refers to the fact that eastern sites sample air
at a flow rate of 1.5 L/min, while western sites sample at 3.0 L/min.
Thus, for the same concentration, the mass loading on western filter
pack samples should be twice that on eastern filter packs. Atmospheric
concentrations at the western collocated site are generally much less
than half those at the eastern sites (at least for these major species),
and, ^therefore, one would not expect significant overlap in mass
loading. Another explanation involving flow rate could simply be that
the MFCs in operation maintain flow more precisely at 3.0 L/min that at
1.5 L/min. Results of quarterly calibrations suggest that MFC drift
might be slightly lower for western sites than eastern sites, but that
this is unlikely, by itself, to completely explain precision
differences . ,
29
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Y-lntercept -0.025
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1 I
28
IS
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Sites I07/I5J/I57
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Linear Regression
Slope
Y-lntercept
R—Squared
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Site 167
A
20
B
28 3 2 i
Figure 5. Scattergrams of 1989 collocated SO* (Mg/m3) for three eastern sites (A)
and one western site (B)
32
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Linear Regression
Slope
Y-lntercept
R-Squared
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0.99
1.0
0,3
l.f
0.2-
O.O-L-1
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One—To—One Relationship
Linear Regression
Slope
Y-lntercept
R-Squored
0.968
0.02
0.969
0.2
1 I '
O.i
Sitt 1(7
7—I—'
O.i
T
12
B
T
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Figure 6. Scattergrams of 1989 collocated NOj (jug/m3) for three eastern sites (A)
and one western site (B)
33
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S.I
One—To-One Relationship
Linear Regression
Slope
Y-lnlercept
R-Squared
0.958
0.036
0.989
Silts 107/153/157
One—To-One Relationship
Linear Regression
Slope
Y-lntercept
R—Squared
0.987
0.01
0.997
r
08
e,2
' i >
0.4
0.8
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1.0
B
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Site 1(7
Figure 7. Scattergrams of 1989 collocated NHj (/ig/m3) for three eastern sites (A)
and one western site (B)
34
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5
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Linear Regression
Slope 0.983
Y-lntercept -0.062
R-Squared 0.955
1 ' ' ' ' I ' ' ' ' '
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Sites 107/153/157
One-To-One Relationship
Linear Regression
Slope 0.945
Y-lntercept 0.038
R-Squared 0.991
0.0 0.2 0.4
' I '
0.6
B
1 I ' ' ' | i . . i ,
1.2 1.4 1.6 1.8 2.0 2.2
Figure 8. Scattergrams of 1989 collocated HNO3 (/ig/m3) for three eastern sites (A)
and one western site (B)
35
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One-To—One Relationship
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Y-lntercept
R—Squared
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One—To-One Relationship
Linear Regression
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R-Squared
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T-
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Site 1(7
2.5
30
3.0
35
3.5
B
Figure 9. Scattergrams of 1989 collocated SO2 (/ig/m3) for three eastern sites (A)
and one western site (B)
36
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0.5
0.4
0.3
0.2
0.1 •
0.0-
One—To—One Relationship
Linear Regression
Slope, 0.971
Y-intercept 0.002
R-Squared 0.938
0.0
0.1
0.2
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0.3
0.5
0.4
0.2
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0.0-
Siles 107/153/157
One-To-One Relationship
Linear Regression
Slope 0.968
Y-lntercept 0.003
R-Squared 0.984
0.0
0. I
0.2
' I '
0.3
Site 167
1 I '
0.4
' I '
0.4
0.5
B
•>—r
0.5
Figure 10. Scattergrams of 1989 collocated Na+ (Mg/m3) for three eastern sites
(A) and one western site (B)
37
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82'
0:8
One—To-One Relationship
Linear Regression
Slope
Y-lntercept
R—Squared
0.243
0.187
0.09
80
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Linear Regression
Slope 0.8
Y-lntercept 0.029
R-Squared 0.619
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9. 19 9.IS
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0.2,0
0.2S
0.30
035 0.40
site
Figure 11. Scattergrams of 1989 collocated K+ Og/m3) for three eastern sites (A)
and one western site (B)
38
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s
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t 0.8
2
0
7
/ 0.6
2
5
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2
5 0.4
0.2-
0.0-1-
One-To-One Relationship
Linear Regression
Slope
Y—Intercept
R—Squared
0.977
0
0.868
0.0
0.2
0.4
0.20
0. 15
0. 10
0.05
0.00-.''
O.i
Sites 107/153/157
0.1
One—To-One Relationship
Linear Regression
Slope
Y-Intercept
R-Squared
1 I '
t.O
0.00
O.OS
0. 10
Sill 167
T
1.2
B
0.20
Figure 12. Scattergrams of 1989 coUocated Ca2+ Otg/m3) for three eastern sites (A)
and one western site (B)
39
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2
S 10
1
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t
581
1
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One—To-One Relationship
Linear Regression
Slope
Y-lntercept
R-Squared
0.969
0.003
0.953
00
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1 ' I ' '
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Sites 107/153/157
One—To-One Relationship
Linear Regression
Slope
Y-lntercept
R-Squared
1.143
-0.024
0.984
B
02
0,4
0.8
Silt 167
0.8
1.0
1.2
Figure 13. Scattergrams of 1989 collocated Mg2* (Mg/m3) f°r three eastern
sites (A) and one western site (B)
40
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-i
« w °°
-------
o
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<
4
f^
vl <
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T— «S
(/I
UJ
t < <
10 <
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<
< ^
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m°
-------
o
ID
CM
CO
LiJ
O S
CO 03
S'o
qj o
*"^
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ttl
45
-------
Differences in environmental conditions could conceivably give rise to
precision differences between eastern and western sites. Located in
southern Arizona, the western collocated site is much drier than the
eastern collocated sites. Low relative humidity and infrequent rain,
fog, and mist could reduce (or eliminate) interferences or loss
mechanisms common to humid environments. Collocation of a site in a
less arid region of the western United States could help resolve this
question.
4.2 FILTER PACK MEASUREMENTS
This section summarizes results of filter pack measurements of SO2/, NOg,
NHj, HN03, and S02 during 1989. Annual and quarterly data are presented
and discussed for eastern NDDN sites. Since the majority of western
NDDN sites were initiated in June 1989, data are addressed as semiannual
averages only. Comparisons of day versus night concentrations, aerosol
cations versus aerosol anions, and 1988 versus 1989 concentrations are
also presented in Sections 4.2.6 through 4.2.8.
4.2.1 SO2-
Annual and quarterly arithmetic mean concentrations of S04T are shown in
Figures 19 through 21. Annual concentrations range from 7.9 Mg/m3 in
western Ohio (Site 122) to 2.7 Mg/m3 in northern Maine (Site 135). Mean
values of 7.0 /ig/m3, or more, extend in a narrow band from southern
Indiana (Site 140) to Maryland (Site 116).
Annual averages of 5.0 jLtg/m3 cover nearly the entire eastern United
States, from New York and Michigan to northern Mississippi and Alabama.
Only sites from northern New York to Maine, northern Michigan,
Wisconsin, and Florida exhibit concentrations below 5.0 /Ug/m3. A strong
concentration gradient (i.e.y greater than a factor of 2) exists between
Pennsylvania and northern New York, and weak gradients extend from Ohio
through Michigan and from Illinois into Wisconsin.
Quarterly data for SO2" show dramatic differences from season to season,
but reasonably consistent locations of peak concentrations (see
Figures 20 and 21). Results for first quarter 1989 exhibit a narrow
range from 5.2 Mg/m3 in northern Alabama (Site 152) and Maryland (Site
116) to 2.4 Mg/m i-n northern Maine. Mean concentrations of 4.0 jLig/m3,
or above, cover the region from the southern Great Lakes southward, with
the exception of a small cluster of sites located in the mountains of
North Carolina, Virginia, and eastern Kentucky.
Concentrations during second|quarter 1989 exhibit a substantial increase
(50 to 75 percent) at most sites relative to first quarter 1989.
Average values range from 8.8 Mg/m3 in western Ohio to 2.8 ^ig/m3 in
Maine and Wisconsin (Site 134). Concentrations above 8.0 MS/™3 occur at
46
-------
iNS = Insufficient samples for the period covered (<755'.)
Figure 19. Annual average SO% concentrations (jig/m3) for the eastern United
States during 1989
47
-------
^J|r—^
\ \
i \
\J
Figure 20. Average SOf' concentrations C/iig/m3) for the first (A) and second (B)
quarter 1989
48
-------
INS = Insufficient samples for the period covered (<75s)
Figure 21. Average SO*' concentrations (/ig/m3) for the third (A) and fourth (B)
quarter 1989
49
-------
a ring of sites in Indiana, Ohio, and Kentucky, while.-concentrations
below 5.0 /ig/m3 occur only in New England and the upper midwest.
Results for third and fourth quarter 1989 show another large increase in
concentrations, followed by a dramatic decline to the lowest levels of
the year. For third quarter 1989, mean 304" ranges from 13.5 MS/m3 in
central Kentucky (Site 129) to 3.2 Mg/m3 in Maine. Values above
12.0 M6/m3 occur at a small number of sites in Indiana, Kentucky, Ohio,
and West Virginia, while values above 8.0 Mg/m3 extend from the southern
Great Lakes into northern Mississippi, Alabama, and Georgia. Fourth
quarter concentrations range from 5.1 Mg/m3 in western Ohio, to
2.1 Mg/m3 i-n Wisconsin and ,are generally similar to those observed
during the first quarter.
