6-1

Chapter 6

Microscale Study

6.1 Introduction

The microscale study utilized mobile platforms to sample for four-week periods at

selected locations, then moved to other sites for similar sampling. The objectives for this 

element were to determine if communities are experiencing localized hot spots not

otherwise determined by the fixed site MATES-II monitoring; to confirm hot spot areas

indicated by modeling; to respond to public concerns; and to assess the localized

representativeness of the monitoring. Two mobile platforms were employed to collect

ambient toxics measurements to meet the objectives. Sampling was conducted on a more 

intensive basis than the MATES-II element, but for only four weeks at a time.

A third mobile platform was used to sample as a microscale site, but limited to only two 

locations, sampling at each site during each seasonal quarter. This platform represented

more of a "hybrid" approach between the MATES-II fixed sites and the microscale sites.

It should be noted that the intent to investigate a number of different sites, with available 

resources, limited the power of the microscale study to detect localized disparities in air 

toxic levels. The microscale study should therefore be regarded as more of a “pilot 

study” than as a study to definitively address possible differences in community air 

pollutant exposures within the South Coast Air Basin. These factors should be taken into 

consideration to avoid possible over-interpretation of the results.

6.2 Site Selection Process

For the microscale sites, a multi-step process was used. First, to determine appropriate

locations for the mobile platforms, locations of known facilities which emit toxic air

contaminants were plotted on maps to determine "clusters" of facilities. Next, aerial

photos were used to determine locations where residential areas abutted, and were

immediately downwind of these clusters. Seasonal wind patterns were considered such

that predominant seasonal flows helped to determine the appropriate season for sampling. 

Most of the microscale sites were in Los Angeles County where the greatest levels of

toxic emissions occur, but at least two microscale locations were selected from each of

the other three counties. Of the 14 microscale sites, three (Montclair, Norwalk and

Rialto) were selected because of influence and proximity to major mobile sources (e.g.

congested freeways).

Two of the microscale sites were semi-fixed. These sites were in Pacoima and

Hawthorne and were used to sample for the species of interest for the MATES-II. These 

two sites were incorporated to study seasonal variability of the pollutants species

measured. Table 6-1 lists the addresses and sampling periods of the monitoring sites;

Figure 6-1 shows the locations of the sites on a map of the Basin. Appendix VI contains 

location maps for each of the sites.
6-2

In Table 6-2, the expected winds used for the siting designs, and the actual prevailing

winds as measured at the sites are shown. (Because of seasonal variability in prevailing

wind directions, neither Hawthorne nor Pacoima were selected based upon expected wind 

condition.) It can be seen that for the most part, actual wind conditions were close to the 

expected conditions. Notable exceptions are at Norwalk and Corona, where prevailing 

winds were almost one quadrant (i.e., 90 degrees) different than expected. Thus at these 

sites, presumed influences from the facility clusters cannot be assumed. (Note that

because the two “hybrid” sites, Hawthorne and Pacoima, were sampled over four 

seasonal months, there were no expected prevailing conditions; hence, these sites are not

included in the table.) Descriptions of wind conditions and accompanying wind “roses,” 

by site, are included in Appendix VI.

