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Sources of Air Emissions

Information and links to data on nonpoint, point, non-road mobile, on-road mobile, and biogenic sources.

Point Source Emissions

The Point Source Emissions Inventory is an annual survey of chemical plants, refineries, electric utility plants and other industrial sites that meet the reporting criteria in the TCEQ emissions inventory rule (30 TAC §101.10 Exit the TCEQ).

Point Source Emissions Data

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Emissions from Non-Point Sources

On-Road Mobile Sources

Emissions from on-road mobile sources are estimated using a sophisticated model called MOBILE, which was developed by the EPA. MOBILE calculates an emissions factor for mobile sources using a set of complex mathematical equations that require several user input values.

Vehicles are segregrated into eight vehicle classes, with MOBILE generating an emissions factor for each class and a composite emissions factor representing all classes.

On-Road Mobile Source Vehicle Classes

  • Light-duty gasoline vehicles (LDGV)
  • Light-duty gasoline trucks up to 6,000 pounds gross vehicle weight (LDGT1)
  • Light-duty gasoline trucks from 6,001 to 8,500 pounds gross vehicle weight (LDGT2)
  • Heavy-duty gasoline vehicles with more than 8,500 pounds gross vehicle weight (HDGV)
  • Light-duty diesel vehicles (LDDV)
  • Heavy-duty diesel vehicles with more than 8,500 pounds gross vehicle weight (HDDV)
  • Motorcycles (MC)
  • Light-duty diesel-powered trucks (LDDI)

After an emissions factor is generated for each vehicle class, the factor is then used in conjunction with the vehicle miles traveled (VMT) estimates, which were developed with the Texas Highway Performance Monitoring System (HPMS) data set for that selected area. This combination determines the contribution of emissions from mobile sources in a city, county, or state. VMT data is maintained by the Texas Department of TransportationExit the TCEQ

The HPMS Program produces VMT estimates for various roadway types categorized into two major population areas—urban and rural.

Roadway Types
Other freeways and expressways
Other principal arterial
Minor arterial
Other principal arterial
Minor arterial
Major collector
Minor collector

In Texas, emissions from mobile sources are estimated on a county-wide basis. With a few exceptions nationwide, on-road mobile sources constitute the largest single source category of air pollution.


Texas Air Emissions Repository (TexAER)


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Non-Road Mobile Sources

Nonroad mobile sources include a wide variety of internal combustion engines not associated with highway vehicles. Emissions calculation methodology is as varied as the categories themselves. A federal computer model using engine types and landing-takeoff cycles is used to calculate most aircraft emissions. Actual fuel usage and track mileage are applied to determine locomotive emissions. Data on ship and barge traffic is used to calculate emissions from ocean vessels.

Nonroad Mobile Source Categories


Vessels (ships and barges)
Small Engines:
Lawn and garden
Airport support vehicles
Recreational marine
Light commercial
Industrial construction
Agricultural logging

The EPA developed emissions factors for a lengthy list of small engine categories, which included research on individual engines and fuel types. Included in this category are two major emissions sources—lawn mowers and recreational boat engines.

Nonroad Mobile Small Engine Categories

Lawn and Garden:
Trimmers/edgers/brush cutters
Lawn mowers
Leaf blowers/vacuums
Rear engine riding mowers
Front mowers
Chainsaws, 4 horsepower (HP)
Shredders, 5 HP
Tillers, 5 HP
Lawn and garden tractors
Wood splitters
Chippers/stump grinders
Commercial turf equipment
Other lawn and garden equipment

Airport Support:
Aircraft support equipment
Terminal tractors

Recreational Vehicles:
All terrain vehicles
Off-road motorcycles
Golf carts
Specialty vehicle carts

Recreational Marine:
Vessels with inboard engines
Vessels with outboard engines
Vessels with stern drive engines
Sailboat auxiliary inboard engines
Sailboat auxiliary outboard engines

Light Commercial:
Generator sets, 50 HP
Pumps, 50 HP
Air compressors, 50 HP
Gas compressors, 50 HP
Welders, 50 HP
Pressure washers, 50 HP

Aerial Lifts
Other general industrial equipment
Other material handling equipment