The seasonal progression of SO^" concentrations, therefore, appear to
follow temperature and solar radiation, which also exhibit, maxima and
minima in summer and winter, respectively. Seasonal variability for
individual sites, however, is much lower around the periphery of the
network than near its center. For example, quarterly average
concentrations for the site in Maine (Site 135) fell within a narrow
range of 2.4 to 3.2 Mg/m3. In contrast, quarterly averages in western
Ohio (i.e., those sites with the highest annual averages) ranged from
4.8 to 12.9 /ig/m3.
Semiannual average 804" concentrations for the nine western sites (see
Figure 22) range from 1.65/ig/m3 in southern Arizona (Site 167) to
0.65 Mg/m3 in southern Idaho (Site 163). Results are invariably lower
than those observed across,the eastern United States. For example, the
range of semiannual concentrations was only 27 to 69 percent of the
lowest quarterly average reported for the site in northern Maine.
4.2.2 NOg
Annual average concentrations for N03 exhibit much more variability than
804" and a definite pattern of higher concentrations in the midwest than
elsewhere (see Figure 23). The lowest concentrations are observed at
forested sites in New England and the southern Appalachian Mountains,
while the highest concentrations (i.e., greater than 2.0 /ig/m3) are
observed in agricultural areas of the midwest. Intermediate values
(i.e., 1.0 to 2.0 Mg/m3) appear to be associated with agricultural sites
anywhere in the eastern United States. This finding supports the idea
of a link between agricultural activity and N03 concentrations. Two
potential mechanisms for N03 production include gas-phase reaction
between HN03 and NH3 and gas-particle reaction of HN03 with soil
particles. Although both .of these reactions are likely to be enhanced
in agricultural areas, the spatial correlation of NH^ and N03
concentrations suggests that the former may be more important.
50
-------
INS = Insufficient samples for the period covered (<75r.)
Figure 22. Average SO^' concentrations (/ig/m3) for western NDDN sites, combined
third and fourth quarters 1989
51
-------
INS = insufficient samples for the period covered (<75%)
Figure 23. Annual average NOj concentrations (jug/m3) for the eastern United
States during 1989
52
-------
Quarterly data for N03 (see Figures 24 and 25) exhibit a seasonal cycle
that runs counter to that of 304" and NH^. That is, the highest
concentrations occur during the first quarter, and the lowest occur in
the third quarter. This cycle is consistent with the temperature-
dependent equilibrium between particulate NH4N03 and gaseous NH3 and
HN03.
The highest quarterly concentrations invariably occur at sites located
in the midwest. These sites also undergo the greatest seasonal
excursions, with typical concentrations decreasing by factors of 2 to 3
from winter to summer. The lowest concentrations occur at forested
sites remote from any agricultural activity. This distinction often
results in fairly sharp concentration gradients. For example, the
forested site in central West Virginia (Site 119) typically exhibits
seasonal concentrations that are lower by a factor of 5 than an adjacent
site in Ohio (Site 114). A gradient of this magnitude suggests that N03
aerosol either deposits rapidly or, more likely, changes phases readily
outside of the region of peak concentration.
Results for the western sites (see Figure 26) indicate semiannual
concentrations similar to the lowest values observed at eastern sites.
As with other variables, the overall range of concentrations is on the
order of 2 (i.e., much narrower than for eastern sites), and the sites
in Arizona (along with that in Idaho) represent the highest average
concentrations.
4.2.3 NHj
Annual and quarterly average concentrations of NH^ are illustrated in
Figures 27 through 29. Annual average values range from 3.2 jtig/m3 in
northern Indiana (Site 133) to 0.69 /ig/m3 in Maine and, in general,
exhibit higher values at agricultural sites than at forested sites.
Since the midwest is primarily agricultural, this results in regional
differences between the midwest, northeast, and southeast.
Concentrations above 2.0 Mg/m3 are found at all sites within Illinois,
Indiana, and Ohio, plus the two southernmost sites in Michigan.
Additional concentrations in this range also are found in northern
Alabama, eastern Pennsylvania, and Maryland. Average values below 1.0
jUg/m3 are found only in New England. Seasonal concentration data follow
a similar, but not as pronounced, pattern as those for SO^". Summertime
(i.e., third quarter) averages are the highest of any season, with
concentrations above 2.0 jUg/m3 encompassing all states from Michigan to
Georgia. Similarly, sites on the periphery of the network (e.g., Maine)
exhibit both the lowest concentrations and the lowest quarter-to-quarter
variability.
Results for the third and fourth quarters for the western sites show
about a factor of 2 range in concentrations (see Figure 30). The
highest value (0.57 /Jg/m3) occurs in southern Arizona and the lowest
53
-------
tNS * Insufficient samples for
Figure 24. Average
quarter 1989
concentrations (jug/m3) for the first (A) and second (B)
54
-------
INS = Insufficient samples for the
B
Figure 25. Average NOj concentrations (Mg/m3) for the third (A) and
fourth (B) quarter 1989
55
-------
-YS = Insufficient samples for the period covered
Figure 26. Average NOg concentrations Oig/m3) for western NDDN sites, combined
third and fourth quarters 1989
56
-------
INS = Insufficient samples for the period covered (<75r.}
Figure 27. Annual average NH+ concentrations (jig/m3) for the eastern United
States during 1989"
57
-------
INS - Insufficient samples for the period covered (<75i)
Figure 28. Average NHj concentrations (jug/m3) for the first (A) and second
(B) quarter 1989
58
-------
INS = insufficient samples for the period covered (<75»)
Figure 29. Average NH+ concentrations Ug/m3) for the third (A) and
fourth (B) quarter 1989
59
-------
INS = Insufficient samples for the perioid covered (<75x)
Figure 30. Average NH^ concentrations (/ig/m3) for western NDDN sites, combined
third and fourth quarters 1989
60
-------
average value (0.28 /ig/m3) occurs in Montana, Wyoming, and Nevada. The
overall pattern suggests a weak north-south gradient for the reporting
period. Since none of the western sites is agricultural in nature, the
apparent gradient could be a reflection of temperature-dependent
emission rates of NH3. Alternatively, it could simply be due to
chemical association with SO^" aerosols.
4.2.4 HN03
Annual averages for HN03 (Figure 31) exhibit a maximum concentration of
approximately 3.6 Hg/m3 in southeastern Pennsylvania (Site 128) and a
minimum of 0.7 /Xg/mJ in northern Maine (Site 135). Concentrations in
excess of 3.0 /ig/m3 occur in two groups of three sites. The first group
is located in central Kentucky (Site 129) and Ohio (Sites 114 and 122);
.the other group is located in an arc extending from northern Virginia
(Site 118) through Pennsylvania and into New Jersey (Site 144). The
majority of NDDN sites fall in the range of 2.0 to 3.0 JJg/m3. Only six
sites exhibit average concentrations of 1.0 Mg/m3 or less. Four of
these sites are located in the northern and southern extremes of the
network (i.e., Wisconsin, Maine, New Hampshire, and Florida). In
contrast, two of the sites with lowest concentrations are located near
the geographic center of the network in eastern Kentucky (Site 121) and
southwestern North Carolina (Site 137).
The overall pattern of HN03 might be influenced by terrain as much or
more than other factors (e.g., emissions). Sites with the highest
average values have good exposure to wind flow (fetch), while those
sites with the lowest values typically have poor exposure, due mainly to
complex terrain. For a species with a large deposition velocity (such
as HN03) , the microclimate in regions of complex terrain could produce
local variability in atmospheric concentrations.
Quarterly data show that HN03 concentrations are relatively constant
from season to season (see Figures 32 and 33). For example, quarterly
averages in southeastern Pennsylvania, the site with the highest annual
average, vary only over a range of 1.2 /ig/m3. The majority of sites
exhibit an even narrower range. For the 41 eastern sites, averages
range from roughly 1.9 Mg/m3 during the fourth quarter to 2.5 Mg/m3
during the second quarter. Thus, seasonal variability is much less than
and slightly out of phase with SOf" (i.e., another secondary pollutant).
Results for the western region indicate concentrations that are somewhat
lower than the lowest values observed in the east (see Figure 34).
Exceptions to this are the two Arizona sites (Sites 167 and 174), which
exhibit semiannual averages similar to those in New England, Florida,
and Wisconsin. In conjunction with data for other species, these
observations suggest the Arizona sites may be influenced by local
emissions and/or long-range transport from neighboring California and
Mexico.
61
-------
.MS = Insufficient samples for the perfiod covered
Figure 31. Annual average HNO3 concentrations (jug/m3) for the eastern
United States during 1989
62
-------
INS = 'nsutfioent samples for the period covered (<75r.)