Table 6-1

 Microscale Monitoring Sites

Abbrev. Site Period of Record Address

Microscale Sites

AN Anaheim 12/31/98 – 02/02/99 1316 Paradise Ct., Anaheim 92806

BH Boyle Heights 09/26/98 – 10/29/99 1100 Spence St., Los Angeles 90023

CO Corona 10/22/98 – 11/25/98 1080 Pomona Rd., Corona 91720

CM Costa Mesa 08/15/98 – 09/08/98 2045 Meyer St., Costa Mesa 92627

MO Montclair 07/02/98 – 08/01/98 5450 Deodar St., Montclair 91763

NO Norwalk 11/13/98 – 12/16/98 12901 Hoxie Ave., Norwalk 90650

RI Rialto 03/19/99 – 04/30/99 200 W. Valley Blvd., Rialto 91720

RV Riverside 03/10/99 – 04/21/99 J. W. North HS 1150 - 3rd. St., Riverside 92507

SP San Pedro 03/25/99 – 04/27/99 202 S. Palos Verdes St., San Pedro 90731

EM South El Monte 05/09/99 – 06/11/99 2550 Edwards Ave., So. El Monte 91733

TO Torrance 07/21/98 – 09/05/98 631 Border Ave., Torrance 90503

VN Van Nuys 01/09/99 – 02/17/99 16101-1/2 Roscoe Blvd., Van Nuys 91406

Microscale Seasonal Sites

HA Hawthorne 05/09/98 – 06/02/98

07/13/98 – 08/13/98

10/07/98 – 11/11/98

01/06/99 – 02/23/99

5234 W. 120th. St., Hawthorne

PA Pacoima 06/27/98 – 09/14/98

11/18/98 – 12/13/98

01/15/99 – 02/23/99

11251 Glenoaks Blvd., Pacoima 91331
6-3
6-4

Table 6-2

Comparison of Expected Versus Actual

 Prevailing Winds at the Microscale Sites

Microscale Site Expected Wind Direction Actual Prevailing Wind

Boyle Heights WSW WSW

Torrance NW W

Van Nuys NNW N

Norwalk WNW N

Montclair W W

Costa Mesa SW WSW

Anaheim NE NNW & WSW

Corona N WNW

Riverside NW W

Rialto WSW WSW

San Pedro NW WNW

South El Monte WSW WNW

6.3 Analytical Approach

The microscale program employed mobile platforms at 14 communities for more

intensive sampling, but for a more limited period of sampling, than occurred at the 10

fixed sites. Typical sampling periods were four to five weeks, with two to three samples 

per week. This differed from the fixed site schedule of one sample every sixth day for a

full year. At the outset, the microscale samples were collected to optimize field

efficiency, irrespective of the fixed site schedule. It was noted by the ATSTRG that a

better approach would be to have as many samples coincide with the fixed site schedule

to gain a greater number of comparative samples. Thus, for the first six months of the

study, two to three samples collected at a microscale site were taken on the same days as 

fixed site sampling. For the latter six months, the number of coincidental samples

increased to five. (Because microscale samples were only analyzed by the AQMD

laboratory, and because some differences in analytical results occurred between AQMD

and ARB laboratories, for purposes of comparisons between microscale and fixed sites,

only those samples analyzed in the AQMD laboratory were used.) Laboratory analytical 

techniques followed the same procedures as described for the MATES-II fixed sites,

except that VOC samples were collected in 8-hour increments over the course of one day, 

from 0000-0800, 0801-1600, and 1600-2400. At the fixed sites, one 24-hour integrated

sample for VOCs was collected.

6.4 Monitoring Results and Findings

As shown previously in the discussion on the MATES-II sites, there are strong seasonal

variations in toxic concentration levels and associated carcinogenic risks. This presents a 
6-5

significant limitation in estimating annualized risk conditions with only a one-month 

period of sampling at the microscale sites. If there were no seasonal differences, then

limited sampling could be used to reasonably approximate an annual condition. With

strong seasonality, the ability to estimate annual conditions is predicated on statistical

analyses to determined confidences in such estimates. These confidences are improved

somewhat where coincident sampling dates occur. When sampling occurs on different

dates, changes in meteorology or emissions on a day sampled at a microscale site as

compared to a different day sampled at a fixed site, add another level of complexity

which cannot be fully accounted for. Given these uncertainties, certain types of limited

analyses can at least yield insights and dimensions which can add useful information to

that obtained from the fixed site network. The most useful information is obtained by

comparing a microscale site to its geographically closest fixed site during the period

when the microscale site was operating. That way, comparisons are made for the same

seasonal time frame without extrapolating to an annual condition. It also means that,

where seasonal variability is large, one microscale site cannot be compared to another

microscale site. It is the microscale-fixed site pairings that are most useful. 

6.4.1 Microscale-Fixed Site Comparisons

To evaluate the data collected for each microscale site and its paired fixed site, statistical 

analyses were conducted and summaries were compiled. These are presented in

Appendix VI, where complete descriptions for each of the microscale sites are presented, 

along with details about the data collected, the emissions inventories compiled, and the

modeling conducted. 

Table 6-3 summarizes the results of the paired analyses, which depict those comparisons 

in which the microscale sites are statistically greater than the paired fixed site. (Since the 

purpose of the microscale program is to focus on localized "hot spots," those situations

where the fixed sites are statistically greater than the microscale sites are not shown.) As 

can be seen in this table, there are relatively few cases where the microscale sites are

statistically greater than its corresponding fixed site. Most of these differences are with

regard to the carbonyls, including formaldehyde, acetaldehyde, acetone, and methylethyl-ketone (MEK). The latter two are not considered carcinogens. At 7 of the 13

paired sites, at least one of these compounds is statistically greater at the microscale sites. 

