CHAPTER 3 

DEVELOPMENT OF THE TOXICS EMISSIONS INVENTORY 
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Chapter 3. Development of the Toxics Emissions Inventory 

3.1 Introduction 

An emissions inventory of air pollutants and their sources is essential in order to identify the 

major contributors of air contaminants and the measures required to reduce air pollution. 

Information necessary to produce an emissions inventory for the Basin is obtained from the 

SCAQMD and other government agencies including: California Air Resources Board (ARB); 

California Department of Transportation (Caltrans); and Southern California Association of 

Governments (SCAG). 

Each of these agencies is responsible for collecting data (e.g., industry growth factors, socioeconomic projections, travel activity levels, emission factors, emission speciation profiles, etc.) 

and developing methodologies (e.g., model and demographic forecast improvements) required to 

generate a comprehensive emissions inventory. SCAQMD is solely responsible for developing 

the point source inventory, and the area source inventory is developed jointly by SCAQMD and 

ARB. ARB is the primary agency responsible for developing the emissions inventory for all 

mobile sources. SCAG is the primary agency for projecting population and activity growth in 

the Basin. ARB provides on-road and off-road inventories from their EMFAC and OFF-ROAD 

Models, respectively. Caltrans provides SCAG with highway network, traffic counts, and road 

capacity data. SCAG incorporates these data into their Travel Demand Model for 

estimating/projecting vehicle miles traveled (VMT) and speed. ARB’s on-road inventory also 

relies on SCAG’s VMT estimates. 

3.2 Overview 

The toxic emissions inventory for MATES III consists of four components: (1) point sources; (2) 

area sources; (3) on-road mobile sources; and (4) off-road (or other) mobile sources. Point 

source emissions are from facilities having one or more pieces of equipment registered and 

permitted with the SCAQMD and with emissions above certain threshold levels. Area sources 

represent numerous small sources of emissions that can collectively have significant emissions 

(e.g., dry cleaners, retail gasoline stations, auto body shops, residential heating, etc.). On-road 

mobile sources include cars, trucks, buses, and motorcycles. All mobile sources not included in 

the on-road mobile source inventory are considered as “off-road” mobile sources, which include 

aircraft, ships, commercial boats, trains, recreational vehicles, construction equipment, etc. 

The 2007 Air Quality Management Plan (AQMP)[1] is the basis for the toxics emissions 

inventory developed for MATES III. The 2005 inventory used for the MATES III modeling 

analysis is projected from the 2002 baseline emissions inventory in the 2007 AQMP. All the 

future year inventories in the 2007 AQMP are also projected from the 2002 baseline emissions 

inventory. A “top-down” approach is used to develop the toxics inventory, that is, toxic 
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emissions are calculated by applying the latest ARB speciation profiles[2] to the hydrocarbon and 

particulate matter emissions. Speciation profiles provide estimates of the emission’s chemical 

composition. The ARB maintains and updates the chemical composition and size fractions of 

particulate matter (PM) and the chemical composition and reactive fractions of total organic 

gases (TOG) for a variety of emission source categories. The source type (e.g., equipment and 

fuel) is used to identify the appropriate speciation profile. 

A top-down approach is preferable for a regional modeling risk analysis, such as MATES III, for 

the following reasons: 

• Speciating the VOC and PM inventory affords consistency with the 2007 AQMP. 

• The photochemistry algorithms in the MATES III modeling system require the complete 

speciation of the VOC emissions to ensure their correct application. An inventory of just 

reported toxic species would be insufficient. 

• Lastly, the computer programs used to grow and control the VOC and PM emissions into 

the future for the 2007 AQMP can also be used for projecting the toxic emissions in 

MATES III. Thus, future air toxics risk reductions resulting from the 2007 AQMP can 

be estimated. 