Asphalt pavers
Plate compactors
Concrete pavers
Paving equipment
Surfacing equipment
Signal boards
Bore/drill rigs
Concrete/industrial Saws
Cement and mortar mixers
Off-highway trucks
Crushing/process equipment
Rough terrain forklifts
Rubber tired loaders
Rubber tired dozers
Crawler tractors
Skid steer loaders
Off-highway tractors
Other construction equipment

2-wheel tractors
Agricultural tractors
Agricultural mowers
Tillers, 5 HP
Hydro power units
Other agricultural equipment

Chainsaws, 4 HP
Shredders, 5 HP




Texas Air Emissions Repository (TexAER)


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Area Sources

Area source inventories generally report emissions by categories rather than by individual source; a common method in reporting point source emissions. Area source emissions are calculated by various methods and depend on the type of data available for each category. For example, whenever fuel use and materials data are not available, employee and county population numbers are used with established emission factors to calculate emissions. Emissions are calculated and reported on a county-wide basis.

Major categories of area sources are:

  • Stationary source fuel combustion such as residential 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)
  • Waste management (e.g., landfills)
  • Miscellaneous area sources (e.g., forest fires, wind erosion, unpaved roads)

The following table gives a more complete list of area sources by category.

Product storage and transport distribution:
Oil and gas production
Oil and gas production—offshore
Aircraft refueling
Marine vessel loading losses
Leaking underground tanks
Catastrophic/accidental releases

Service stations:
Vehicle refueling
Tank truck unloading
Tank trucks in transit
Tank breathing losses
Other (spillage)

Solvent use:
Barge, tank, tank truck, rail car, drum cleaning
Consumer/commercial solvent use
Surface cleaning (degreasing)
Dry cleaning
Graphic arts
Cutback asphalt
Emulsified asphalt

Waste management:
Municipal waste landfills
Municipal wastewater treatment (POTW)
Industrial wastewater treatment
Wastewater package plants

Light industrial and commercial sources:
Commercial bakeries

Agriculture and miscellaneous area sources:
Pesticide application
Orchard heaters
Agricultural burning
Prescribed burning
Structure fires
Forest fires
Slash burning
Open burning

Painting operations:
Architectural coatings
Auto refinishing
Traffic markings
Furniture fixtures
Metal containers
Automobiles (new)
Machinery and equipment
Other transportation equipment
Sheet, strip, and coil
Factory finished wood
Electrical insulation
Other product coatings
High-performance maintenance
Marine coatings
Other special purpose coatings

Stationary source fuel combustion:
Fuel oil—residential
Fuel oil—commercial/distillate
Fuel oil- commercial/residual
Fuel oil—industrial/residual
Natural gas—residential
Natural gas—commercial
Natural gas—industrial





Texas Air Emissions Repository (TexAER)


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Biogenic Sources

Biogenic emissions account for 30 percent of all the volatile organic compounds (VOCs) emitted in urban areas in the eastern half of Texas. For the purposes of photochemical modeling, biogenic VOC emissions are estimated using a computer model that takes into account the species of trees present, the density of their foliage, the temperature and solar radiation on the day in question, and the distribution of vegetation throughout the modeling domain. It is important to measure these parameters accurately if the biogenics inventory is to be correct. The TCEQ has hired specialists to measure some of these variables in north-central and southeastern Texas.

Most plants emit some VOCs, but the largest emitters are oaks, pines, sweet gums, eucalypti, and poplars. Some VOCs are easily detected by their aroma. Pines, sycamores, and eucalypti emit fragrant monoterpenes, while other VOCs—such as isoprene—are not as aromatic.

Isoprene is a byproduct of photosynthesis. Scientists still debate the purpose of its emission but some evidence suggests that plants can cope better with heat if isoprene is present. Because it is generated by photosynthesis, isoprene emissions are not generated at night.

Monoterpenes are known as “essential oils.” There is solid evidence that plants make monoterpenes, which are found in small reservoirs in the leaves or needles of plants, to ward off herbivores. When an insect feeds on the leaf, the monoterpenes are released and can adversely affect the insect’s health. Because the monoterpenes are always present in the leaves, their emission rate depends mostly on the temperature. Higher temperatures will evaporate larger amounts into the atmosphere.

There are a few other important organic compounds emitted by plants. Alcohols are often emitted by damaged vegetation; there is some evidence that these alcohols act as an antiseptic. A few recent studies suggest that alkenes are also emitted by some plants.


Texas Air Emissions Repository (TexAER)


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