We believe this observation may be an artifact of the difference in sampling times. At

the fixed sites, one 24-hour sample was collected on each sampling day, while at the

microscale sites, three 8-hour samples were collected on each sampling day, with the

results averaged to represent 24 hours. Experts on sampling carbonyls have suggested

that reactions can take place over 24 hours which can reduce the level of some

compounds by the time the laboratory analyses are conducted. Thus there may be a bias 

in favor of lowered measurements at the fixed sites, resulting in the higher number of

significant differences for these compounds. Further, formaldehyde and acetaldehyde are 

more commonly associated with mobile sources, not stationary sources. With no

significant differences observed for either benzene or 1,3 butadiene (key mobile sourcerelated compounds) at any of the 13 paired sites, it is more likely that methodological

differences are accounting for these results.
6-6

Table 6-3

Summary of Cases: Microscale Site > Fixed Site with 90% Confidence Anaheim Boyle Heights Costa Mesa Corona Hawthorne Montclair Norwalk Pacoima Rialto Riverside San Pedro Torrance Van Nuys

Pollutant

Chloromethane 1

Chloroethane

1,3 Butadiene

1,1 Dichloroethane 1

Methylene Chloride x

1,1 Dichloroethane

Chloroform x

Ethylene Dichloride

Benzene

Carbon Tetrachloride

Trichloroethene x x

Toluene 1

Ethylene Dibromide

Perchloroethylene

Ethylbenzene 1

(m+p)-Xylene 1

x

Styrene 1 x x

o-Xylene 1

p-Dichlorobenzene

o-Dichlorobenzene 1

Formaldehyde * x x x x x

Acetaldehyde * x x x

Acetone * 1 x x x x x x x

MEK * 1

x

Hexavalent Chromium

Arsenic

Nickel

Selenium 1

Cadmium

* Measurement methodological differences may be causing the statistical differences shown for 

these compounds.

1 No risk factors established for these pollutants.

x Levels at Microscale site > levels at Fixed Site with 90% confidence.

 x Subset of x, but with substantially greater concentrations at the microscale site
6-7

There is one exception, and that is for formaldehyde at San Pedro. Relatively few

samples are available for the paired analyses because the sampling period extended

beyond March 1999 when the fixed site network completed one year of sampling. Levels 

observed at San Pedro not only are statistically higher than its paired site at Wilmington,

but the levels measured during this period were considerably higher than at any other 

fixed site in the network. Examination of the data from all samples for the carbonyls at

San Pedro are shown in Table 6-4. What is unusual about the formaldehyde data is that

there appears to be decreasing levels over time. If there was a local source, there would

likely be significant changes among the three time periods as winds changed from

daytime to nighttime flows. Since the within-day levels are reasonably constant, but the

changes from the beginning of the sampling period in March to the end of the period in

April are very substantial, an extremely close source is suspected. The San Pedro site

was located on a vacant lot near an apartment complex, so there are no known nearby

sources to account for the observed levels. Initially, it was suspected that modifications

made to the platform may have caused "outgassing" of formaldehyde. Building materials 

and certain adhesives found in carpeting outgass formaldehyde for a limited period of

time. However, modifications to the platform in response to meeting city permit criteria

involved only electrical modifications which would not be a source of formaldehyde. It

is suspected that contamination of the sampling equipment is the cause of this anomaly. 

Decreasing levels of formaldehyde over time coupled with insensitivity to wind

directional changes are not characteristics of local source influences. Also, the

relationship between formaldehyde and acetaldehyde, which was shown in the fixed site

network to be highly correlated, did not exhibit the same relationship at the San Pedro

site. Therefore, we believe these data are not representative of ambient conditions in the

Basin, but rather an artifact of sample contamination. 

Aside from the carbonyls, it can be seen from Table 6-3 that there are only seven cases of 

statistical significance for microscale compounds out of a total of 325 possible pairs (24

compounds times 13 paired sites.) For methylene chloride, one case was observed at the 

Anaheim-Anaheim pair. Because this compound does not show strong seasonal variation 

(as shown in Figure 3-5) it is reasonable to be able to compare to other fixed sites without 

introducing distortions that would otherwise occur for compounds which exhibit seasonal 

variations, such as benzene or 1,3 butadiene. Levels of methylene chloride measured at

the Anaheim microscale site were close to levels observed at Huntington Park, Compton, 

and Burbank. Furthermore, the levels observed at the Anaheim fixed site were the lowest 

of any of the fixed sites used in the paired analyses. Thus we can conclude that the

statistical difference observed was more a result of very low levels at the Anaheim fixed

site than as a result of having unusually high levels at the microscale site.