3.3 Point Sources 

The 2002 point source emissions inventory is based on the emissions data reported by the point 

source facilities in the 2002/2003 Annual Emissions Reporting (AER) Program and is the basis 

for the 2005 inventory used for MATES III modeling analysis. This program applies to facilities 

emitting four tons or more of VOC, NOx, SOx, or PM or emitting more than 100 tons of CO per 

year. Facilities subject to the AER Program calculate and report their emissions primarily based 

on their throughput data (e.g., fuel usage, material usage), appropriate emission factors or source 

tests, and control efficiency (if applicable). Under the 2002/2003 AER Program, approximately 

3,200 facilities reported their annual emissions to the SCAQMD. Emissions from smaller 

industrial facilities not subject to the AER Program, which represent a small fraction of the 

overall stationary source inventory, are included as part of the area source inventory (see Section 

3.3). 

In order to prepare the point source inventory, emissions data for each facility are categorized 

based on U.S. EPA’s Source Classification Codes (SCCs) for each emission source category. 

Since the AER collects emissions data on an aggregate basis (i.e., equipment and processes with 

the same emissions factor are grouped and reported together), facility’s equipment permit data 

are used in conjunction with the reported data to assign the appropriate SCCs and develop the 

inventory at the SCC level. For modeling purposes, facility location is specified in Universal 

Transverse Mercator (UTM) coordinates. The business operation activity profile is also recorded 

so that the annual emissions can be distributed temporally throughout the day, week, and year. 
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Toxic emissions are calculated by applying the latest ARB speciation profiles[2] to the 

hydrocarbon and particulate matter emissions. The SCC is used to identify the appropriate 

speciation profile for the source. 

3.4 Area Sources 

The area source emissions developed for the 2007 AQMP and projected to the year of interest 

(i.e., 2005) are used for MATES III. SCAQMD and ARB shared the responsibility for 

developing the 2002 area source emissions inventory for approximately 350 area source 

categories. Specifically, SCAQMD developed the area source inventory for about 93 categories 

and ARB developed the remaining area source categories (of which 239 categories are associated 

with consumer products, architectural coatings, and degreasing). For each area source category, 

a specific methodology is used for estimating emissions. Emissions are spatially allocated to 2 

km by 2 km grids using spatial surrogates. Some commonly used spatial surrogates are listed in 

Table 3-1. As with the point source inventory, toxic emissions are calculated by applying the 

latest ARB speciation profiles to the hydrocarbon and particulate matter emissions. 

3.5 On-Road Mobile Sources 

On-road emissions are the product of emission factors and vehicular activity. The emissions 

developed for the 2007 AQMP and projected to calendar year 2005 are used for MATES III. For 

the 2007 AQMP, ARB’s EMFAC2007 (v2.3) emission factors[3] were used and link-based traffic 

volumes and speeds were obtained from the SCAG regional transportation modeling. The Direct 

Travel Impact Model (DTIM) was used to link emission factors and transportation modeling 

results and generate hourly gridded emissions of criteria pollutants (i.e., TOG, NOx, PM, CO, 

and SOx). Toxic emissions are calculated by applying the latest ARB speciation profiles for 

mobile sources to the hydrocarbon and particulate matter emissions. A flow chart illustrating 

this process is provided in Figure 3-1. Some of the key steps in the process are discussed in 

more detail below. 

EMFAC stands for EMission FACtor. It is a FORTRAN computer model that estimates the onroad emissions of hydrocarbons (HC), CO, NOx, PM, lead (Pb), SO2, and CO2 for calendar years 

1970 to 2040. EMFAC considers 1965 and newer vehicles powered by gasoline, diesel, or 

electricity and reports for 13 broad vehicle classes as shown in Table 3-2. Over 100 different 

technology groups are accounted for within each class (e.g., catalyst, non-catalyst, three-way 

catalyst, carbureted, multi-port fuel injection, LEV, TLEV, SULEV, etc.). 