B
Figure 32. Average HNO3 concentrations Gig/m3) for the first (A) and second
(B) quarter 1989
63
-------
INS = Insufficient samples for the period covered (<75»)
B
Figure 33. Average HNO3 concentrations (Mg/m3) for the third (A) and
fourth (B) quarter 1989
64
-------
INS = Insufficient samples for the period covered (<75%)
Figure 34. Average HNO3 concentrations (/ig/m3) for western NDDN sites, combined
third and fourth quarters 1989
65
-------
4.2.5 S02 "
Annual average data for S02 range from 23.2 £tg/m3 in southwestern
Pennsylvania (.Site 117) to 2.4 Mg/m3 in Maine (Site 135) (see
Figure 35). Concentrations of 15 |ig/m3 or greater occur in a small area
encompassing Pennsylvania and Maryland, as well as at isolated sites in
northern Illinois, southern Indiana, and western Ohio. A much larger
area extending from Kentucky and Indiana eastward to New York exhibits
concentrations in the range of 10 to 15 /ig/m3. Sharp concentration
gradients occur from Pennsylvania to northern New York, from the lower
midwest to upper midwest, and around isolated sites in Kentucky and
North Carolina. As suggested earlier for HN03, relatively rapid dry
deposition of S02 could account for large differences (i.e., a factor
of 3) between rolling terrain sites in central Kentucky, central
Tennessee, central North Carolina, and neighboring complex terrain sites
in eastern Kentucky and western North Carolina. Relatively low emission
densities for S02 in the central Appalachian Mountains also might
account for low S02 concentrations in Kentucky and North Carolina.
Inspection of quarterly averages shows dramatic changes in
concentrations from season to season (see Figures 36 and 37).
Domainwide mean concentrations are nearly a factor of 3 higher in the
first quarter (16.2 Jlg/m3) than in the third quarter (6.2 Mg/m3) .
Despite large relative changes in concentrations, the locus of peak
concentrations remains more or less stationary from season to season.
In general, the highest values occur in western Pennsylvania (Sites 113
and 117) and at source-influenced sites in Illinois and Indiana.
Another persistent feature of the S02 field is marked depressions at
complex terrain sites in Kentucky and North Carolina. As suggested
previously, this appears to be the result of meteorological isolation of
these sites.
Concentration data for the western sites range from 1.8 jUg/m3 in
southern Arizona (Site 167) to 0.4 MS/1"3 in central Colorado (Site 161)
and northern Nevada (Site 164) and southern Idaho (Site 163) (see
Figure 38). Consistent with the other species measured, relatively high
concentrations at the Arizona site appear to be attributable to large
point sources in the southwest or Mexico. Despite the absence of point
sources within 50 km of NDDN sites, intermediate S02 concentrations in
Montana and eastern Wyoming appear to reflect small-scale industrial
activities within their respective airsheds. Low S02 concentrations in
Nevada (Site 164) and Colorado (Site 161), in turn, suggest that sites
in the Great Basin and central Rocky Mountains are isolated from local
or regional emissions.
66
-------
INS = insufficient samples for the period covered (<75z)
Figure 35. Annual average SO2 concentrations Og/m3) for the eastern United
States during 1989
67
-------
I
SNS » Insufficient somples for the period covered (<75*)
Figure 36. Average SO2 concentrations (/ig/m3) for the first (A) and
second (B) quarter 1989
68
-------
INS = Insufficient samples for the
B
Figure 37. Average SO2 concentrations (jug/m3) for the third (A) and
fourth (B) quarter 1989
69
-------
!NS = Insufficient samples for the period covered
Figure 38. Average SO2 concentrations (jug/m3) for western NDDN sites, combined
third and fourth quarters 1989
70
-------
4.2.6 Day Versus Night Concentration Data
Day/night filter pack samples were collected at NDDN sites through
September 1989 to determine whether diurnal variations could affect the
uncertainty of dry deposition estimates. Results are summarized in this
section for the period October 1988 through September 1989 for selected
sites that exemplify the day/night variability observed throughout the
network.
Day/night concentration data for 804', N0;
3.
NH+,
HN03, total N03 (i.e.,
N03 plus HN03) , and S02 are tabulated, by quarter, for 19 NDDN sites in
Tables 9 through 14. Data for S0^~ show appreciable seasonality in
day/night concentration differences. Results for fourth quarter 1988
and first quarter 1989 (approximately equivalent to fall and winter,
respectively) ,show nearly identical averages for day and night at
virtually all sites. During this period, only two sites exhibited
statistically significant day/night differences. In both cases,
differences were detected during the fall, and day values were higher
than night values by 0.3 jUg/m3, or about 10 percent.
During second and third quarter 1989 (spring and summer, respectively),
differences become increasingly more pronounced. Daytime concentrations
during the summer, for example, are typically 2.0 Mg/m3 greater than
those at night and are, with only two exceptions, statistically
significant.
Results for the presumptive aerosol species NO3 and NH^ also show
relatively low day/night variability. For N03, day/night excursions
occur primarily during winter and spring at a few midwestern sites. In
this case, concentrations at night are typically greater than those
during the day, frequently by 1.0 jUg/m3 or more. This behavior is not
unexpected, based on the temperature dependence of the NH4N03
equilibrium with NH3 and HN03. If NH4N03 is forming at night, it is not
clear why the behavior of NH^ does not follow that of N03 more closely.
One explanation for this behavior is that variability in ammonium
sulfate (or bisulfate) concentrations obscures any day/night variability
in NH4N03. y
Data for HN03 and S02 show more frequent and pronounced day/night
differences than the aerosol- species. Concentrations of HN03 are lower
at night for nearly every site-season combination. This is especially
apparent during summer, when all sites show statistically significant
nocturnal reductions ranging from 35 to 80 percent. Given the high
affinity of HN03 for virtually any surface, nocturnal depletion is not
surprising. However, day/night differences of this magnitude imply a
shallow nocturnal boundary layer or minimal nocturnal production rates
of HN03 (e.g., from N205) or both.
71
-------
,2-
TABLE 9. DAY (D) VERSUS NIGHT (N) CONCENTRATIONS OF PARTICUIATE SOf
Quarter
04/88
Region
Northeast
Midwest
Southeast
West
State
ME
NY
PA
PA
PA
VA
VA
WV
IL
IN
KY
KY
MI
OH
GA
NC
NC
MT
WY
Site
135
104
106
117
128
108
120
119
130
133
121
129
115
114
153
101
137
168
165
D
N/0
3.4
3.9
3.8
3.8*
3.1
3.7
3.2
3.6
3.6
3.0*
3.5
2.8
3.6
4.2
3.8
2.8
N/0,
N/0:
N
N/0
3.4
3.7
3.5
3.5
2.9
3.7
3.2
3.3
3.7
2.7
3.5
2.7
3.5
3.8
3.6
2.8
N/0
N/0
01/89
D
2.4
3.8
4.2
4.4
4.9
4.3
3.5
4.1
4.5
4.7
3.8
4.6
3.7
4.7
3.9
3.9
2.5
0.59
0.42
N
2.4
4.0
4.3
4.4
4.7
4.3
3.7
4.2
4.5
4.7
3.8
4.2
4.0
4.6
3.7
4.0
2.6
0.65
0.42
02/89
D
2.9
5.9*
7.0*
7.4
8.3*
6.8*
7.4
7.6
7.3
7.8
7.7*
8.7*
7.0*
8.8
7.5*
7.2 +
6.2
0.74
0.70
N
2.6
5.1
5.9
7.1
7.2
5.7
7.8
7.0
7.3
7.3
6.7
8.1
6.1
8.2
6.3
6.1
6.2
0.74
0.67
03/89
D
3.4
8.1+
10.
13.
11.
12.
11.
13.
11.
10.
11.
14.
8.
13.
10.
9.
9.
0.
5 +
8 +
7*
2+
4*
1+
4+
f -4-
6 +
2 +
4+
9*
6+
0+
4_
5 +
0 +
,73
0.95*
N
3.0
6.1
8.2
10.1
10.0
9.6
10.3
8.9
9.8
9.1
8.6
12.5
8.1
11.3
8.4
7.4
7.6
0.69
0.89
Note: N/0 - not operational.
*Day greater than night at the 95-percent confidence level.
*Day greater than night at the 99-percent confidence level.
Source: ESE, 1990.
72
-------
TABLE 10. DAY (D) VERSUS NIGHT (N) CONCENTRATIONS OF PARTICULATE NHj
Region
Northeast
Midwest
Southeast
West
Quarter
04/88 01/89 02/89 03/89
State
ME
NY
PA
PA
PA
VA
VA
WV
IL
IN
KY
KY
MI
OH
GA
NC
NC
MT
WY
Site
135
104
106
117
128
108
120
119
130
133
121
129
115
114
153
101
137
168
165
D
N/0
1.1
1.6
1.2
1.6
1.1
1.1
1.0
1.6
1.9
0.9*
1.4
1.5
1.5
1.4
1.3
0.8
N/0
N/0
N D
N/0 0.6
1.1 1.3
1.7 1.7
1.2 1.3
1.5 2.2
1.0 1.4
1.1 1.1
1.1** 1.2
1.8 2.4
2.2** 3.1
0.8 1.0
1.5 1.6
1.5 2.0
1.8** 2.1
1.3 1.3
1.3 1.4
0.8 0.8
N/0 0 . 24
N/0 0 . 15
N D
0.6 0.7
1.4** 1.7
2.0 2.7
1.5** 1.9
2.2 3.3
1.5** 2.0
1.2 2.2
1.2 2.0
2.7** 2.7
3.5** 3.4
1.0 1.9
1.7 '2.7
2.4** 2.6
2.5** 3.2
1.4 1.9
1.5 2.2
0.8 1.6
0.23 0.27
0.16 0.28
N D
0.6 0.9
1.6 2.0
2.7 3.3
2.2 2.5
3.0 3.8
1.9 2.6
2.1 2.2+
2.2** 2.7
3.2** 3.4
3.8** 3.9
1.9 2.1
2.8 3.5
2.7 2.9
3.7 3.6
1.9 2.7
2.1 2.3
1.8 1.9
0.28 0.28
0.30 0.38
N
0.8
1.8
3.4
2.7
3.5
2.7
2.0
2.5
3.6
3.6
2.1
3.4
2.9
4.0
2.6
2.3
1.9
0.30
0.39
Note: N/0 = not operational.