V 
6-8

Table 6-4

Carbonyl Results at San Pedro Site

Date 

Field

see below Formaldehyde Acetaldehyde Acetone MEK

Station Interval Cartridge (ppbv) MDL (ppbv) MDL (ppbv) MDL (ppbv) MDL

3/25/99 San Pedro 8A 826A 32.0 0.1 6.6 0.1 16.3 0.1 1.1 0.1

3/25/99 San Pedro 8B 827A 33.9 0.1 5.2 0.1 10.8 0.1 0.8 0.1

3/25/99 San Pedro 8C 828A 36.0 0.1 6.7 0.1 16.0 0.1 1.2 0.1

3/28/99 San Pedro 8A 845A 36.6 0.1 8.8 0.1 20.1 0.1 1.7 0.1

3/28/99 San Pedro 8B 846A 34.0 0.1 5.3 0.1 10.3 0.1 1.2 0.1

3/28/99 San Pedro 8C 847A 35.2 0.1 6.0 0.1 12.9 0.1 1.1 0.1

4/3/99 San Pedro 8A 862A 22.2 0.1 4.6 0.1 11.2 0.1 0.7 0.1

4/3/99 San Pedro 8B 863A 20.6 0.1 3.5 0.1 6.5 0.1 0.6 0.1

4/3/99 San Pedro 8C 864A 13.8 0.1 2.5 0.1 4.1 0.1 0.4 0.1

4/6/99 San Pedro 8A 857A 17.1 0.1 3.6 0.1 7.7 0.1 0.6 0.1

4/6/99 San Pedro 8B 858A 20.0 0.1 3.5 0.1 7.1 0.1 0.7 0.1

4/6/99 San Pedro 8C 859A 17.8 0.1 3.3 0.1 6.2 0.1 0.5 0.1

4/9/99 San Pedro 8A 855A 11.3 0.1 2.4 0.1 4.6 0.1 0.4 0.1

4/9/99 San Pedro 8B 884A 11.4 0.1 2.5 0.1 4.8 0.1 0.4 0.1

4/9/99 San Pedro 8C 885A 12.1 0.1 2.7 0.1 4.5 0.1 0.4 0.1

4/12/99 San Pedro 8A 873A 14.9 0.1 3.9 0.1 8.6 0.1 0.6 0.1

4/12/99 San Pedro 8B 874A 14.3 0.1 3.0 0.1 5.9 0.1 0.5 0.1

4/12/99 San Pedro 8C 875A 14.0 0.1 2.9 0.1 5.9 0.1 0.5 0.1

4/15/99 San Pedro 8A 887A 17.7 0.1 5.6 0.1 11.3 0.1 1.4 0.1

4/15/99 San Pedro 8B 888A 19.3 0.1 8.4 0.1 12.9 0.1 3.6 0.1

4/15/99 San Pedro 8C 889A 13.6 0.1 5.1 0.1 10.0 0.1 1.4 0.1

4/18/99 San Pedro 8A 890A 14.0 0.1 6.7 0.1 14.5 0.1 1.6 0.1

4/18/99 San Pedro 8B 891A 17.4 0.1 8.1 0.1 13.7 0.1 2.5 0.1

4/18/99 San Pedro 8C 892A 12.5 0.1 4.9 0.1 10.4 0.1 1.3 0.1

4/21/99 San Pedro 8A 893A NS 0.1 NS 0.1 NS 0.1 NS 0.1

4/21/99 San Pedro 8B 894A NS 0.1 NS 0.1 NS 0.1 NS 0.1

4/21/99 San Pedro 8C 895A 7.6 0.1 2.0 0.1 3.9 0.1 0.6 0.1

4/24/99 San Pedro 8A 896A 6.5 0.1 2.0 0.1 3.6 0.1 0.4 0.1

4/24/99 San Pedro 8B 897A 6.5 0.1 1.8 0.1 2.9 0.1 0.5 0.1

4/24/99 San Pedro 8C 908A 6.6 0.1 2.0 0.1 3.5 0.1 0.5 0.1

4/27/99 San Pedro 8A 910A 6.6 0.1 2.0 0.1 4.1 0.1 0.7 0.1

4/27/99 San Pedro 8B 911A 8.1 0.1 2.9 0.1 5.4 0.1 0.9 0.1

4/27/99 San Pedro 8C 912A 6.1 0.1 1.7 0.1 3.2 0.1 0.5 0.1

Interval = 8-hour, A=0000 to 0800; B=0800 to 1600 and C=1600 to 2400 hours
6-9

One statistical difference was observed for chloroform at the Montclair site. Chloroform

measurements here were just slightly above the minimum detection limit of the

instrument (0.05 ppb). The levels at Montclair were 0.06 ppb. The statistical inference is 

associated with the measurement detection limit, since the paired site at Fontana did not

have any samples above the detection limit, and hence the Fontana site had no statistical

variability. Levels at Montclair are consistent with levels measured at other fixed sites,

and hence there is no unusually elevated level of chloroform at the Montclair site.

