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Sources of Air Emissions

Air emissions are emitted from point sources (large stationary such as fossil fuel fired power plants, smelters, industrial boilers, petroleum refineries, and manufacturing facilities) and non-point (area, on-road mobile, non-road mobile, and biogenic) sources. The Texas Commission on Environmental Quality (TCEQ) maintains an estimate of these emissions.

Point Source Emissions

Stationary point source emissions data are collected annually from sites that meet the reporting requirements of Title 30 Texas Administrative Code (TAC) § 101.10. To collect the data, the TCEQ sends emissions inventory questionnaire courtesy notification letters to all sites identified as meeting the reporting requirements of 30 TAC § 101.10. Companies either download and complete the EIQs or submit emissions inventory data using a Web-based system. Companies are required to report emissions data and to provide sample calculations used to determine the emissions. Information characterizing the process equipment, the abatement units, and the emission points is also required to be reported. The point source emissions data submitted to the TCEQ is stored in the State of Texas Air Reporting System (STARS) database.

More information on the point source emissions inventory including state level emissions trends is available on the Point Source Emissions Inventory webpage.

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Non-Point Source Emissions

Area Source Emissions

Stationary source emissions data from sites and processes that do not meet the reporting requirements for point sources are classified as area sources. Area sources are small-scale industrial, commercial, and residential sources that generate emissions. Statewide area source emissions are calculated and recorded on the county level every three years. With some exceptions, area source emissions are calculated by multiplying an established emissions factor (emissions per unit of activity) by the appropriate activity or activity surrogate responsible for generating emissions. Population is one of the more commonly used activity surrogates for area sources. Other activity data include the amount of gasoline sold in an area, employment by industry type, and crude oil and natural gas production.

Area sources include but are not limited to:

  • Stationary source fuel combustion;
  • Solvent use (e.g. small surface coating operations);
  • Product storage and transport distribution (e.g. gasoline);
  • Light industrial/commercial sources;
  • Agriculture (e.g. feedlots, crop burning, tilling);
  • Waste management (e.g. landfills) and
  • Miscellaneous area sources (e.g. forest fires, wind erosion, unpaved roads)

A complete list of area source types can be found in the non-point emissions inventory database.

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On-Road Mobile Source Emissions

On-road mobile emission sources consist of automobiles, trucks, motorcycles, and other motor vehicles traveling on public roadways. On-road mobile source emissions are usually categorized as either combustion-related emissions or evaporative hydrocarbon emissions. Combustion-related emissions are estimated for vehicle engine exhaust. Evaporative hydrocarbon emissions are estimated for the fuel tank and other evaporative-leak sources on the vehicle. To estimate on-road mobile source emissions, emissions rates calculated by the Motor Vehicle Emissions Simulator (MOVES) model must be multiplied by the level of vehicle activity. On-road mobile source emissions factors are expressed in units of grams per mile; therefore, the activity information that is required to complete the inventory calculation is vehicle miles traveled (VMT) in units of miles per day. The level of vehicle travel activity is developed using a travel demand model (TDM) run by the Texas Department of Transportation or by the local metropolitan planning organization. The TDMs are validated against a large number of ground counts, i.e., traffic passing over counters placed in various locations throughout a county or area. For state implementation planning (SIP) and reporting inventories, VMT estimates are calibrated against outputs from the federal highway performance monitoring system, a model built from a different set of traffic counters. In addition to the number of miles traveled on each roadway link, the speed on each roadway type or segment is also needed to complete an on-road EI. Roadway speeds and other required inputs for the MOVES model are calculated by using the activity volumes from the TDM and a post-processor speed model. A statewide on-road mobile source emissions inventory is calculated and recorded on the county level every three years.

On-road mobile sources include the following vehicle classes:

  • Light-duty gasoline vehicles;
  • Light-duty gasoline trucks up to 6,000 pounds gross vehicle weight;
  • Light-duty gasoline trucks – 6,001 to 8,500 pounds gross vehicle weight;
  • Heavy-duty gasoline vehicles greater than 8,500 pounds gross vehicle weight;
  • Light-duty diesel vehicles;
  • Light-duty diesel-powered trucks;
  • Heavy-duty diesel vehicles greater than 8,500 pounds gross vehicle weight; and
  • Motorcycles.

A complete list of on-road mobile sources by road type can be found in the non-point emissions inventory database.

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Non-Road Mobile Source Emissions

A Texas-specific version of the EPA’s latest NONROAD 2008a model, called the Texas NONROAD (TexN) model, is used to calculate emissions for all non-road mobile source equipment and recreational vehicles, with the exception of airports, locomotives, CMVs, and drilling rigs. Airport emissions including aircraft engines, auxiliary power units, ground support equipment, and aircraft take offs and landings is calculated using approved models and survey analysis. Locomotive emissions are developed using survey data and activity data from rail yards. CMV emissions are developed using data from local ports and the United States Department of Transportation Maritime Administration. Drilling rig emissions are developed from activity data from the Railroad Commission of Texas. A statewide non-road mobile source emissions inventory is calculated and recorded on the county level every three years.

Non-road vehicles do not typically operate on roads or highways and are often referred to as off-road or off-highway vehicles. Non-road emissions sources include but are not limited to:

  • Agricultural equipment;
  • Construction and mining equipment;
  • Lawn and garden equipment;
  • Aircraft and airport equipment;
  • Locomotives;
  • Commercial marine vessels (CMV); and
  • Drilling rigs.


A complete list of non-road mobile source types can be found in the non-point emissions inventory database.

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Biogenic Source Emissions

Biogenic sources include VOC emissions from crops, lawn grass, and trees as well as a nitrogen oxides from soils. Plants are sources of VOC such as isoprene, monoterpene, and alpha-pinene. The TCEQ develops episode specific biogenic inventories for SIP development. Annual biogenic emissions estimates are available from the EPA Clearinghouse for Inventories and Emissions factors website.

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