*Day greater than night at the 95-percent confidence level.
Day greater than night at the 99-percent confidence level.
**Night greater than day at the 95-percent confidence level.
Source: ESE, 1990.
73
-------
TABLE 11. DAY (D) VERSUS NIGHT (N) CONCENTRATIONS OF PARTICIPATE
Quarter
04/88
Region
Northeast
Midwest
Southeast
West
State
NE
NY
PA
PA
PA
VA
VA
WV
IL
IN
KY
KY
MI
OH
GA
NC
NC
MT
WY
Site
135
104
106
117
128
108
120
119
130
133
121
129
115
114
. 153
101
137
168
165
D
N/0
0.6
1.4
0.6
1.7
0.5
0.7
0.4 ;
2.4
2.8
0.8*
1.4
2.1
1.8
0.6
0.9
0.3
N/0
N/0
N
N/0
0.5
1.6
0.7
1.8
0.7
0.9 +
0.5
3.4+
3.9 +
0.6
1.5
2.3
2.3+
0.7+
0.8
0.2
N/0
N/0
01/89
D
0.4
1.1
2.0
0.7
2.6
0.6
0.6
0.5
3.5
5.5
0.9*
1.1
3.3
2.4
0.6
0.7
0.3
0.19
0.16
N
0.4
1.3 +
2.2
0.8
2.6
0.8
0.8
0.4
4.6 +
6.7
0.7
1.4+
4.0+
3.3 +
0.6
0.9 +
0.2
0.26
0.17
02/89
D
0.3
0.4
0.9
0.4
1.4
0.4
0.6
0.3
2.2
2.7
0.6
1.1
1.5
1.4
N/0
0.6
0.3*
+ 0.15
0.27
N
0.3
0.4
1.9 +
0.4
1.4
0.4
0.7
0.3
3.6 +
4.7 +
0.6
1.4+
2..3 +
3.0+
N/0
0.6
0.2
0.13
0.22
03/89
D
0.
0.
0.
0.
0.
0.
0.
0.
1.
1.
0.
0.
0.
0.
0.
0.
0.
2
5*
8
3*
9
4
4
2
1
1
4*
4
8
7
4*
6*
,2*
0.15
0.25*
N
0.2
0.3
1.0
0.2
1.2
0.2
0.3
0.2 .
1.5+
1.8 +
0.2
0.5
0.8
1.2 +
0.2
0.4
0.1
0.13
0.22
Note: N/0 - not operational.
*Day greater than night at the 95-percent confidence level.
+Night greater than day at the 95-percent confidence level,
Source: ESE, 1990.
74
-------
TABLE 12. DAY (D) VERSUS NIGHT (N) CONCENTRATIONS OF
HNO, (Ug/m3) FOR SELECTED SITES
Quarter
04/88
Region
Northeast
Midwest
Southeast
West
State
NE
NY
PA
PA
PA
VA
VA
WV
IL
IN
KY
KY
MI
OH
GA
NC
NC
MT
WY
Site
135
104
106
117
128
108
120
119
130
133
121
129
' 115
114
153
101
137
168
165
D
N/0
2.1*
2.1
2.2*
2.5
2.3
2.6
1.5 +
1.3*
1.2
0.8 +
2.1
1.2
2.0*
2.4+
2.4+
1.1+
N/0
N/0
N
N/0
1.5
1.8
1.4
2.2
2.0
3.0**
0.7
0.7
0.8
0.4
1.6
1.0
1.3
1.3
1.3
0.6
N/0
N/0
01/89
D
1.0
2.3*
2.2
2.9 +
3.0
3.0
2.5
2.5 +
2.9*
2.1
1.2 +
3.2*
1.9
2.9*
2.4
2.4+
1.2 +
0.41
0.33 +
N
1.0
1.8
2.0
2.0
3.1
2.8
3.4**
1.3
1.7
1.3
0.6
2.6
1.6
2.0
2.2
1.4
0.7
0.46
0.24
02/89
D
0.8 +
3.8 +
3.3 +
3.6 +
4.7 +
3.4+
3.2
2.8 +
4.3 +
3.9+
1.9 +
4.3 +
4.0 +
4.8 +
N/O
3.7+ '
1.7 +
0.39 +
0.41 +
N
0.5
2.2
1.5
1.8
3.3
1.8
3 . 5**
1.0
2.2
1.6
0.6
2.5
2.0
2.4
N/0
1.6
0.8
0.21
0.27
0
3
3
4
5
3
2
2
5
4
1
5
4
5
3
3
1
0
0
03/89
D
.7 +
.4+
.6 +
.1+
,3 +
.5 +
.9 +
.2 +
.2+
.4+
.4+
.1+
.4+
.4+
.1+
.6+ •
.3 +
.46 +
.95 +
N
0.5
1.5
1.8
1.3
3.2
1.5
2.3
0.5
2.3
1.5
0.3
2.2
1.6
2.5
1.2
1.4
0.3
0.18
0.52
Note: N/0 = not operational.
*Day greater than night at the 95-percent confidence level.
+Day greater than night at the 99-percent confidence level.
**Night greater than day at the 95-percent confidence level.
Source: ESE, 1990.
75
-------
TABLE 13. DAY (D) VERSUS NIGHT (N) CONCENTRATIONS OF TOTAL
FOR SELECTED SITES
Quarter
04/88
Region
Northeast
Midwest
Southeast
West
State
NE
NY
PA
PA
PA
VA
VA
WV
IL
IN
KY
KY
MI
OH
GA
NC
NC
MT
WY
Site
135
104
106
117
128
108
120
119
130
133
121
129
115
114
153
101
137
168
165
D
N/0
2.7
3.5 :
2.8
4.2
2.8
3.3
2.0
3.6
3.9
1.6
3.5
3.3
3.8
3.0
3.2*
1.4
N/0
N/0
N
N/0
2.2
3.3
2.1
4.0
2.7
3.9
1.3
4.1
4.7
1.0
3.1
3.3
3.6
2.0
2.2
0.8
N/0
N/0
01/89
D
1.4
3.3
4.1
3.6*
5.6
3.5
3.1
2.9 +
6.4
7.5
2.0+
4.2
5.1
5.3
3.0
3.1*
1.4+
0.60
0.48
N
1.
3.
4.
2.
5.
3.
4.
1.
6.
8.
1.
3.
5.
5.
2.
2.
0.
0.
0.
4
2
3
9
6
5
1**
7
1
0
3
9
6
3
8
3
9
70
41
02/89
D
1.1*
4.1+
4.2 +
4.0 +
6.0+
3.8 +
3.8
3.0+
6.4
6.5
2.5 +
5.4+
5.4*
6.1
N/0
4.2 +
2.0+
0.54+
0.67 +
N
0.8
2.6
3.3
2.2
4.6
2.2
4.1
1.2
5.8
6.2
1.1
3.8
4.3
5.3
N/0
2.2
0.9
0.34
0.49
03/89
D
0.9*
3.8 +
4.4+
4.3 +
6.1+
3.8 +
3.3 +
2.4+
6.3 +
5.4+
1.8 +
5.4+
5.2 +
6.0+
3.4+
4.2 +
1.5 +
0.60+
1.18 +
N
0.6
1.8
2.8
1.4
4.4
1.7
2.6
0.7
3.8
3.3
0.6
2.7
2.3
3.6
1.4
1.8
0.5
0.30
0.73
Note: N/0 = not operational.
*Day greater than night at the 95-percent confidence level.
+Day greater than night at the 99-percent confidence level.
**Night greater than day at the 95-percent confidence level.
Source: ESE, 1990.
76
-------
TABLE 14. DAY (D) VERSUS NIGHT (N) CONCENTRATIONS OF SO,
Quarter
04/88
Region
Northeast
Midwest
Southeast
West
State
NE
NY
PA
PA
PA
VA
VA
WV
IL
IN
KY
KY
MI
OH
GA
NC
NC
MT
WY
Site
135
104
106
117
128
108
120
119
130
133
121
129
115
114
153
101
137
168
165
D
N/0
16.5 +
19.5*
28. 1+
16.7
14. 0 +
14.2
16.1+
9.6*
11.3
5.2 +
13.7*
8.0
16.7*
13.3*
11.9 +
3.6*
N/0
N/0
N
N/0
11.7
15.8
18.8
17.3
10.8
14.7
7.1
6.2
10.5
2.5
10.8
7.4
12.6
6.6
9.0
2.2
N/0
N/0
01/89
D
4.7
24.8*
28.1
35.7*
26.8*
14.6*
12.7
26.8 +
15.1
17.3
8.3 +
19.1
15.0
21.1*
10.8
11. 2 +
3.3
1.1
0.41
N
4.5
19.6
21.1
30.0
24.1
11.5
13.0
15.7
12.8
15.1
4.1
16.4
12.6
16.1
9.8
8.4
2.2
1.0
0.36
02/89
D
1.8 +
12. 4+
10.4+
21. 0+
10.7
6.3 +
7.6
13. 4+
8.5 +
7.7
5.5 +
11.0+
8.2
12.7 +
N/0
6.4+
2.9*
0.49
0.46
N
0.9
7.0
6.9
11.4
10.9
3.5
1.