Trichloroethene (TCE) was found to be significantly higher at two sites, Montclair and

Norwalk. Interestingly, these sites were selected because of their proximity to mobile

sources, yet TCE is not associated with mobile source emissions. Examination of the

data reveal a similar situation to that described above for chloroform: levels at these two

microscale sites were barely above detection limits (0.07 ppb and 0.06 ppb, respectively) 

while their corresponding fixed sites did not exceed detection limits. Comparing to other 

fixed sites, the levels observed at these microscale sites were lower than those observed

at other fixed sites. Hence, no unusually high levels of TCE are evident. Another

compound found significantly higher at Montclair, as compared to the Fontana fixed site, 

is (m+p)-xylene. This compound is not considered an air toxic, and is primarily

associated with mobile source emissions. Because mobile source emissions tend to have 

substantial seasonal variation, it is directly comparable to other fixed sites. The

proximity of the Montclair site to the I-10 Freeway is likely the factor accounting for the 

statistical significance.

Lastly, styrene is statistically higher at Anaheim and Corona. Styrene has not currently 

been assigned a carcinogenic risk factor, although it has been associated with certain

acute health effects. The levels at Anaheim are the highest (5.04 ppb) observed at any of 

the sites, either microscale or fixed. Additional analysis reveals the existence of three

localized sources of styrene in the vicinity of the monitoring site, outside the prescribed

wedge (see Section 6.5), contributing to elevated levels there. (See Figure 6-2) At

Corona, though statistically higher, the measured levels (1.39 ppb) are not unusual

compared to other fixed sites.

Discussions and general observations regarding each of the pairs are as follows:

1) Anaheim (micro) - Anaheim (fixed): Microscale site has localized influence from 

three sources of styrene emissions, which happened to be outside of the

prescribed wedge (as shown in Figure 6-2), and exhibits higher influence from

mobile sources than is observed at the fixed site.

2) Boyle Heights (micro) - Huntington Park (fixed): No significant differences

observed for any compounds except for acetone, not considered to be an air toxic. 

Huntington Park has greater mobile source influence.

3) Corona (micro) - Rubidoux (fixed): Corona may have higher levels of carbonyls

(formaldehyde), but in general, mobile source influences are about the same at

both sites. Higher levels of styrene observed at Corona, but not unusually high.
6-10

4) Costa Mesa (micro) - Anaheim (fixed): Generally similar, except that the Costa

Mesa site has very low levels of benzene and 1,3 butadiene, indicating only small 

contributions from mobile sources.

5) Hawthorne (micro) - Compton (fixed): Nine compounds significantly higher at

the fixed site (Compton) as compared to the microscale site. Both stationary and

mobile source influence greater at Compton.

6) Montclair (micro) - Fontana (fixed): Montclair has greater mobile source

influence as compared to Fontana, especially for carbonyls (formaldehyde). 

Chloroform and TCE are significantly higher than Fontana, but levels are just

barely above instrument detection limits, and the levels of these two compounds

are similar to observed at other fixed sites.

7) Norwalk (micro) - Compton (fixed): TCE and carbonyls higher at Norwalk. 

Although Norwalk was selected because of its proximity to mobile sources (at the 

convergence of the 105 and 605 freeways), there are no appreciable differences in 

observed levels of key mobile source compounds. 

8) Pacoima (micro) - Burbank (fixed): Very similar, except that Burbank has

significantly greater levels of perchloroethylene and formaldehyde.

9) Rialto (micro) - Fontana (fixed): Almost identical. No significant differences

observed for any compounds.

10) Riverside (micro) - Rubidoux (fixed): Very similar. No significant differences

observed for any compounds, although key mobile source compounds tend to be

higher at Rubidoux.

11) San Pedro (micro) - Wilmington (fixed): This pair had a very limited number of

comparative samples. Very high levels of formaldehyde were observed at San

Pedro. (See earlier discussion.)

12) Torrance (micro) - Compton (fixed): Compton shows greater influence from

mobile sources. Levels of mobile source emissions, mainly benzene and 1,3

butadiene, are especially low at Torrance.

13) Van Nuys (micro) - Burbank (fixed): Very similar. No significant differences

observed for any compounds, although Burbank shows greater mobile source

influence.

(Note: There is no comparison made for the South El Monte microscale site since that

sampling did not occur until after the fixed site network had completed its one full year of 

sampling, and therefore there were no comparative sites available. However, as shown in 
6-11

the next section, the relationship between mobile and stationary sources at South El

Monte is similar to other microscale sites.)

6.4.2 Mobile/Stationary Source Influences

Although the cancer risks at the microscale sites are not being estimated due to the

limited nature of the sampling, the risk factors can be used to determine the relative

importance, on a risk-weighted basis, of mobile versus stationary sources at each of the

microscale sites. Using benzene, 1,3 butadiene, and 50% of the carbonyls as indicators of 

mobile source influences, and all other compounds as indicators of stationary source

emissions, the relative risk-weighted importance of each source group is shown in Figure 

6-3 for all 14 microscale sites*. There are some important caveats with this analysis. 

