?
United States Office of Air Quality EPA-450/3-78-02t
Environmental Protection Planning and Standards June 1978
Agency Research Triangle Park NC 27711
Air
v>EPA Comparison of
Ambient NMHC/
NOx Ratios with
NMHC NOX Ratios
Calculated from
Inventories
-------
EPA-450/3-78-026
Comparison of Ambient NMHC/NOx Ratios
with NMHC/NOx Ratios Calculated
from Emission Inventories
by
Peter J. Dnvas
Pacific Environmental Services, Inc
1930 14th Street
Santa Monica, California 90404
Contract No 68-02-2583
Assignment No 4
EPA Project Officer James H Wilson. Jr
Prepared for
U S ENVIRONMENTAL PROTECTION AGENCY
Office of Air and Waste Management
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina 27711
June 1978
-------
This report is issued by the Environmental Protection Agency to report
technical data of interest to a limited number of readers. Copies are
available free of charge to Federal employees, current contractors and
grantees, and nonprofit organizations - in limited quantities - from the
Library Services Office (MD-35), U.S. Environmental Protection Agency,
Research Triangle Park, North Carolina 27711; or, for a fee, from the
National Technical Information Service, 5285 Port Royal Road, Springfield,
Virginia 22161.
This report was furnished to the Environmental Protection Agency by
Pacific Environmental Services, Inc., 1930 14th Street, Santa Monica,
California 90404, in fulfillment of Contract No. 68-02-2583, Assignment
No. U. The contents of this report are reproduced herein as received
from Pacific Environmental Services, Inc. The opinions, findings, and
conclusions expressed are those of the author and not necessarily those
of the Environmental Protection Agency. Mention of company or product
names is not to be considered as an endorsement by the Environmental
Protection Agenc1
Publication No. EPA-450/3-78-026
ii
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TABLE OF CONTENTS
Section Page
1.0 INTRODUCTION AND SUMMARY 1-1
2.0 LOS ANGELES DATA ANALYSIS 2-1
2.1 Ambient Air Monitoring Data 2-1
2.2 Emission Inventory Data 2-3
2.3 Ratio Comparison 2-5
3.0 SAN FRANCISCO DATA ANALYSIS 3-1
3.1 Ambient Air Monitoring Data 3-1
3.2 Emission Inventory Data 3-3
3.3 Ratio Comparison 3-7
4.0 ST. LOUIS DATA ANALYSIS 4-1
4.1 Ambient Air Monitoring Data 4-1
4.2 Emission Inventory Data 4-3
4.3 Ratio Comparison 4-5
5.0 DISCUSSION OF RESULTS 5-1
5.1 Correlations Versus Numerical Values 5-1
5.2 Reasons for Ratio Differences 5-4
5.2.1 Meteorological Factors 5-4
5.2.2 Accuracy of Emission Inventory Data .... 5-5
5.2.3 Elevated Point Sources 5-6
5.2.4 Other Factors 5-6
6.0 REFERENCES 6-1
APPENDIX A. LOS ANGELES AMBIENT AIR MONITORING DATA. . . . A-l
APPENDIX B. SAN FRANCISCO AMBIENT AIR MONITORING DATA. . . B-l
APPENDIX C. ST. LOUIS AMBIENT AIR MONITORING DATA C-l
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LIST OF TABLES
Table Page
2-1 Los Angeles NMHC/NOX Ratio Summary 2-2
2-2 Los Angeles Gridded Emission Inventory Data 2-4
2-3 Los Angeles NMHC/NOX Ratio Comparison Using Gridded
Emission Data 2-6
3-1 San Francisco MMHC/NOX Ratio Summary 3-2
3-2 San Francisco Gridded Emission Inventory Data .... 3-4
3-3 California Air Resources Board Hydrocarbon Reactivity
Classes 3-5
3-4 San Francisco Countywide Emission Inventory Data . . . 3-6
3-5 San Francisco NMHC/NOX Ratio Comparison 3-8
4-1 St. Louis NMHC/NOx Ratio Summary 4-2
4-2 St. Louis Countywide Emission Inventory Data 4-4
4-3 St. Louis NMHC/NOX Ratio Comparison Using Countywide
Emission Data 4-6
5_1 Averaged Ambient Air NMHC/NOX Ratios Compared with
Emission Ratios in the Central Downtown Area . . . 5-2
Figure Page
2-1 Best-Fit Regression Lines for Five High-Oxidant
ays in Los Angeles, Comparing Gridded Emission
N^HC/NOX Ratios with Ambient Ratios 2-7
3-1 Best hit Regression Lines for Five High-Oxidant
Days in San Francisco, Comparing Gridded Emission
NMHC/NOX Ratios with Ambient Ratios 3-9
3-2 Best-Fit Regression Lines for Five High-Oxidant
Days in San Francisco, Comparing Countywide
Emission NMHC/NOX Ratios with Ambient Ratios ... 3-10
4-1 Best-Fit Regression Lines for Five High-Oxidant
Days in St. Louis, Comparing Countywide Emission
NMHC/NOX Ratios with Ambient Ratios 4-7
IV
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1.0 INTRODUCTION AND SUMMARY
One method for revising State Implementation Plans to meet the
ambient oxidant standards is the Empirical Kinetic Modeling Approach
(EKMA). This method requires the determination of the ratio of non-
methane hydrocarbons (NMHC) to oxides of nitrogen (NOV). It is
/\
recommended in the EKMA method (Environmental Protection Agency 1977,
EPA-450/2-77-021a) that the ratio of NMHC/NOX be calculated with
ambient air monitoring data. However, due to the lack of adequate
air monitoring data in some locations, local agencies have proposed
using emission inventory data to calculate the appropriate NMHC/NO
/\
ratio. The purpose of this project was to determine what differences
result from calculating NMHC/NOX ratios from emission inventory data
compared with the EKMA recommended technique of calculating NMHC/NOX
ratios from ambient monitoring data, and to rationally explain the
differences between the two methods.