10.
9.
16.
9.
4.
9 . 9** 4 .
5.5
6.3
6.3
1.3
8.0
6.9
8.0
N/0
3.9
1.3
0.42
0.47
9.
10.
7.
2.
14.
6.
15.
7.
4.
1.
0.
0.
03/89
D
0+
9 +
1 +
1 +
3
6 +
7
0+
1+
5 +
5+
3 +
6 +
0+
4+
6 +
0 +
68 +
87 +
N
0.8
4.3
5.6
7.0
8.3
2.4
5.1
1.3
6.4
4.2
0.5
6.8
3.5
6.6
2.5
2.8
0.4
0.33
0.65
Note: N/0 = not operational.
*Day greater than night at the 95-percent confidence level.
+Day greater than night at the 99-percent confidence level,
**Night greater than day at the 95-percent confidence level.
Source: ESE, 1990.
77
-------
The day/night behavior of S02 is similar to that of HN03.
Statistically significant differences occur at most sites for most
seasons, and the magnitude of the differences increases from winter to
summer. Site 120, in southwestern Virginia, represents one exception to
the previous pattern. In this case, day/night concentrations differ
significantly only in the summer, with night averages greater than those
during the day. As will be discussed later for 03, this phenomenon
appears to be related to the unique circulation patterns around the
mountaintop NDDN sites. •
The fractional depletion of S02 at night frequently equals that of
HN03. This is especially apparent at complex terrain sites (such as
Site 119), where day/night ratios are greater than 5.0 during the
summer. One explanation for this behavior involves surface wetness via
condensation. Although long-term average deposition velocities of HN03
are likely to be much greater than S02, short-term deposition velocities
are thought to be similar, if the surface is wet with dew. Inspection
of limited surface wetness data from NDDN sites for the summer of 1989
suggests that dew occurs almost every night. The frequency of dew
occurrences also appears to increase from flat and rolling terrain sites
to sites in isolated valleys or hollows. Coupled with shallow nocturnal
boundary layers, dew formation could be responsible for essentially
complete depletion of S02 at night. Figure 39 illustrates weekly
day/night concentrations of S02 at a complex terrain site (Site 119), a
rolling terrain site (Site 129), and a mountaintop site (Site 120).
Although day/night differences are apparent for the rolling terrain and
mountaintop sites, nighttime S02 varies appreciably from week to week.
In contrast, nighttime concentrations at the complex terrain site
exhibit minimal week-to-week variability, especially during the summer
months. An alternative explanation for the observed S02 could involve
plume-like behavior. An unknown fraction of the long-term average S02
concentration could be due to brief concentration spikes. If this
fraction is large, and if it occurs primarily during the day, then
day/night concentration differences would occur in the absence of dry
deposition. In this case, a stable nocturnal boundary layer would
isolate the monitoring site from plumes aloft.
4.2.7 Aerosol Ion Balances
The analysis of NH^ and base metal cations on Teflon® filters
permits evaluation of various ionic relationships in aerosol samples.+
This section presents data comparing total measured cations (i.e. NH4,
Na+, K+, Ca2+, Mg2+) with total measured anions (i.e., N03 and S04") and
NHl'with SO2;. The first of these relationships is designed to evaluate
whether there are significant unmeasured species in the samples
collected and to provide insight into what those species, if any, might
be. The second comparison (i.e., NH^ versus SO2.") focuses on the
simultaneous behavior of what appear to be the dominant anionic and
cationic species throughout NDDN. As a cautionary rote, it should be
78
-------
SITE 119
KEY
SAMPLE TYPE
° DAY SAMPLE
• NIGHT SAMPLE
WONIH/11Afl
SHE 120
B
s
0 It
1
SITE IJ'3
Figure 39. Weekly day/night SO2 concentrations for sites in complex
terrain (A), mountaintop (B), and rolling terrain (C), October 1988
through September 1989
79
-------
recalled that the filter pack sampling approach used for NDDN does not
unambiguously separate particles and gases and does not have a well
characterized outpoint for aerosol size. Thus, gas/aerosol interactions
could be important and a wide range of aerosol sizes with widely
differing chemical composition could be sampled by the current filter
pack samples. Results should therefore be interpreted to reflect the
chemical makeup of the filter extract, rather than as a definitive
indication of aerosol chemistry at NDDN sites.
Ion balances, in nanoequivalents per cubic meter (neq/m3), from
Teflon® filter extracts at nine NDDN sites are illustrated in Figures 40
through 42. The sites presented in Figure 40 appear to be
representative of the eastern seaboard (Site 108), forested northeast
(Site 117), and agricultural midwest (Site 133). In general, the data
for these sites indicate that SO2; and NH$ are the dominant anion and
cation species, respectively, at both forested and agricultural sites in
the eastern United States, and that the nature of the ion balances
differs between forested and agricultural sites. The forested sites
exhibit fairly minor ionic contribution for NOg and the base metal
cations and a clear excess of anions over cations. The agricultural
midwestern site, in contrast, exhibits significant ionic contributions
from NOg, Mg2+, and Ca2+ and an apparent excess of cations.
Results shown in Figure 141 suggest that similar relationships hold
for forested sites in complex terrain (Site 119) and at high elevation
(Site 120). Data for the urban-agricultural site in Maryland (Site 116)
and the midwestern agricultural site exhibit similar relationships
between cations and anions. Figure 42 shows ionic balances for
relatively pristine sites in the northeast (Site 135) and the southeast
(Site 156), as well as a typical western site (Site 165). Results for
the eastern sites indicate nearly identical concentrations of anions and
cations and, on a percentage basis, reduced dominance of NH4 as the
primary cation relative to other sites. For the western site, there is
a clear excess of cations in! the aerosol samples, with NH4 comprising
only about 50 percent of the total cations. Results from all three
sites suggest that NOg is a minor contributor to the overall ion
balance.
The spatial variability of aerosol ion ratios is illustrated in
Figure 43. Note that data for the eastern United States represent the
period October 1988 through September 1989, while those for the western
sites represent the period June through September 1989. Results
indicate a general dichotomy between midwestern sites and the majority
of northeastern and southeastern sites that apparently is related to
agricultural activity. With only one exception, midwestern sites
exhibit cation/anion ratios above unity; and with only six exceptions,
northeastern and southeastern sites exhibit ratios below unity. Five of
the six sites are located in agricultural or urban-agricultural areas,
and the sixth (Site 121 in eastern Kentucky) is apparently influenced by
80
-------
250
225
108
117
SITE NUMBER
133
Figure 40. Aerosol ion balances for Sites 108, 117, and 133
81
-------
250
225
200
175
150
f125
100
75
50
25
LEGEND
2-
S04
33 N03
NHj
C«2*
Mfl2*
116
119
SITE NUMBER
120
Figure 41. Aerosol ion balances for Sites 116, 119, and 120
82
-------
200
175
150
125
.100
75
50
25
LEGEND
^^wj 2-
NH4
Ca2*
Na+
135
156
SITE NUMBER
165
Figure 42. Aerosol ion balances for Sites 135, 156, and 165
83
-------
B
Figure 43. Aerosol cation/anion ratios for eastern (A) and western (B) NDDN sites
84
-------
road dust from a coal mining operation. Western sites invariably show a
substantial excess of cations over anions .
Although the previous relationships refer to average values taken
over a year or a quarter, they are generally adhered to on a sample -by-
sample basis. Figure 44 shows scattergrams of total anions versus total
cations for weekly day, night, or composite samples taken at a
northeastern site (Site 117), midwestern site (Site 133), and western
site (Site 165). With few exceptions, samples from the midwestern and
western sites exhibit ion ratios of 1.0 or greater. The northeastern
site shows considerable scatter around a 1:1 relationship, but a clear
tendency for ratios to fall below unity with increasing total anions.
Periods of high concentration, therefore, appear to drive ion balances
below unity at northeastern sites.
Ratios of NH^ to SO^" in aerosol samples are shown for all NDDN
sites in Figure 45. Results show very similar spatial variability to
those for the cation/ anion ratios. This is not surprising, since NH^
and SO^" are the dominant cation and anion, respectively, at all NDDN
sites. However, the similarity of ratios strongly suggests that the
chemical composition of aerqsols containing SO^" control ion balances
and ratios .
In the majority of northeastern sites, ion balances and
ratios fall below unity, and an unmeasured cation (possibly H+) must be
present to satisfy electroneutrality conditions. For midwestern* and
western sites, cations are in excess, and one or more unmeasured anions
(possibly HCC>3 or COf") must therefore be present. Unfortunately, the
NDDN filter pack cannot definitively quantify certain atmospheric
species (including H+ and HCOg) due to the potential for gas -particle
interactions within the sampler itself.
4.2.8 1988 Versus 1989 Concentration Data
One of the principal objectives of the NDDN is to evaluate long-
term trends in dry deposition rates and atmospheric concentrations.