Since mobile source compounds vary substantially by season, whereas the stationary

source compounds tend to be more seasonally invariant, those microscale sites sampled

during the fall and winter months should show greater mobile source influences than

would be expected on an annual basis. Conversely, those sites sampled during the spring 

and summer months would tend to show less mobile source influence than would be

expected annually. Figure 6-3 is therefore arranged such that the first group of sites are

those sampled in the fall/winter months; the second group in the spring/summer months;

and the third group representing all seasons at Hawthorne and Pacoima. 

In general, it can be seen that the mobile source influences are indeed greater at those

sites sampled in the fall/winter months as compared to those sampled in the

spring/summer months. In all but two cases (Torrance and Cost Mesa), all microscale

sites show greater mobile source influence than from stationary sources. (The evaluation 

for San Pedro is not considered part of this statement because of the concerns about the

local influence from the monitoring platform, as described previously.) Thus the efforts

to locate monitors in neighborhood areas expected to have substantial stationary source

influences from toxic emissions did not reveal such influences. (The two sites shown to

have dominant fractions from stationary sources were not so much from elevated levels

of compounds emitted from such sources, but rather from a noticeable lack of emissions

from mobile sources.)

*Elemental carbon was not measured at microscale sites; hence contributions from diesel 

particulates cannot be estimated for these sites.
6-12

Figure 6-2 

Styrene Emitters Close to Anaheim Site
6-13

Figure 6-3. Relative Influence of Mobile and Stationary Sources at Microscale 

Sites, On a Risk-Weighted Basis, Excluding Diesel Particulates

0%

20%

40%

60%

80%

100%

Anaheim

Boyle Heights

Corona

Norwalk

Van Nuys

Costa Mesa

Montclair

Rialto

Riverside

San Pedro

Torrance

El Monte

Hawthorne

Pacoima

Microscale Site

Source Apportionment of Risk

Stationary

Mobile
6-14

6.5 Emissions Inventory

The purpose of the microscale emissions inventory effort was three-fold: 1) to inform

the air monitoring staff about the possible presence of any unusual compounds so that

specialized instrumentation could be included in the air monitoring station; 2) to correct 

for inconsistencies after the regional modeling effort is completed; and 3) to conduct

microscale air dispersion modeling to determine the local impact of the emission sources.

For each of the fourteen microscale study sites, a wedge of influence was defined as a

function of predominant wind direction and frequency. The sources of toxic emissions

for non-AB2588 facilities within the wedge were determined utilizing numerous sources

of information including District data, personal drive-by, and interviews with the

owners/operators of the facilities. Quantification of emissions relied on estimates,

facility feedback, District permit data, product information, and Material Safety Data

Sheets. For facilities identified as AB2588 sources, District's AB2588 data base was

used to generate the most recent emissions inventories. 

Table 6-5 presents a summary of toxic emissions for all fourteen sites. Four of the sites

(Anaheim, Hawthorne, Norwalk and Riverside) had no identified sources of toxic

emissions within the prescribed wedge. Boyle Heights had the most number of sources

of emissions identified within the wedge. Appendix VI contains all the emissions

inventory information pertaining to the Microscale Study.
6-15

Table 6-5

 Summary of Microscale Emissions Inventories by Site

Emissions (lbs/year)