Three geographic locations were considered in analyzing the
differences between NMHC/NOX ratios calculated from emission inventory
data and those from ambient monitoring data. The three locations were
chosen due to the availability of detailed gridded emission inventories
and the large number of air monitoring sites in each location. The
following three locations were used:
1. Los Angeles, California
2. San Francisco, California
3. St. Louis, Missouri
Ambient monitoring data from five high oxidant days in each of the
three locations were used to calculate 6-9 a.m. NMHC/NOX ratios, as
specified in the EKMA method.
The emission inventory data involved detailed hourly gridded
emission data in Los Angeles and San Francisco in addition to county-
wide emission data in each of the three locations. Because the EKMA
1-1
-------
method requires volumetric NMHC/NOY ratios in units of ppmC/ppm, it
/\
was necessary to adjust the emission NMHC/NOX ratios from units of
mass to units of volume. Since NOX emission rates are normally
expressed as N02 (molecular weight = 46) and hydrocarbon emission
rates are normally expressed as CH4 (molecular weight = 16), all
emission NMHC/NOX ratios were multiplied by 46/16 or 2.875 to con-
vert to the volumetric units of ppmC/ppm.
The relationship between ambient monitoring NMHC/NOX ratios
and emission inventory NMHC/NOX ratios was tested by applying a linear
regression analysis to air quality and emission data from a number of
monitoring stations in each region. The relationship was tested for
each of five high-oxidant days in each of the three locations - Los
Angeles, San Francisco, and St. Louis. The basic result found in
all three locations was that there was little correlation between
NMHC/NOX ratios calculated from ambient monitoring data and the
corresponding NMHC/NGX ratios calculated from emission inventory
data. There was sc j improvement in correlation when gridded
emission ratios wer^- jsed instead of countywide emission ratios in
the San Francisco area, but the correlations in general were not
significant.
Since the EKMA method normally requires ambient monitoring
data from the main downtown urban center in a region, a brief analysis
was made comparing the numerical values of averaged ambient NMHC/NOX
ratios with emission NMHC/NOX ratios in the central downtown areas
of the three locations. Perhaps fortuitously, the countywide emission
NMHC/NOV ratio compared reasonably well with the averaged ambient
X
NMHC/NO ratio in each of the three downtown locations. This rela-
A
tionship should be further studied for a larger number of cities
to determine its usefulness.
1-2
-------
Due to a lack of correlation between the two methods of ratio
calculation, it was difficult to assess a definite reason for the
discrepancy between the two methods. Probably the most important
reason for the differences between ambient NMHC/NCL ratios and
A
emission ratios is the influence of meteorology on ambient pollu-
tant concentrations. Emissions and ambient pollutants from
different regions can be transported to the location of an air moni-
toring station. Another major reason for the discrepancy between
the two methods of ratio calculation is probably the lack of detail
and inaccuracy in most emission inventories, since day-to-day
variations in emissions are normally not represented and some
important sources may be omitted from inventories.
Because of the lack of correlation between NMHC/NOX ratios
calculated from ambient air monitoring data and NMHC/NOX ratios
calculated from emission inventory data, it is recommended that
only ambient monitoring data be used in the EKMA technique to
calculate the ratio of nonmethane hydrocarbons to oxides of nitrogen.