Since the network has recently been fully deployed, little historical
data, are available to evaluate trends. This section compares annual
concentration data from 1988 and 1989 and determines whether interannual
differences are statistically significant, or merely random fluctuations
in observations.
Annual average concentrations of SO^' and NO^ for 1988 and 1989 are
shown in Figure 46. Results for 804" indicate what appear to be
distinct regional patterns. In general, annual averages for the 2 years
are within ±0.2 to 0.3 Hg/ro3 for the easternmost sites in the network,
from North Carolina to New York. The midwestern sites from central
Kentucky to northern Illinois invariably exhibit higher (i.e., greater
than or equal to 0.5 JLig/m3) concentrations in 1989, while two sites in
85
-------
SITE 11 7
o'oja a
o D a
o*r a a a ncm
loo tot loo nao
SITE 133 ,
B
•-•• One-To-On« R
HI 101 ((I SOS (01 700 100
SOI i N01 t.M/iJ)
SITE 165
' ''
On*-To-One Retottonship
II II II
10 it ii 'o to
S04 t IOJ (•«/•*)
Figure 44. Total cations versus total anions for a northeastern site (A), a midwestern
site (B), and a western site (C), October 1988 through September 1989
86
-------
B
Figure 45. NH^/SO^" ratios" for eastern (A) and western (B) NDDN sites
87
-------
B
Figure 46. Annual average concentrations (Mg/m3) of SO^ (A) and NOJ (B) for
1988/1989
88
-------
West Virginia and Virginia exhibit somewhat higher values in 1988. Only
the largest interannual difference of 1.9 Hg/m* (in western Ohio) is
statistically significant at the 95-percent confidence level. The
inherent noise in observed concentrations thus appears to prevent
statistical discrimination between successive years of concentration
differences on the order of 1.0 Hg/ra3 or less.
Combination of data across sites (within regions), adjusting for
seasonal variability or relaxation of statistical confidence levels may
be necessary for detection of statistically significant interannual
differences.
Results for NOj show, if anything, a more dramatic contrast between
the northeast and midwest. Northeast data for 1988/1989 show remarkable
temporal stability across a broad range of concentrations. Midwestern
data show not only higher concentrations, but larger differences between
years. As with S04", the higher concentrations were associated with
1989. Relative differences at midwestern sites ranged from 27 percent
in western Ohio (Site 122) to 49 percent in central Illinois (Site 130).
Differences between years were significant for all midwestern sites
except Site 122.
Annual average concentrations of HN03 (see Figure 47) show higher
values in 1988 than 1989 for all sites, except Site 122. However, .
differences exceeded 15 percent only at a cluster of three sites in
eastern Kentucky, West Virginia, and Virginia, and at a single site in
central Pennsylvania. Absolute differences for these sites ranged from
0.17 Mg/m to 0.70 jtig/m3 and in all cases were statistically
significant.
Results for S02 show small differences between years and no
evidence of a consistent regional pattern. Data for eastern
Pennsylvania and New York show essentially no detectable differences
between years. Sites in western Pennsylvania, Ohio, Virginia, and North
Carolina exhibit annual differences of 10 to 20 percent. Sites in Ohio
and Pennsylvania suggest an increase in concentrations from 1988 to
1989, while those in Virginia and North Carolina suggest the opposite
Despite interannual differences of 2 to 3 Mg/m3 at some sites, no site
exhibited a statistically significant difference between years. As for
S04 , it appears that variability in concentrations, rather than
measurement uncertainty, can mask statistical detection of relatively
large year-to-year variations.
4.3 OZONE
As described in Section 3.0, continuous 03 concentrations were
monitored throughout the year at a height of 10 m at all NDDN sites
Annual averages, valid observations, and peak observed concentrations
for 1989 are summarized in Table 15. Annual averages among eastern
89
-------
KEY
2.53/2.36 1988/1989 DATA
Figure 47. Annual average concentrations (/ig/m3) of HNO3 (A) and S02 for
1988/1989
90
-------
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93
-------
sites range from 22.1 ppb in eastern Kentucky,(Site 121) to 45.3 ppb in
northern Virginia (Site 118). The highest annual averages (i.e., 43 to
45 ppb) occur at mountaintop sites along the Blue Ridge and Appalachian
Mountains, while the lowest annual averages occur in sites located in
sharp valleys (e.g., Sites 119 and 121) and in semiurban areas (e.g.,
Sites 101, 116, and 146).
Hourly average concentrations equal to or greater than the NAAQS
were relatively rare during 1989. Eight sites exhibited one or more
hourly values greater than or equal to 120 ppb, and only 15 days of
exceedances occurred at the 43 sites operational throughout the year.
Results for western sites, which were typically operational for
half of 1989, showed average 03 concentrations ranging from 31.2 ppb in
northern Montana (Site 168) to 48.9 ppb in northern Arizona (Site 174).
In general, average concentrations exceeded those in the east. Peak
concentrations, in contrast, were almost invariably less than those
observed across the eastern part of the network. The average elevation
for the western sites is well above the highest elevation of any eastern
site. Thus, relatively high annual averages among western sites are
consistent with similarly high averages for the eastern sites located on
mountaintops.
Daily average and daily maximum 03 concentrations for selected
pairs of northeastern, midwestern, southeastern, and western sites are
shown in Figures 48 through 51. Each figure also shows a horizontal
line at 120 ppb, which represents the NAAQS for 03. The NAAQS for 03 is
violated if four or more daily maxima greater than or equal to 120 ppb
are observed during a 3-year period. In general, results for the
majority of sites reflect the classical seasonal cycle of 03 production.
Maximum concentrations are usually observed during the period June
through September, and minimum concentrations are observed in November,
December, and January. Seasonal variability of this nature has been
observed previously over widespread areas of the eastern United States
(Altshuller, 1987; Meagher et al. , 1987). Results for the two western
sites, in contrast, show limited seasonal variability in both daily
average and maximum 03 concentrations.
Inspection of daily 03 averages for 1989 shows considerable fine
structure in the overall annual pattern. 03 concentrations at nearly
all sites rise and decay with a period of approximately 5 to 10 days,
which is probably meteorologically driven. For example, examination of
daily maxima listed in Table 15 shows that greater than 50 percent of
the first, second, and third highest hourly observations occurred during
two episodes (i.e., June 23 through June 27 and'July 1 through July 4)
within a 2-week period.
As mentioned previously, annual average 03 values appear to show
deterministic differences between mountaintop sites and valley sites and
94
-------
120
NAAOS for Ozone
i i i i 1 i 1 1 1 1 1 1 r
OIJAN 01FE8 01UAR 01APR 01HAY 01JUH 01JUL 01AU5 OISEP OIOCT OINOV OIOEC OUAH
NAAQS for Ozone
OU»H OIFEB 01UAR OUPR 01UAY 01JUN OUUl OUUC OISEP OIOCT OINOV OIOEC OUAH
Figure 48. O3 daily averages and maxima for two northeastern sites:
Site 106 (A) and Site 135 (B)--1989
95
-------
ICO-
no-
HO
NAAQS for Ozone
.A
OIJAN OIFEI OIUAR OIAPR 01VAY 01JUH OIJUl 01AUG OISEP OIOCT 01NOV 01DEC OIJAK
NAAOS for Ozone
Ufl
too
, «
e
B
OIJA« OlfEB OIHAR 01AP« OI«AT 01JUN OIJUL OIAUC OISEP OIOCT OIHOV 01SEC 01 IA X
Figure 49. O3 daily averages and maxima for two southeastern sites:
Site 127 (A) and Site 150 (B)--1989
96
-------
140-
NAAQS for Ozone
T i i i i 1 r 1 1 1 1 1 r
01JAN Olf£8 OIWAR 01AP* OIBAY OIJUH OIJUl OUUC OIS£P OIOCT 01KOV 010EC OUAH
DATE
NAAQS for Ozone
0 I JAN 0 IfEB
B
01AI>« 01KAT OIJUN OtJUL OIAUC OISEP OIOCT OIKOV 010EC 01JAN
Figure 50. O3 daily averages and maxima for two midwestern sites:
Site 122 (A) and Site 134 (B)--1989
97
-------
I
uo
uo
100
NAAOS for Ozone
A
GUAM OlFEt OIUAR OIAPR 01MAY 01JUN 01JUL 01AUC 01SEP 010CI OIHOV 01DEC OUAX
1(9
130
NAAQS for Ozone
(0
OUAH OIFEB OIUAR OIAPR 01UAT 01JUK 01JUI OIAUC 01SEP OIOCT OIHOV OldEC 01JAN
DATE
Figure 51. O3 daily averages and maxima for two western sites: Site 165 (A)
and Site 168 (B)--1989
98
-------
between semiurban sites and nearby rural sites. These relationships are
exhibited in Figures 52 and 53, which present frequency distributions
for a rolling terrain site (Site 108 in south-central Virginia), a
complex terrain site (Site 119 in West Virginia), a mountaintop site
(Site 118 in northern Virginia), and a semiurban site (Site 116 in the
Baltimore-Washington corridor). Results for these sites are typical of
the majority of NDDN sites located in similar settings.