Pollutant

Boyle

Heights

Corona Costa

Mesa

Montclai

r

Pacoima Rialto San

Pedro

South

El Monte

Torrance Van

Nuys

1,3-Butadiene -- -- 0.70 -- -- -- -- 0.01 2.29 --

1,4-Dioxane -- -- -- -- -- -- -- -- 10.48 --

2- butoxyethanol -- -- -- -- -- -- -- 131.04 -- --

Acetaldehyde 0.27 -- -- -- -- -- -- -- -- --

Acetone 116.48 1.25 -- -- 61.15 -- -- 4630.08 2812.99 --

Acrylonitrile -- -- 0.00 -- -- -- -- -- 2.91 --

Aldehydes -- -- -- -- 0.05 -- -- -- -- --

Ammonia -- -- -- -- 124.80 -- -- 2.75 -- --

Benzene 0.36 5.85 0.25 8.74 1.84 0.19 0.43 0.01 0.01 5.85

Benzo[a]pyrene -- 0.00 -- -- 0.00 0.01 0.01 -- -- --

Cadmium -- -- -- -- -- -- -- -- 0.02 --

Carbon black 0.96 -- -- -- 0.06 -- -- -- 0.01 --

Chromic acid -- -- -- -- -- -- -- 1.41 -- --

Chromium, 

hexavalent

0.92 -- -- -- -- -- -- -- 0.01 --

Cobalt -- -- -- -- -- -- -- 0.01 -- --

Copper -- -- -- -- 0.01 -- -- -- 0.04 --

Cyclohexane -- -- -- -- -- -- -- 778.44 -- --

Dibutyl phthalate -- -- -- -- 22.71 -- -- -- -- --

Diesel, particulate 87.36 -- -- -- -- 0.52 -- -- -- --

EGBE -- -- -- -- -- -- -- 148.51 -- --

Ethyl benzene -- -- -- -- -- -- -- 122.30 -- --

Formaldehyde 1.19 0.06 0.00 -- 0.00 0.08 0.04 -- 691.44 --

Furan -- 0.38 -- -- 0.09 0.76 1.12 -- -- --

Glycol ethers -- 15.60 -- -- 10.48 -- -- -- 76.88 --

Hexamethylene-1,6 -- -- 0.05 -- -- -- -- -- -- --

Hexane -- -- -- -- 18.35 -- -- -- 2795.52 --

Hydrofluoric acid -- -- -- -- -- -- -- 0.05 -- --

Isopropanol 95.68 -- -- -- 200.93 -- -- 410.59 480.48 --

Lead -- -- -- -- 0.44 -- -- -- -- --

Manganese 0.78 -- -- -- -- 0.64 -- 0.01 0.05 --

Methanol -- -- -- -- 79.50 -- -- -- 1135.68 --

Methyl ethyl ketone -- -- 194.69 -- 253.34 -- -- -- -- --

Methylene chloride 95.68 24.96 -- -- -- -- 13.00 -- -- --

Naphthalene 1.86 -- -- -- -- -- -- -- -- --

Nickel 1.13 -- -- -- -- -- -- -- 0.00 --

Nickel acetate -- -- -- -- -- -- -- 0.79 -- --

Nitric acid -- -- -- -- -- -- -- 0.79 -- --

PAH 2.04 0.09 -- -- 0.02 0.17 -- -- -- --

Perchloroethylene 1787.50 -- -- -- 234.00 -- -- 0.73 -- --

Phosphoric acid -- -- -- -- 0.42 -- -- 0.00 -- --

POM 0.09 -- 0.00 -- -- -- -- -- 0.00 --

Propylene glycol -- -- -- -- 60.19 -- -- -- -- --

Propylene oxide -- -- -- -- 22.71 -- -- -- 0.06 --

Silica 0.64 -- 0.00 -- -- 0.52 -- -- 0.14 --

Silicon 0.05 -- -- -- -- 0.04 -- -- -- --

Silver -- -- -- -- -- -- -- -- 0.00 --
6-16

Table 6-5 Cont'd.

Summary of Microscale Emissions Inventories by Site

Emissions (lbs/year)

Pollutant

Boyle

Heights

Corona Costa

Mesa

Montclai

r

Pacoima Rialto San

Pedro

South

El Monte

Torrance Van

Nuys

Sodium hydroxide -- -- -- -- -- -- -- 18.30 -- --

Styrene -- -- -- -- -- -- -- 0.00 0.16 --

Sulfates -- -- -- -- -- -- -- 11.65 -- --

Sulfuric acid -- -- -- -- 2.50 -- -- 41.60 -- --

Toluene 372.69 0.62 -- 1.92 410.99 0.43 0.05 227.14 59.40 175.40

Trichloroethane1,1,2-- -- -- -- -- -- -- -- 366.91 --

Trichloroethylene -- -- -- -- -- -- -- 7.25 -- --

Vinyl acetate -- -- -- -- -- -- -- 0.00 -- --

Xylenes 132.11 1.31 85.28 0.96 139.98 0.21 0.04 1257.98 -- 1.31
6-17

6.6 Local-Scale Modeling

For purposes of local-scale modeling, the Industrial Source Complex (i.e., ISC3) air

quality model is used for the subgrid level modeling presented here. The ISC3 model is

included in the U.S. EPA “Guideline on Air Quality Models” (U.S. EPA, 1996a) and has 

been widely used for regulatory air quality assessment. The model is also recommended 

by the California Air Pollution Control Officers Association (CAPCOA) for estimating

exposure to toxic air contaminants (CAPCOA, 1993). The model is a steady-state 

Gaussian plume model, which can be used to assess pollutant concentration from a wide

variety of sources, associated with an industrial source complex. The model simulates

the dispersion of emissions from point, area, and volume sources and can account for

building downwash, dry deposition, and decay of chemicals. The short-term version of

the model (ISCST3) accepts hourly meteorological data records to define the conditions

of plume rise, transport, and diffusion. The model estimates hourly concentrations for

each source and receptor combination and calculates averages for various user-selected 

short-term periods and for annual or longer averaging periods. The model is appropriate 

for transport distances less than 50 kilometers. The short-term version of the model is

applied using hourly meteorological data at the microscale monitoring sites in the Basin. 