1-3
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2.0 LOS ANGELES DATA ANALYSIS
2.1 AMBIENT AIR MONITORING DATA
Ambient monitoring data for Los Angeles, California for non-
methane hydrocarbons (NMHC) and oxides of nitrogen (NOX) were
derived from the records of the Southern California Air Pollution
Control District, Metropolitan Zone. A search was made of data
from the year 1975 to determine the 5 days with the highest hourly
oxidant readings in the region, as recommended in the Empirical
Kinetic Modeling Approach (EKMA). When these five high oxidant
days were determined, hourly data for NMHC and NOX for the hours
of 6-9 a.m. (local daylight time) were examined at eight air moni-
toring stations in the region. All of the eight air monitoring
stations were in one large county, Los Angeles County. For each
station, the hourly values from 6-9 a.m. were averaged for NMHC
and NOX concentrations, and these average values were used to
calculate the NMHC/NOX ratio for each station. The ambient air
monitoring data for each of the 5 days in Los Angeles are shown
in the five tables in Appendix A.
An examination of the individual hourly concentration data
showed that the NOX data were fairly uniform and did not
exhibit extreme fluctuations from day to day; however, the NMHC
values varied widely from one day to the next. NMHC concentrations
are not measured directly, but are derived by subtracting measured
methane concentration values from measured total hydrocarbon con-
centration values; thus, NMHC values are normally less accurate
than measured NOX concentration values. Because of possible
inaccuracies in low NMHC values, it was decided to exclude data
from a station where any of the three hourly NMHC values was 0.3
ppmC or less. The remaining 3-hour averaged ambient NMHC/NOX
ratios are summarized in Table 2-1 for the eight stations in Los
Angeles. Site descriptions of the Los Angeles monitoring stations
are listed in Table 2-2. The 6-9 a.m. ambient air monitoring
NMHC/NOX ratios ranged from a low of 1.39 to a high of 13.30, in
units of ppmC/ppm.
2-1
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2.2 EMISSION INVENTORY DATA
A gridded emission inventory developed by Nordsieck (1974)
was used to calculate the corresponding emission NMHC/NOX ratios.
The emission inventory was representative of a typical summer day
in Los Angeles. Emissions were projected for the year 1975 from
the base year of 1972. The emission inventory was based on a
2 mile x 2 mile square grid system, and emissions were adjusted
to represent 6-9 a.m. (local daylight time) values by using appro-
priate diurnal patterns (CALTRANS 1975). The 6-9 a.m. emissions,
due to the traffic rush hour, represented a higher fraction of
mobile source emissions than the average daily emissions.
The position of each air monitoring station was located on
the grid system, and a weighted emission average was calculated
based on an inverse-square distance relationship among the four
closest grid squares to the air monitoring station location. This
technique was used so that the calculated emissions would represent
6-9 a.m. emissions in a 2 mile x 2 mile square centered on the air
monitoring location. Thus, if an air monitoring station were
located at the intersection point of four grid squares, emissions
from the four grid squares would be averaged; conversely, if a
monitoring station were located near the center of a grid square,
the emissions from that square would be weighted most heavily.
The calculated Los Angeles gridded emissions and the adjusted
NMHC/NO ratios (in units of ppmC/ppm) are shown in Table 2-2. The
A
calculated emission NMHC/NO ratios ranged from a low of 3.08 to
X
a high of 10.81. These emission ratios are representative of a
typical summer day in Los Angeles.
The Los Angeles NMHC/NO ratios from ambient monitoring data
X
and the corresponding ratios from the gridded emission inventory
are summarized in Table 2-1. Also shown in Table 2-1 is the average
countywide NMHC/NO emission ratio (adjusted to represent ppmC/ppm),
X
which is calculated from annual emission estimates for Los Angeles
County (Kinosian 1977). However, since all eight ambient monitoring
stations were in the same large county, only the gridded emission
NMHC/NO ratios were used in the following statistical analysis.
A
2-3
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2.3 RATIO COMPARISON
A linear least-squares regression analysis (Bevington 1969)
was carried out on the data in Table 2-1. Ambient NMHC/NOX ratios
for each day in Los Angeles were compared with the corresponding
gridded emission NMHC/NOX ratios. Only gridded emission data were
analyzed since all eight monitoring stations were in the same
county. The results are shown in Table 2-3.
There was no significant correlation for any of the 5 days
in Los Angeles between NMHC/NOX ratios calculated from ambient air
monitoring data and NMHC/NOX ratios calculated from the gridded
emission inventory. The linear correlation coefficients ranged
from -0.53 to 0.31, which are not significant for the number of
data points analyzed. For seven data points to be significant at
the 5 percent level, a correlation coefficient of 0.75 is necessary.
The best fit linear regression lines through the data points are
plotted for each day in Figure 2-1, with the emission ratio as the
independent variable.