Results for Site 108 show a peak in the frequency distribution
around 20 to 30 ppb and approximately equal numbers of observations in
the 0- to 10-ppb and 50- to 60-ppb ranges. Hourly values greater than
or equal to 80 ppb (see inset in Figures 52 and 53) represent less than
5 percent of total observations. Results for Site 119, a typical valley
site, show a peak in the frequency distribution in the 0- to 10-ppb
range and a monotonic decrease at successively higher concentration
levels. Interestingly, the frequency of observations greater than or
equal to 80 ppb is nearly identical to that at the rolling terrain site.
Data for the mountaintop site in northern Virginia show a peak in
the frequency distribution in the 30- to 40-ppb range and very few
observations (i.e., less than 5 percent) below 20 ppb. The number of
hourly values greater than or equal to 80 ppb exceeds that at the
rolling terrain and valley sites by approximately a factor of 3. Urban
NDDN sites, in general, show a unique distribution of 03 observations.
In this case, a. pronounced peak in the frequency distribution is
observed in the 0- to 10-ppb range, but a secondary peak also occurs in
the 20- to 30-ppb range. The distribution also exhibits a much longer
tail than other classes of sites and a much higher frequency of values
greater than or equal to 80 ppb than even the mountaintop site.
Differences in diurnal 03 cycles appear to offer partial
explanation for the different frequency distributions previously
described. Hour-by-hour average 03 concentrations for January, July,
and calendar year 1989 for two rolling terrain sites, two complex
terrain sites, two mountaintop sites, and two urban or semiurban sites
in the eastern United States are plotted in Figures 54 through 57.
Results show generally consistent differences between types of sites,
both for the summer and winter months and the entire annual period. The
rolling terrain sites exhibit moderate day/night variability, with night
minima typically on the order of 50 to 60 percent of the day maxima.
Day maxima during the summer period of maximum photochemical activity
show a fairly broad plateau between the hours of 1200 through 1800.
Hourly averages for the complex terrain sites show markedly different
behavior. In this case, nocturnal minima (especially during July) are
much less than half the daily maxima. In addition, periods of minimum
and maximum concentrations la«t appreciably longer and shorter,
respectively, than at rolling terrain sites. However, maximum
concentrations are similar at the two types of sites.
99
-------
Observotions >= 80 ppb
I! I) 111 I II 111 Ul 111 111 III 111
5 15 25 35 45 55 65 75 85-95 105 115 115 '35 '* 5 ' • 5 '55 ' • 5
Observations >— 80 ppb
II It 111 111 'It
-------
Observations >= 80 ppb
5 15 25 35 45 55 65 75 85 95 105 115 125 :J5 :4S '55 '65 175
Observations >= 80 ppb
5 15 25 35 45 55 55 75 85 95 105 115 '25 "35 "5 '55 '55 175
B
Figure 53. O3 frequency distribution for Sites 116 (A) and 118 (B) during 1989
101
-------
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91 9) g] 94 gj 91 01 OS 09 10 II 12 U H IS 16 U I! 19 20 21 22 21 2<
HOUB
UOKIH a A A JA»Um ODD JUU
O O O ANNUAL
B
Figure 54. Hourly average O3 concentrations for typical sites in rolling terrain: Site
108 (A) and Site 129 (B)
102
-------
3 a
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a o
o
2 S
~l 1 1 1 1 1 1 1 1 1 1 1 ! 1 1 1 1 i 1 1 1 1 r
01 01 Oi 0< OS 01 07 01 09 10 II 12 I] I* li IS II II 19 It II 11 2J 24
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02 0} 04 OS 0( 07 01 01 10 II 12 13 14 IS IS 17 It I! 20 21 22 21 24
HOUR
A A A JANUARY O D D JULY O O O ANNUAL
B
Figure 55. Hourly average O3 concentrations for typical sites in complex terrain:
Site 119 (A) and Site 121 (B)
103
-------
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B
Figure 56. Hotarly average O3 concentrations for typical mountaintop sites:
Site 118 (A) and Site 126 (B)
104
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i i ii i i i i i i i ] ; 1 [ 1 1 | 1
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01 02 01 01 OS 01 92 01 09 10 II 12 I] U IS If 17 I! II 20 21 22 21 2<
HOUI
HONIK a a a JAMUART a a D JIHT o o o AIKUAL
B
Figure 57. Hourly average O3 concentrations for typical urban or semi-urban
sites: Site 116 (A) and Site 146 (B)
105
-------
Mountaintop sites exhibit a unique diurnal pattern, or lack of one.
For these sites, there is no distinct period of maximum or minimum
concentration. Rather, hourly average values consistently remain within
a few parts per billion of hourly maxima observed at the other sites.
July and January hourly averages differ dramatically (approximately a
factor of 2) over the entire 24-hour cycle. Finally, results for the
semiurban sites show depressed nocturnal values (especially during
winter) and pronounced day maxima during the summer. In general, day
maxima are 5 to 10 ppb higher than at the other sites but similar in
duration to the complex terrain sites. Both of these semiurban sites
are located in rolling terrain. Thus, similarities between semiurban
and complex terrain sites could have distinctly different explanations.
One explanation for the observed behavior involves the relationship
between sampler location and the nocturnal inversion layer. The
mountaintop sites presumably sit above the inversion layer and,
therefore, are always in contact with a large reservoir for 0;!. The
rolling terrain and complex terrain sites are situated below the
inversion layer, within which 03 is subject to a variety of depletion
processes. Longer lasting and/or shallower inversions could result in
rapid decay of 03 in complex terrain than in rolling terrain. The end
result of this day/night variability is a gradient in integrated
exposure (i.e., average concentrations) from mountaintop to rolling
terrain to complex terrain. Although no continuous data support this,
the same terrain effect might also account for the S02 and HN03 pattern
described previously for these sites. For the semiurban sites, broad
nocturnal minima could be the result of destruction by nitric oxide
(NO), while sharp day maxima could be due to enhanced photochemical
production in the presence of NOX and natural or manmade volatile
organic gases.
As mentioned previously, relatively few 03 concentrations greater
than or equal to 120 ppb were observed during 1989 (i.e., 15 exceedances
from 43 sites). This contrasts sharply with data from 1988, which
showed 98 exceedances from only 18 sites. Differences between 1988 and
1989 are clearly illustrated on examination of frequency distributions
for the 2 years. Frequency distributions for a rolling terrain site in
central Kentucky (Site 129), a complex terrain site in eastern Kentucky
(Site 121), and a mountaintop site in southwestern Virginia (Site 120)
are shown in Figures 58 through 60. As a rule, the differences between
terrain types previously discussed appear to hold for both years.
However, the frequency distribution for each site is skewed toward
higher concentrations in 1988 than in 1989. In addition, Site 121
exhibits a dramatic increase in frequency of observations in the 0- to
10-ppb interval from 1988 to 1989, while Sites 120 and 129 show only
modest changes in this range.
106
-------
Observations >= 80 ppb
5 15 25 35 <5 55 55 75 85 95 105 115 125 US !
145 155-165 '75
Observations >= 80 ppb
5 I' 25 35 <5 55 65 75 85 95 105 115 125 135 H5
B
Figure 58. 1988 (A) versus 1989 (B) O3 frequency distribution for Site 121
107
-------
Observations >= 80 ppb
5 \i 25 55 (5 55 65 75 J5 95 105 '15 !25 '35 ''5 '55 :i5 -175
rttouCHcr
Observations >= 80 ppb
5 15 25 35 45 55 85 75 85 95 105 115 25 '!5 '
-------
Observations >— 80 ppb
5 15 25 35 45 55 85 75 85 95 105 t 15 125 135 MS 155 1-65 175
Observations >= 80 opb
5 15 25 35 45 55 65 75 85 95 105 115 '.25 !J5 M5 :'55 '55 175
B
Figure 60. 1988 (A) versus 1989 (B) O3 frequency distribution for Site 120
109
-------
Differences in annual frequency distributions are most obvious in
the concentration range above 80 ppb (see insets). For all three sites,
the number of observations greater than or equal to 80 ppb is at least a
factor of 5 higher in 1988 than in 1989. This phenomenon was generally
observed across the midwestern and northeastern United States, as shown
in Figure 61.
Other measures of integrated 03 exposure or concentration also
suggest that 1988 and 1989 were markedly different years. One such
measure is the 7-hour growing season average, which has been used by the
National Crop Loss Assessment Network (NCLAN) to assess 03 effects on
various agricultural crops (Heck et al.. 1982). This measure represents
an arithmetic average of observations taken during the period 0900 to
1559 during the relevant growing season for a particular location. The
7-hour growing season averages for 1988 and 1989 are shown in Figure 62
for the 16 sites completely operational during both years. Since
growing season varies over the NDDN domain, May through September was
selected as a likely period of biological activity at all sites.
Results show a fairly consistent 10- to 20-percent difference between
1988 and 1989. The only significant exception to this occurred in
extreme northern New York (Site 105), which showed essentially no
difference between years.
The sigmoidally weighted 03 concentration (W126) is a statistic
recently proposed by Lefohn and Runeckles (1987) as a tool for examining
03 damage to forests and crops. This function weights each 03
concentration in a manner that emphasizes high values (e.g., greater
than or equal to 80 ppb) and deemphasizes low values (e.g., less than or
equal to 30 ppb). Thus, concentrations believed to be more harmful to
crops are given greater weight in the averaging scheme than low
concentrations. Calculations of W126 for 1988 and 1989 (see Figure 63)
uniformly show higher values for the earlier year. In fact, differences
are generally on the order of a factor of 2, suggesting that, by at
least one measure, 03 exposure in 1988 was twice that in 1989.