Important model options employed include: urban dispersion parameters (i.e., URBAN)

and no calm wind processing (i.e., NOCALM). The URBAN option assumes

atmospheric dispersion rates typical of an urban environment. The NOCALM option

simulates dispersion even under calm wind conditions by assuming a minimum wind

speed of 1 m/s. This is important because winds are often calm or near calm in southern 

California. All other model options assumed the default values.

Where detailed stack information was not available, for the surveyed sources, facility

emissions are simulated using a ground-based volume source treatment in ISCST3. The

volume source dimensions for all the sources are assumed to be 15m by 15m horizontal

dimensions and 6m vertical dimension. The operating hours per day are unique for each 

facility but all facilities are assumed to operate 365 days per year.

A cartesian coordinate receptor grid is used to estimate peak concentrations in the local

area within and surrounding the facilities modeled. The receptor spacing assumed is 25m 

with all receptors placed at ground level. The horizontal extent of the modeling domain

is such that all the sources are within its boundaries. Each facility is assumed to have a

25m property line from the center of the volume source. In other words, impacts from 

the facility are estimated at receptors greater than 25m from the center of the source. Flat 

terrain is assumed, since emissions are treated as a non-buoyant volume source.

ISCST3 is applied with two distinctly different meteorological data sets: one based on

the 1981 calendar year and another for the period from April 1998 to March 1999. The

District has 1981 meteorological data (i.e., hourly winds, temperature, atmospheric

stability, and mixing heights) at 35 sites in the Basin and vicinity. These data are

available at the District’s web site (www.aqmd.gov/metdata) and are in a format that can 

be directly read by ISCST3. These data are typically used by permit applicants to satisfy

the modeling requirements of Regulations XIII, XIV, XVII, and XX. 
6-18

Hourly three-dimensional fields of wind and temperature and hourly two-dimensional 

fields of mixing depth and atmospheric stability were developed for the period from 

April 1998 to March 1999 for the UAM simulations discussed in Chapter 5. The

meteorological model called CALMET was used to develop these fields. These hourly

meteorological data are extracted from each grid cell containing the microscale site and

reformatted for ISCST3 input. Therefore annual simulations are performed using both

the 1981 meteorological data and the 1998/99 meteorological data. Results of both

simulations are provided in Table 6-6 and in Appendix VI. Appendix VI contains the

complete modeling results and relevant background information for each microscale site

including location, emissions, wind data, comparison to fixed sites, and model results for 

concentration as well as cancer risk.

From Table 6-6, it can be seen that none of the predictions for the monitoring sites

exceeded a risk level of 10 in a million. (The highest risk from local influences at a

monitoring site is only about 5 in a million at Boyle Heights.) This means that regional

conditions overwhelm local influences. The model also shows that significant influences 

from local sources may occur (e.g., estimated 588 in a million at Boyle Heights), but that 

rapid decreases in concentrations occur over relatively short distances. In the case of

Boyle Heights, the predicted maximum near the source was over 100 times higher than

the level at the monitor, less than one mile away. At Torrance, the predicted maxima 

were approximately 10 times greater than those predicted for the monitor. 

Table 6-6

Summary of ISCST3 Predicted Cancer Risks* at the Microscale Sites

Predicted Risks Using

1981 Meteorology

Predicted Risks Using

1998 Meteorology Microscale 

Site At Monitor Maximum At Monitor Maximum

Pollutants contributing

90% of cancer risk

Anaheim 0.0 0.0 0.0 0.0 No carcinogens emitted

Boyle Heights 4.8 464.0 4.5 588.0 Hexavalent chromium

perchloroethylene, diesel PM

Corona 0.5 1.3 0.6 1.7 PAH, benzene

Costa Mesa 0.0 1.1 0.0 1.3 1,3 butadiene

El Monte 0.0 1.9 0.0 2.4 nickel acetate,

trichloroethylene

Hawthorne 0.0 0.0 0.0 0.0 No carcinogens emitted

Montclair 0.0 3.2 0.0 2.6 benzene

Norwalk 0.0 0.0 0.0 0.0 No carcinogens emitted

Pacoima 0.1 14.0 0.1 21.0 benzene, PAH,

perchloroethylene

Rialto 0.2 2.3 0.1 1.9 PAH, benzo(a)pyrene,

diesel PM

Riverside 0.0 0.0 0.0 0.0 No carcinogens emitted

San Pedro 0.0 3.6 0.0 2.4 benzene, PAH,

Torrance 2.6 36.6 3.7 43.9 formaldehyde, trichloroethana

Van Nuys 0.0 1.9 0.0 2.4 benzene

*Per Million