Although the correlations were not significant, it is interesting
to examine the numerical values of the ratios for the downtown Los
Angeles station, since this station might be used for an EKMA analysis
over the entire Los Angeles area. For this station, the gridded
emission NMHC/NOX ratio was 3.71 and the countywide emission ratio
was 3.51, as shown in Table 2-1. The averaged ambient monitoring
NMHC/NOV ratio for four high oxidant days at the Central Los Angeles
A
station, as shown in Table 2-1, was 3.98, only 7 percent higher
than the gridded emission ratio and 13 percent higher than the county-
wide emission ratio.
2-5
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20
Figure 2-1. BEST-FIT REGRESSION LIMES FOR FIVE HIGH-
OXIDANT DAYS IN LOS ANGELES, COMPARING
GRIDDED EMISSION NMHC/NOX RATIOS WITH
AMBIENT RATIOS. THE DASHED LINE REPRE-
SENTS PERFECT THEORETICAL AGREEMENT.
2-7
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3.0 SAN FRANCISCO DATA ANALYSIS
3.1 AMBIENT AIR MONITORING DATA
Ambient air data for San Francisco, California were obtained
from the records of the San Francisco Bay Area Pollution Control
District. Although both total hydrocarbon concentrations and
methane concentrations are measured in the Bay Area, only total
hydrocarbon concentration data are reduced and reported on a
regular basis. Fortunately, the Association of Bay Area Govern-
ments (ABAG) had reduced NMHC data for a number of stations for
one month, October 1976. Thus, the ambient monitoring data used
in the analysis were based on the five highest oxidant days for
October 1976. Total hydrocarbon concentrations in October were
similar to those measured during the summer months of 1976. For
the five highest oxidant days in October, 6-9 a.m. (local standard
time) averaged concentrations were calculated for NMHC and NOX.
These values were used to calculate the ambient ratios of NMHC/NOX.
The ambient air monitoring data for each day in the San Francisco
area are shown in the five tables in Appendix B.
An examination of the hourly concentration data revealed that
the NOY data were much more uniform than the NMHC data, as was found
J\
in Los Angeles. Again, it was decided to eliminate data from a
station in which any hourly value of NMHC was 0.3 ppmC or less.
Also, it was decided to eliminate a very anomalous NMHC/NOX ratio
of 38.57 found at the Gilroy station on October 7, 1976. The
remaining 3-hour averaged ambient air NMHC/NOX ratios are summarized
in Table 3-1. Site descriptions of the San Francisco monitoring
stations are listed in Table 3-2. The 6-9 a.m. ambient air moni-
toring NMHC/NOV ratios for nine stations in the San Francisco area
A
ranged from a low of 1.31 to a high of 7.65, in units of ppmC/ppm.
3-1
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3.2 EMISSION INVENTORY DATA
A gridded emission inventory for the San Francisco area for the
year 1973 was provided by the Bay Area Air Pollution Control District
(Robinson 1977). Emissions were distributed on a 2 kilometer x 2
kilometer square grid system, and were representative of a typical
summer day in San Francisco. It should be noted that emissions for
the month of October may be different than for a typical summer day.
As was done in Los Angeles, the air monitoring stations were located
on the grid system, and a weighted emission average was calculated
using an inverse-square distance relationship among the four closest
grid squares to the air monitoring location. Thus, the resulting
emissions represented values for a 2 kilometer x 2 kilometer square
centered on the air monitoring station. Since hourly gridded emis-
sions were available, averaged emissions of NO and NMHC were
A
calculated for 6-9 a.m. (local standard time). The resulting gridded
emission data and the corresponding adjusted NMHC/NO ratios (in
/\
units of ppmC/ppm) are shown in Table 3-2. The gridded emission
ratios ranged from a low of 1.12 to a high of 12.05.
Average countywide emission inventory data were obtained from
the California Air Resources Board (Kinosian 1977). These emissions
data represented hydrocarbon emissions in three different reactivity
categories, as shown in Table 3-3. It was assumed in the analysis
that Classes II and III represented NMHC emissions; since some
compounds other than methane are included in Class I, this assump-
tion may result in a somewhat lower estimate of NMHC emissions.
The average countywide emissions for the San Francisco area and
the resulting adjusted NMHC/NOX ratios (in units of ppmC/ppm) are
shown in Table 3-4. The countywide emission ratios ranged from a
low of 2.79 to a high of 4.74. The gridded emission NMHC/NOX
ratios and the countywide emission ratios, with the ambient air
monitoring ratios, are summarized in Table 3-1 for the San Francisco
area.
3-3
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3.3 RATIO COMPARISON
A linear least-squares regression analysis (Bevington 1969)
was conducted on the San Francisco data, comparing the ambient air
monitoring NMHC/NOX ratios with the gridded emission ratios in
Table 3-1. A linear regression analysis was also conducted com-
paring the ambient air monitoring ratios with the countywide
emission ratios; the ambient air ratios were averaged if two or
more air monitoring stations were in the same county. The results
are shown in Table 3-5.