EMFAC currently considers the following county-specific information when calculating 

emissions: 

• Ambient air temperature (denoted by T in Figure 3-1); 

• Relative humidity (denoted by RH in Figure 3-1); 

• Vehicle population; 
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• Fleet composition; 

• Fleet growth rates; 

• Mileage accrual rates; 

• Vehicle age distribution; 

• Distribution of VMT by speed; 

• Smog check regulations; 

• Fuel properties; and 

• Altitude. 

Some on-road activity information for the four counties in the Basin is summarized in Table 3-3. 

Four of the top seven counties in California in terms of vehicle population, VMT, and trips are in 

the Basin. 

The output from EMFAC is a text file containing HC, CO, NOx, PM, lead, SO2, and CO2

emission rates for 45 model years for each vehicle class within each calendar year, for 24 hourly 

periods, and for each month of the year for each county/air basin specified. Processing continues 

with the DTIM modeling system, which prepares gridded hourly on-road emissions for 

photochemical grid modeling. 

The DTIM processing system consists of three FORTRAN Program modules: CONVIRS4, 

IRS4, and DTIM4. The main function of CONVIRS4 is to re-format the emission rate file 

output from EMFAC into a form compatible with IRS4. IRS4 creates fleet average emission 

rates by ambient air temperature, relative humidity, and vehicle speed. 

The DTIM4 module prepares gridded, hourly on-road emissions of HC, CO, NOX, PM, lead, 

SO2, and CO2 link by link in the transportation network. SCAG’s Travel Demand Model 

provides the following for each link in the transportation network: the number of vehicles, their 

average speed, and time on the link. Separate files containing hourly gridded temperature (T in 

Figure 3-1) and relative humidity (RH in Figure 3-1) are provided as input to DTIM4. Knowing 

the air temperature and relative humidity representative of the link and the average vehicle speed 

on the link, DTIM4 looks up the fleet average emission rate in the file prepared by IRS4, and 

multiplies these by the number of vehicles and the average time on the link. 

ARB speciation profiles are used to speciate the on-road HC and PM emissions into its toxic 

components. 
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3.6 Off-Road Mobile Sources 

The off-road emissions developed for the 2007 AQMP are used for MATES III. For the 2007 

AQMP, ARB’s OFF-ROAD model[4] was used to estimate emissions for all off-road categories 

(100+ source categories) except commercial ships, aircraft, locomotive, and recreational 

vehicles. This model incorporates various aspects of off-road elements, such as the effects of 

various adopted regulations, technology types, and seasonal conditions on emissions. The model 

combines population, activity, horsepower, load factors, and emission factors to yield the annual 

equipment emissions by county, air basin, or state. Spatial and temporal features are 

incorporated to estimate seasonal emissions. Emissions are spatially allocated to 2 km by 2 km 

grids using spatial surrogates. Toxic emissions are calculated by applying the latest ARB 

speciation profiles for off-road mobile sources to the hydrocarbon and particulate matter 

emissions. 

The diesel particulate emissions from the major source category of “Ships and Commercial 

Boats” differ between the 2007 AQMP and MATES III. “Ships and Commercial Boats” consists 

of 16 Category of Emission Source (CES) codes. For the 2007 AQMP, 13 out of 16 CESs were 

assumed to fall into the category of diesel internal combustion. It has since been determined and 

assumed for MATES III that all 16 CESs involve diesel internal combustion. The fuels 

identified in the three ship emissions categories (82172 -military and coast guard ships, 82214-

crew and supply boats, offshore oil production, and 47654 -tugboats) were previously 

categorized as unspecified liquids and consequently are not included as diesel. Further inventory 

reconciliation determined that the ships in these categories were actually burning diesel fuels. 

This resulted in an additional 0.42 ton/day of diesel particulate attributed to “Ships and 

Commercial Boats” in the MATES III 2005 inventory. 