The underlying reasons for differences between 1988 and 1989 are
undoubtedly complex; however, various observations at NDDN sites suggest
a meteorological link. Solar radiation and surface temperature (9 m)
are routinely measured by NDDN. Growing season data for 1988 and 1989
for these variables are compared in Figure 64. Results for solar
radiation show that, on average, 20 percent more sunlight was received
at the surface in 1988 than in 1989. The same may not be exactly the
case for the more important UV radiation, which drives atmospheric
photochemical cycles, but it should have been at least a few percent
greater in 1988 than 1989. The only site that does not show an
appreciable difference in solar radiation was the one exhibiting the
smallest differences in exposure statistics between years (i.e.,
Site 105).
110
-------
B
Figure 61. Number of hourly observations greater than or equal to 80 ppb, 1988 (A)
versus 1989 (B)
ill
-------
B
Figure 62. Seven-hour growing season averages (ppb), 1988 (A) versus 1989 (B)
112
-------
Note: Units ore ppm/hour.
B
Figure 63. Integrated O3 exposure indices (W126) for 1988 (A) and 1989 (B)
113
-------
Note: Units ate In °C.
B
Figure 64. Ratios of 1988 versus 1989 growing season solar radiation (A) and
1988/1989 growing season temperatures (B)
114
-------
Growing season temperatures also showed marked differences between
years, with higher temperatures in 1988 than 1989. This is especially
apparent in the midwest, where growing season averages differ by as much
as 2 percent or more. This difference between years is many times the
uncertainty of the measurement. The 1988 growing season appears to have
been significantly hotter, drier, and sunnier than 1989. Although these
results do not prove cause and effect, they reinforce the notion of
meteorology as an important forcing function to 03 exposure.
4.4 ESTIMATED DRY DEPOSITION
Another NDDN objective is to produce long-term data on patterns and
trends of dry deposition across the continental United States.
Ultimately, dry deposition rates will be calculated using one or more
algorithms that couple land use or vegetation data, meteorological data,
and air-quality data to produce weekly, seasonal, and annual fluxes.
This section presents estimates of dry deposition rates for 1989 using
assumed values for annual average deposition velocities taken from the
literature and annual average concentration data from 28 NDDN sites in
the eastern United States that were operational throughout 1989. They
are specifically not intended to be an approximation of, or substitute
for, deposition estimates based on the inferential approaches described
by Hicks et al. (1985) and Wesely (1988). Calculated dry deposition
rates are then compared with average wet deposition rates obtained by
NADP and other precipitation chemistry networks in the vicinity of NDDN
sites. The wet deposition sites used in this comparison, and distances
to neighboring NDDN sites, are listed in Table 16. Results are intended
only to illustrate a possible range of dry deposition rates across the
network.
The estimated deposition velocities used in these calculations are
listed in Table 17. Tabulated values reflect a variety of theoretical
estimates, experimental tests, and modeling results. Wu and Davidson
(1988) used the resistance model of Hicks et al. (1985) to estimate
weekly and annual deposition velocities of S02, SO2;, and HNO3 for three
sites during 1986. Ranges of annual average deposition velocity for Oak
Ridge, TN, Penn State University, PA, and Whiteface Mountain NY (all
current or former NDDN sites), were 0.07 to 0.13 centimeter per second
(cm/sec) for SO;2, 0.18 to 0.33 cm/sec for S02, and 0.96 to 1.9 cm/sec
for HN03. Sheih et al. (1979) and Wesely and Lesht (1988) have
developed computer routines for estimating deposition velocity as a
function of land use, season, and meteorological stability class.
Wesely and Lesht (1988) recently compared their model with a site-
specific inferential technique similar to that which may be used for the
NDDN (Hicks et al., 1988). Calculated deposition velocities were
consistent with data shown in Table 17 and, on average, within ±30 to
40 percent of deposition velocities provided by the inferential
technique. Little information is available on the deposition velocity
for atmospheric NOg. Since, by definition, both NOj and SO2; occur in
115
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TABLE 17. ESTIMATED DEPOSITION VELOCITIES FOR AEROSOLS AND GASES
Species
S02-
NOj
S02
HN03
Vd
(cm/sec) Reference
0.1 - 0.2 Sehmel, 1980; Sheih et al. , 1979;
Voldner et al. . 1986; Wesely and Lesht,
1988; Wu and Davidson, 1988
0.1 - 0.2 Assumed to be the same as SO^"
0.2 - 0.4 Sehmel, 1980; Sheih e_t al. , 1979; Cadle
et al., 1987; Wesely and Lesht, 1988;
Wu and Davidson, 1988
1.0 - 2.0 Heubert, 1983; Wesely and Lesht, 1988;
Wu and Davidson, 1988
Note:cm/sec = centimeter per second.
Vd = deposition velocity.
Source: ESE, 1990.
117
-------
the particulate phase, the same range of deposition velocities was used
for these species. However, differences in particle size and reactivity
could result in substantial differences in deposition velocities for N03
and S0;|".
Estimates of annual dry deposition for 804", S02, N03, and HN03 are
listed in Table 18. Estimated dry deposition for SO^" plus S02 appears
to be highest in western Pennsylvania (Site 117) and lowest in
southwestern North Carolina (Site 137), northern Maine (Site 135), and
northern Florida (Site 156). This pattern reflects the annual average
S02 concentration, since it appears unlikely that SO^" contributes more
than 30 percent of the estimated dry deposition at any site. Relatively
high deposition in northern Illinois, southern Indiana, and eastern
Tennessee may be the result of local S02 emissions. Similarly high
values in western Pennsylvania and West Virginia seem to reflect more
widespread sources.
Estimated dry deposition of N03 plus HN03 shows similar values over
much of the northeast and midwest but considerable variability in the
vicinity of the Appalachian Mountains. Deposition at the mountaintop
site in Virginia (Site 120) appears to be approximately three times that
at complex terrain sites in eastern Kentucky and North Carolina.
(Site 121 and Site 137). The overall pattern is almost an exact
transformation of annual HN03 concentration due to large differences in
concentration and deposition velocity (assumed) between HN03 and N03.
At no site was N03 responsible for more than 15 percent of the estimated
dry deposition of N03 plus HN03. As stated previously, N03 deposition
could be significantly greater if it exists in large particles with
appreciable settling velocities, ;
Wet and dry deposition data from a variety of NDDN sites are
compared in Figures 65 and 66. The data shown for dry deposition
reflect the midpoint of the 50% plus S02 fluxes listed in Table 17 and,
therefore, are uncertain by at least +50 percent. In addition,
estimated dry deposition rates refer to calendar year 1989, while wet
deposition rates are averaged over the period 1984 through 1988 (or 1983
through 1987). Thus, the data illustrate possible, rather than actual,
relationships between wet and dry deposition. Results for sulfur
deposition suggest that wet deposition is the dominant process in
northern New York (Site 105), but that at most other sites wet and dry
deposition could be similar, especially if the upper limit for the
deposition velocity of S02 is approached. Results for nitrogen suggest
regional differences in the comparability of wet and dry deposition.
For the northeast, dry deposition would be comparable to wet deposition
only if the upper limit deposition velocity for HN03 is attained. Data
for the midwest and parts of the southeast, in contrast, suggest that
wet and dry deposition are comparable even if the lower limit deposition
velocity for HN03 is attained.
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Figure 65. Observed wet (A) and estimated dry (B) deposition (eq/ha-yr)
of SO|- at selected sites
121
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Figure 66. Observed wet (A) and estimated dry (B) deposition (eq/ha-yr)
of NOj at selected sites
122
-------
REFERENCES
Altshuller, A.P. 1987. Estimation of Natural Background of Ozone
Present at Surface Rural Locations. Journal of the Air Pollution
Control Association, 37(12):1409-1417.
Barnes, H.M., Hansen, D.A., and Lusis, M. 1987. Regional Field Study
Design for Evaluation of Eulerian Acidic Deposition Models
Presented at 80th Air Pollution Control Association Conference
June 21-26, 1987. New York, NY.
Cadle, S.H., Dasch, J.M., and Mulawa, P.A. 1985. Atmospheric
Concentrations and the Deposition Velocity to Snow of Nitric Acid,
Sulfur Dioxide and Various Particulate Species. Atmos Envir
19:1819-1827. '
Environmental Science & Engineering, Inc. (ESE). 1989a. National Dry
Deposition Network (NDDN) Laboratory Operations Manual. Prepared
for U.S. Environmental Protection Agency (EPA). Contract No 68-
02-4451. Gainesville, FL.
Environmental Science & Engineering, Inc. (ESE). 1989b. National Dry
Deposition Network (NDDN) Quarterly Data Report (January through
March). Prepared for U.S. Environmental Protection Agency (EPA)
Contract No. 68-02-4451. Gainesville,' FL.
Environmental Science & Engineering, Inc. (ESE). 1989c. National Dry
Deposition Network (NDDN) Quarterly Data Report (April through
June). Prepared for U.S. Environmental Protection Agency (EPA)
Contract No. 68-02-4451. Gainesville, FL.
Environmental Science & Engineering, Inc. (ESE). 1989d. National Dry
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«0& GOVERNMENT PRINTING OFFICE: 1991-5118-187/25622
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