Again, the correlations between NMHC/NOV ratios calculated
A
from ambient air monitoring data and NMHC/NOX ratios calculated
from emission inventory data were mostly not significant. However,
the gridded emission inventory ratios provided somewhat better
correlation than the countywide emission inventory ratios. The
gridded emission correlation coefficients ranged from 0.35 to
0.89. As shown in Table 3-5, the ambient air NMHC/NOX ratios on
October 6, 1976 and the gridded emission ratios had a 0.74 correla-
tion coefficient, which for seven data points is significant at
the 5-percent level. Likewise, the ambient air ratios on October 9,
1976 and the gridded emission ratios had a correlation coefficient
of 0.89, which for four data points is significant at the 10-percent
level. The other 3 days did not have significant correlations
with the gridded emission ratios. The regression lines for the
gridded emission ratios are plotted for each day in Figure 3-1.
The comparison of the ambient air monitoring NMHC/NOX ratios
with the countywide emission ratios produced less significant
correlations than with the gridded emission ratios. The county-
wide emission correlation coefficients ranged from -0.51 to 0.16,
which are not significant for the number of data points analyzed.
Regression lines for the countywide emission ratios are plotted
in Figure 3-2.
3-7
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20
Figure 3-1. BEST-FIT REGRESSION LINES FOR FIVE HIGH-OXIDANT
DAYS IN SAN FRANCISCO, COMPARING GRIDDED
EMISSION NMHC/NOX RATIOS WITH AMBIENT RATIOS.
THE DASHED LINE REPRESENTS PERFECT THEORETICAL
AGREEMENT.
3-9
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COUNTYWIDE EMISSION NMHC /NO RATIO
A
Figure 3-2. BEST-FIT REGRESSION LINES FOR FIVE HIGH-OXIDANT
DAYS IN SAN FRANCISCO, COMPARING COUNTYWIDE
EMISSION NMHC/NOX RATIOS WITH AMBIENT RATIOS.
THE DASHED LINE REPRESENTS PERFECT THEORETICAL
AGREEMENT
3-10
-------
Although the correlations between ambient air and emission
NMHC/NOX ratios were not especially significant, it is interesting
to examine the numerical values of the ratios for the downtown San
Francisco station, since this station might be used for an EKMA
analysis over the entire San Francisco area. For this station,
the gridded emission NMHC/NOX ratio was 11.64 and the countywide
emission ratio was 4.08, as shown in Table 3-1. The averaged
ambient air monitoring NMHC/NOX ratio for three high-oxidant days
at the downtown San Francisco station, as shown in Table 3-1,
was 3.81, far lower than the gridded emission ratio but only 7
percent lower than the countywide emission ratio.
3-11
-------
4.0 ST. LOUIS DATA ANALYSIS
4.1 AMBIENT AIR MONITORING DATA
Ambient air monitoring data for St. Louis, Missouri were ob-
tained from the St. Louis Regional Air Pollution Study (Richter
1977). Ambient air data for 24 stations in the St. Louis area
were examined to find the five highest oxidant days in 1976.
For these five highest oxidant days, hourly concentration values
of NMHC and NOX were averaged over the time period 6-9 a.m. (local
daylight time). The averaged NMHC and NOX concentration values
were used to calculate the NMHC/NOX ratio for each station. The
ambient air monitoring data results for each day in the St. Louis
area are given in Appendix C.
An examination of the individual hourly averaged data points
at each station revealed that the NOX data were very uniform and
the NMHC data were more uniform than the data from Los Angeles
and San Francisco. Instead of rejecting NMHC values less than
0.3 ppmC, it was decided to eliminate all hourly averaged data
which showed a change greater than 100 percent from hour to hour;
17 NMHC concentration values were eliminated by this procedure.
Also, it was decided to eliminate a very anomalous NMHC/NOX ratio
of 48.5 reached at station 114 on September 24, 1976. The remain-
ing NMHC/NOX ratios were averaged if two or more stations were in
the same county. The resulting amoient air averaged ratios for
the St. Louis area are shown in Table 4-1. The 6-9 a.m. ambient
air monitoring NMHC/NOX ratios ranged from a low of 1.33 to a
high of 15.17, in units of ppmC/ppm.