3.7 Summary of Toxic Emissions 

Table 3-4 presents the emissions from selected compounds apportioned by the on-road, off-road, 

point, and area source categories. Chemicals that are considered potential or known human 

carcinogens are denoted with a check mark. Toxic emissions by major source categories are 

provided in Appendix VIII. 

Species and source apportionment are shown in Table 3-5 and Figure 3-2, respectively. In those 

illustrations, the emissions of the carcinogenic pollutants in Table 3-4 are weighted by the ratio 

of their cancer potency to the cancer potency of diesel particulate matter (DPM). (Cancer 

potencies developed by the State of California Office of Environmental Health Hazard 

Assessment were used). Thus, emissions from species less potent than DPM (e.g, benzene, 

perchloroethylene, etc.) are weighted less, while emissions from species more potent than DPM 

(e.g., hexavalent chromium, arsenic, etc.) are weighed more. DPM has a weighting factor of 

one. 

As shown in Table 3-5, DPM accounts for over 85% of the overall potency weighted emissions. 

The other significant compounds (i.e., contributions >1%) are 1,3-butadiene, benzene, 
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perchloroethylene, and hexavalent chromium. On-road and off-road mobile sources contribute 

nearly 93% of the potency weighted air toxics emissions and stationary (i.e., point and area) 

sources contribute about 7% of the potency weighted risk (Figure 3-2). 

Carcinogenic potency weighted emissions have decreased by 11% since MATES II (1998 

inventory year) as shown in Figure 3-3. Emissions from on-road, point, and area source 

categories decreased by 12%, 66%, and 42%, respectively and off-road emissions are essentially 

unchanged (an increase of 1%). To perform the comparison, the 2002 inventory used in the 2007 

AQMP was back-cast to 1998 in order to account for changes to the EMFAC and OFF-ROAD 

models that have occurred since MATES II. These emissions for air toxics are tabulated in 

Appendix VIII. 

3.8 Selected Emissions and Air Quality Changes Since MATES II 

Table 3-6 compares emission and air quality changes since MATES II for selected toxics. The 

air quality change is compared to the MATES III year 1 average from the ten fixed monitoring 

sites. Emissions have decreased and air quality has improved since the MATES II (i.e., 1998 

timeframe). It should be noted that the difference for some pollutants, such as cadmium and 

arsenic, may also be due to lower method reporting limits in MATES III, as discussed in Chapter 

2. 

Several caveats are appropriate when comparing the changes in inventory emissions and ambient 

measurements. For example, weather and dispersion of emissions are not considered, which can 

influence the relationship between emissions and ambient concentrations. Also, the inventory is 

a regional estimate of total emissions throughout the Basin, whereas ambient measurements are 

from the ten fixed monitoring locations where there may be influences from local sources. 

Another difference is that secondary formation or degradation of substances is not accounted for. 

Nonetheless, comparing emissions estimates with air quality measurements can provide 

information on whether expected emissions changes are reflected in ambient measurements, can 

be used to help calibrate emissions, and may suggest where emissions inventory methods can be 

improved. 

3.9 References 

1. A copy of the 2007 AQMP can be viewed or downloaded at the following SCAQMD link: 

http://www.SCAQMD.gov/aqmp/07aqmp/index.html. 

2. ARB speciation profiles can be viewed or downloaded from the following ARB link: 

http://www.arb.ca.gov/ei/speciate/speciate.htm. 

3. EMFAC2007 model and its documentation can be obtained at the following ARB link: 

http://www.arb.ca.gov/msei/onroad/latest_version.htm. 

4. The OFF-ROAD Model and its documentation can be obtained at the following ARB link: 

http://www.arb.ca.gov/msei/offroad/offroad.htm. 
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Table 3-1. Commonly Used Spatial Surrogates. 

Population Total employment 

VMT Industrial employment 

Length of rail per grid cell Retail employment 

Locations of unpaved rural roads Single dwelling units 

Total housing Rural land cover – range land 

Agricultural land cover Rural land cover – forest 

National forest > 5000 ft 

Source: http://eos.arb.ca.gov/eos/projects/surrogates/ 

Table 3-2. Broad Vehicle Classes Considered by EMFAC. 