4-1
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-------
4.' f^;r-ION INVENTORY DATA
iJu-.- .0 the unavailability of a detailed griddded emission
inventory for the St. Louis area, only annual averaged countywide
en;i; si on: were used in the data analysis. The countywide emissions
we-& obtained from the National Emission Data System (NEDS). The
eniss';cr,s from one county, St. Louis County, were broken down into
emissions from St. Louis City and emis_:ons from the remainder of
St. Louis County. The NEDS hydrocarbon emissions were given as
tote1 hydrocarbons; it was assumed that these emissions represented
mainly NMHC emissions. The emissions for each county and the
resulting adjusted NMHC/NOX ratios (in units of ppmC/ppm) for the
St. Lr.'is area are given in Table 4-2. The countywide emission
rcr, i.;.i ranged from a low of 1.64 to a high of 9.86. The county-
M.J6 evi ion NMHC/NOx ratios are compared with the ambient air
monitoring ratios in Table 4-1.
4-3
-------
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4.3 RATIO COMPARISON
A linear regression analysis (Bevington 1969) was conducted on
the St. Louis data in Table 4-1, comparing the county-averaged
ambient air monitoring NMHC/NOX ratios with the countywide emission
ratios. The ratios for St. Louis City and the remainder of St.
Louis County were treated as separate data points. The results of
the regression analysis for St. Louis are shown in Table 4-3.
Except for one day, there was no significant correlation
between the NMHC/NOX ratios calculated from ambient air monitoring
data and the NMHC/NOX ratios calculated from the countywide emission
inventory data. However, on October 12, 1976, the correlation
coefficient was 0.98,which is significant at the 2-percent level.
It is likely that this corrrelation was fortuitous, since only
four data points were involved in the analysis. The regression
lines for the countywide emission ratios are plotted in Figure 4-1.
Although the correlations between ambient air and emission
NMHC/NOV ratios were not especially significant, it is interesting
J\
to examine the numerical values of the ratios for St. Louis City,
since these ratios might be used for an EKMA analysis over the
entire St. Louis area. For St. Louis city, the countywide NMHC/NOX
emission ratio was 9.86, as shown in Table 4-1. The averaged
ambient air monitoring NMHC/NOX ratio for five high oxidant days
in the St. Louis city area, as shown in Table 4-1, was 7.68, or
22 percent lower than the countywide emission ratio.
4-5
-------
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-------
5.0 DISCUSSION OF RESULTS
5.1 CORRELATIONS VERSUS NUMERICAL VALUES
The relationship between ambient air monitoring NMHC/NO
/\
ratios and emission inventory NMHC/NO ratios was tested by
/\
applying a linear regression analysis to ambient air and emis-
sion data from a number of air monitoring stations in a region.
The relationship was tested for each of 5 high-oxidant days in
each of 3 locations. The basic result found in all 3 locations -
Los Angeles, San Francisco, and St. Louis - was that there was little
correlation between NMHC/NO ratios calculated from ambient air
/\
monitoring data and the corresponding NMHC/NO ratios calculated
/\
from emission inventory data. There was some improvement in
correlation when gridded emission ratios were used instead of
countywide emission ratios in the San Francisco area, but the
correlations in general were not significant.
However, since the EKMA method normally requires ambient
air monitoring data from the main downtown urban center in a
region, a brief analysis was made comparing the numerical values
of averaged ambient NMHC/NO ratios with emission ratios from
/\
the central downtown areas of the 3 locations. The results
are shown in Table 5-1. The gridded emission NMHC/NO ratio
A
compared well with the ambient ratio in Los Angeles but poorly
in San Francisco (a gridded emission inventory for St. Louis
was unavailable). The countywide emission NMHC/NO ratio com-
/\
pared reasonably well with the ambient ratio in each of the 3
locations.
It is statistically possible to have poor correlation yet
have numerical values be relatively close. However, it appears
somewhat fortuitous that the three countywide emission NMHC/NO
5-1
-------
Table 5-1. AVERAGED AMBIENT AIR NMHC/NOX RATIOS COMPARED WITH
EMISSION RATIOS IN THE CENTRAL DOWNTOWN AREA
(units in ppmC/ppm)
Location
Los Angeles
San Francisco
Averaged
Ambient Air
NMHC/NOV
A
3.98
3.81
St. Louis 7.68
Gridded
Emissions
NMHC/NO
j\
3.71
11.64
(no data)
Countywide
Emission
NMHC/NOV
A
3.51
4.08
9.86*
St.Louis City only. Entire St.Louis County ratio is 5.12.
5-2
-------
ratios matched up well with averaged ambient air ratios from
the central downtown areas. For one reason, Los Angeles
County has many sources over a very large area, 10,534 sq.km
(4,069 sq.mi), and it seems unlikely that one air monitoring
station could be representative of the entire county. Also,
the gridded emission ratio for the San Francisco station,
which should be more representative of the station during
stagnant conditions than the countywide ratio, was 3 times
greater than the ambient air ratio. Without additional work on
this relationship for a larger number of cities, it cannot be
recommended to use countywide emission NMHC/NO ratios in lieu
/\
of ambient air ratios in the EKMA method.
5-3
-------
5.2 REASONS FOR RATIO DIFFERENCES
If emissions from one area were completely responsible
for the ambient pollutant concentrations in that area, then
ambient air and emission NMHC/NO ratios should compare favor-
X
ably. However, little correlation was found in 3 different
locations between NMHC/NO ratios calculated from ambient air
A
monitoring data and the corresponding NMHC/NO ratios calcu-
A
lated from emission inventory data. There are a number of
possible reasons for this discrepancy, which are discussed
below in order of importance.
5.2.1 METEOROLOGICAL FACTORS
Probably the most important reason for the discrepancy
between ambient air NMHC/NO ratios and emission NMHC/NO
A X
ratios is the great influence of meteorology on ambient pollu-
tant -oncentrations. The relationship between emission rates
and ; mient air concentrations is quite complex, depending on
many reteorological parameters such as wind speed and direction,
atmospheric stability, temperature inversion heights, and hori-
zontal and vertical diffusion rates.
The day-to-day variations in wind can advect pollutants
from different regions to the location of an air monitoring
station. As an example of possible variations,
as shown in Table 4-1, ambient air monitoring ratios ranged from
1.3 to 14.3 during high oxidant days in one location. Even in
locations which are dominated by automobile emissions, differ-
ing wind conditions on a given day could advect emissions from
a major freeway to an air monitoring location, and these freeway
emissions would have different NMHC/NO ratios than traffic emis-
A
sions from surface streets. Since NMHC traffic emissions decrease
5-4
-------
with car speed while NOX emissions increase with speed, freeway
emission NMHC/NOX ratios may be lower than surface street ratios
by a factor of two.
Also important is the transport of background pollutants
from one location to another. Most air quality simulation models,
which try to relate emissions to ambient air concentrations, are
very sensitive to the effect of initial or background concentra-
tions. Even the very simple linear rollback model (deNevers and
Morris 1975), which assumes that pollutant concentrations are
directly proportional to emission rates, considers the effect of
background pollutants. Background concentrations above an inversion
layer can also be entrained into a mixing layer if the inversion
base rises.
5.2.2 ACCURACY OF EMISSION INVENTORY DATA
Another major reason for the discrepancy between ambient air
NMHC/NOX ratios and emission ratios is probably the lack of detail
in emission inventories. Certainly annual averaged countywide
emission estimates cannot be assumed to be representative of a
specific day with high oxidant levels. Even detailed gridded
emission inventories are normally representative only of a typical
summer day and do not reflect the actual day-to-day variations
which occur in a particular location.
Most emission inventories are also somewhat inaccurate or
omit certain types of sources. For example, natural sources of
hydrocarbons are normally emitted from most emission inventories.
Also, fugitive sources of hydrocarbons, e.g., from pumps and valves
in the oil and gas industries, may be omitted in inventories.
The NOX emission inventory, as well as the organic inventory,
may be inaccurate; for example, the percentage of cold starts in
a location greatly influences automobile NO emissions. However,
A
5-5
-------
even if emission inventories were perfectly accurate, ambient
and emission NMHC/NOY ratios would probably still not correspc
A
due to the influence of meteorological factors.
5.2.3 ELEVATED POINT SOURCES
Another minor reason why ambient air NMHC/NOX ratios do not
correspond to emission ratios is the fact that large elevated
point sources near an air monitoring site may not impact that
site significantly, since the site is at ground level while the
elevated emissions may be above an inversion level. For example,
in the San Francisco data analysis, the Pittsburg air monitoring
station was located near a large power plant with considerable
NOX emissions. Thus, the NMHC/NOX ratio calculated from the
gridded emission inventory data was fairly low, 0.39. However,
the ambient air monitoring NMHC/NOX ratios for Pittsburg were
4.00 and 3.53, indicating that the elevated NOX emissions were
not inpacting the air monitoring site significantly.
5.2.4 OTHER FACTORS
Another possible reason for the discrepancy between ambient
ratios and emission ratios is the inaccuracy of the ambient air
monitoring data. Nonmethane hydrocarbon concentrations are
normally less reliable than NO concentrations, since NMHC con-
A
centrations are derived by subtracting measured methane concen-
trations from measured total hydrocarbon concentrations. Also,
the flame ionization detector (FID) normally used for hydro-
carbon measurement has different sensitivity to different
types of hydrocarbons. Likewise, NO measurements may vary
/\
in accuracy depending on the measurement technique used. In the
data analysis, all hourly NMHC concentrations were examined for
uniformity, and any very low or obviously anomalous values were
eliminated from the data set. However, in general, the NMHC
concentration values were less consistent than the NO ambient
A
monitoring values.