Vehicle Class Weight (lbs) Vehicle Class Weight (lbs) 

Passenger cars All Heavy-Heavy-Duty Truck 33,001 – 60,000 

Light Truck I 0 – 3,750 Motorcycle All 

Light Truck II 3,751 – 5,750 Urban Diesel Bus All 

Medium-Duty Truck 5,751 – 8,500 School Bus All 

Light-Heavy-Duty Truck I 8,501 – 10,000 Other bus All 

Light-Heavy-Duty Truck II 10,001 – 14,000 Motor Homes All 

Medium-Heavy-Duty Truck 14,001 – 33,000 

Source: Adopted from the User’s Guide for EMFAC2007. 

Table 3-3. Vehicle Activity Information for the Counties in the Basin. 

County 

Vehicle 

Population VMT/day Trips/day 

Miles per 

Vehicle-Day 

Mean Age of 

Passenger Car 

Fleet (years) 

Los Angeles 5,811,255 197,059,000 39,896,020 33.91 9.36 

Orange 2,071,490 65,359,000 14,058,600 31.55 10.46 

Riverside 1,014,703 42,170,000 6,890,676 41.56 11.91 

San Bernardino 1,064,007 39,152,000 7,298,540 36.80 9.99 

Source: http://www.arb.ca.gov/msei/onroad/briefs/activity.pdf 
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Table 3-4. 2005 Annual Average Day Toxic Emissions for the South Coast Air Basin.

Pollutant Emissions (lbs/day) 

On-road Off-road Point Area Total 

 Acetaldehyde* 4857.0 8622.4 125.8 505.1 14110.3 

 Acetone** 4020.5 7189.1 552.4 28904.9 40666.9 

 Benzene 13244.8 7808.3 906.5 609.3 22568.9 

 1,3 Butadiene 2723.1 1755.6 537.1 108.7 5124.5 

 Carbon tetrachloride 0.0 0.0 11.2 0.0 11.2 

 Chloroform 0.0 0.0 206.9 0.0 206.9 

 1,1 Dichloroethane 0.0 0.0 0.5 0.0 0.5 

 1,4 Dioxane 0.0 0.0 0.8 0.7 1.5 

 Ethylene dibromide 0.0 0.0 2.2 0.0 2.2 

 Ethylene dichloride 0.0 0.0 67.2 0.0 67.2 

 Ethylene oxide 0.0 0.0 16.1 52.6 68.7 

 Formaldehyde* 12596.6 19889.0 1488.8 1302.0 35276.4 

 Methyl ethyl ketone* 745.6 1366.0 1244.3 6466.7 9822.6 

 Methylene chloride 0.0 0.0 325.1 13548.3 13873.4 

 MTBE 0.0 4.4 89.6 0.0 93.9 

 Naphthalene 573.4 376.8 16.6 568.1 1534.9 

 p-Dichlorobenzene 0.0 0.0 115.4 5553.9 5669.3 

 Perchloroethylene 0.0 0.0 940.4 9685.3 10625.7 

 Propylene oxide 0.0 0.0 2.2 0.1 2.3 

 Styrene 681.7 326.3 1332.5 76.5 2417.0 

 Toluene 37707.9 15369.2 8724.3 21029.4 82830.8 

 Trichloroethylene 0.0 0.0 587.1 633.0 1220.1 

 Vinyl chloride 0.0 0.0 51.1 0.0 51.1 

 

 Arsenic 0.2 3.9 13.4 24.8 42.3 

 Cadmium 1.5 2.1 3.2 7.2 14.0 

 Chromium 21.1 9.2 49.2 77.3 156.8 

 Diesel particulate 22164.5 37406.2 489.5 618.3 60678.5 

 Elemental carbon*** 10498.2 9337.4 4850.4 14197.3 38883.3 

 Hexavalent chromium 1.1 0.6 0.6 0.5 2.8 

 Lead 2.4 4.8 13.7 180.9 201.8 

 Nickel 15.3 5.8 44.2 23.4 88.7 

 Organic carbon 19972.7 18073.3 371.0 69230.1 107647.1 

 Selenium 0.5 0.5 41.4 2.2 44.6 

 Silicon** 838.7 136.5 1211.9 218527.2 220714.3 

 Denotes potential or known human carcinogen. 