5-6
-------
Another possible explanation for the lack of correlation
between ambient NMHC/NOX ratios and emission ratios is the role
of photochemical reactions in changing the ambient concentra-
tions. Both nonmethane hydrocarbons and oxides of nitrogen
react significantly in photochemical smog formation. However,
since the ambient air monitoring data are taken between the
hours of 6 and 9 a.m., when solar radiation is low, the effect
of photochemical reactions at this time on ambient concentrations
should be negligible.
Since the significance of the correlation coefficient is
dependent on the number of data points, the lack of correlation
cannot be attributed to the small number of data points (limited
by the number of monitoring stations) which were used in each
analysis. However, a very restricted range of variables will
normally lead to poor correlation. In most cases, this was
not important, since the NMHC/NOX ratios ranged over a factor
of ten. However, the countywide emission NMHC/NOX ratios in
the San Francisco area ranged only from 2.8 to 4.5, and thus
this comparison might be expected to have poor correlation.
5-7
-------
6.0 REFERENCES
Bevinqton, P.R. (1969) Data Reduction and Error Analysis for
the Physical Sciences. McGraw-Hill, New York.
CALTRANS (1975) Trips in Motion: Methodology and Factors for
Estimating Hourly Traffic Volumes for Average Daily Traffic.
California Department of Transportation, LARTS Branch, Report
TR/4.
Kinosian, J.R. (1977) Base-Year Emission Inventories for State
Implementation Plans. California Air Resources Board, Sacra-
mento, California.
deNevers, N. and Morris, J.R. (1975) "Rollback Modeling: Basic
and Modified", Journal of Air Pollution Control Association,
Vol. 25, pp. 943-947.
Nordsieck, R.A. (1974) Air Quality Impacts of Electric Cars in
Los Angeles, Appendix A: Pollutant Emissions Estimates and Pro-
jections for the South Coast Air Basin. General Research
Corporation, Santa Barbara, California, No. RM-1905-A.
Richter, H.G. (1977) Personal Communication: Preliminary RAPS
Data Base. U.S. Environmental Protection Agency, Office of
Air Quality Planning and Standards, Research Triangle Park,
North Carolina.
Robinson, L.H. (1977) Personal Communication: 1973 San Francisco
Emission Inventory. Bay Area Air Pollution Control District, San
Francisco, California.
U.S. Environmental Protection Agency (1977) Uses, Limitations and
Technical Basis of Procedures for Quantifying Relationships
Between Photochemical Oxidants and Precursors. Office of Air
Quality Planning and Standards, Research Triangle Park, North
Carolina, No. EPA-450/2-77-021a.
6-1
-------
APPENDIX A
LOS ANGELES AMBIENT AIR MONITORING DATA
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing}
1. REPORT NO.
EPA-450/3-78-026
2.
3. RECIPIENT'S ACCESSION NO.
4. TITLE ANDSUBTITLE
COMPARISON OF AMBIENT NMHC/NO RATIOS WITH NMHC/NO
RATIOS CALCULATED FROM EMISSlSN INVENTORIES x
5 REPORT DATE
June 1978
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Peter J. Drivas
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Pacific Environmental Servics, Inc.
1930 14th Street
Santa Monica, California 90404
10 PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO
68-02-2583
Assignment No. 4
12. SPONSORING AGENCY NAME AND ADDRESS
U.S. Environmental Protection Agency
Office of Air and Waste Management
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina 27711
13 TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16 ABSTRACT
The Empirical Kinetic Modeling Approach (EKMA) requires the
determination of the ratio of nonmethane hydrocarbons (NMHC) to oxides
nitrogen (NOX). Three geographic locations were used in analyzing the
differences between NMHC/NOX ratios calculated from emission inventory
of
data and those from ambient monitoring data. The three
Los Angeles, California; San Francisco, California; and
Missouri.
locations were:
St. Louis,
The relationship between ambient monitoring NMHC/NOX ratios and
emission inventory NMHC/NOX ratios was tested by applying a linear
regression analysis to air quality and emission data from a number of
monitoring stations in each region. The relationship was tested for each
of five high-oxidant days in each of the three locations. The basic
result found in all three locations was that there was little correlation
between NMHC/NOX ratios calculated from ambient monitoring data and the
corresponding NMHC/NO ratios calculated from emission inventory data
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b IDENTIFIERS/OPEN ENDED TERMS
COSATI } icId/Group
Air Pollution
Hydrocarbons
Nitrogen Oxides
Ozone
4B
7A
13B
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19 SECURITY CLASS (This Reportj
UNCLASSIFIED
21 NO. OF PAGES
58
20 SECURITY CLASS (This page/
UNCLASSIFIED
22 PRICE
EPA Form 2220-1 (R.v. 4-77)
PREVIOUS EDITION IS OBSOLETE
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