* Primarily emitted emissions. These materials are also formed in the atmosphere as a result of photochemical reactions. 

** Acetone and silicon are not toxic compounds. Their emissions are included here because they were measured in the 

sampling program and were subsequently modeled for the purpose of model evaluation. 

*** Includes elemental carbon from all sources (including diesel particulate). 
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Table 3-5. Cancer Potency Weighted Species Apportionment for 2005 Emissions.

Toxic Contribution 

(%) Toxic Contribution (%) 

Diesel particulate 85.64 Ethylene oxide 0.03 

1,3-butadiene 3.94 Vinyl chloride 0.02 

Benzene 2.90 Trichloroethylene 0.01 

Perchloroethylene 2.86 Lead 0.01 

Hexavalent chromium 1.83 Ethylene dichloride < 0.01

Formaldehyde 0.95 Chloroform < 0.01 

Arsenic 0.65 Carbon tetrachloride < 0.01 

p-dichlorobenzene 0.29 Ethylene dibromide < 0.001 

Cadmium 0.27 MTBE < 0.001 

Naphthalene 0.24 1,4-dioxane < 0.0001 

Acetaldehyde 0.18 Propylene oxide < 0.0001 

Nickel 0.10 1,1-dichloroethane < 0.00001 

Methylene chloride 0.06 

Table 3-6. Selected Emissions and Air Quality Changes Since MATES II. 

Toxic Gases Change in 

Emissions 

Change in 

Air Quality Toxic Particulates Change inEmissions 

Change in 

Air Quality 

Acetaldehyde -9% -8% Arsenic** -20% -54% 

Benzene -36% -47% Cadmium** -20% -74% 

1,3-butadiene -31% -67% Elemental carbon* -3% -28%*

Formaldehyde -21% -9% Hexavalent chromium -53% -5% 

Methylene chloride -38% -43% Lead -14% -47% 

Perchloroethylene -58% -77% Nickel -22% -31% 

Trichloroethylene -65% -79% 

Notes: 

* Adjusted for instrumentation changes in MATES III; see Section 2.6.3. 

** Difference in air quality may be in part due to lower laboratory reporting limits in MATES III. 

Emissions: 2005 compared to 1998. 

Air Quality: MATES III year 1 compared to MATES II annual averages from 10 fixed sites. 
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EMFAC2007

DTIM4

Gridded hourly

emissions of

HC, CO, NOx, PM,

Pb, SO2, & CO2

ARB TOG & PM

Specification

Profiles

Gridded hourly

toxic emissions

IRS4

CONVIRS4

Meteorological

Model

Gridded fields

of

T & RH

SCAG Travel

Demand Model

Link data:

• Vehicle count

• Speed

• Time on link

Figure 3-1. Flow Diagram for On-Road Emissions Processing. 

On-road

36.3%

Off-road

56.4%

Point

2.3%

Area

5.0%

Figure 3-2. Cancer Potency Weighted Source Apportionment for 2005 Emissions. 
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Figure 3-3. Cancer Potency Weighted Emission Comparison of MATES II and 

MATES III. 

0

20,000

40,000

60,000

80,000

MATES-II (1998) MATES-III (2005)

Emissions as DPM (lbs/day)

On-road Off-road Point Area

Area 42% decrease

Point 66% decrease

Off-road 1% increase 

On-road 12% decrease

Percent 

Change 

Source 

Category