Sources of Air Emissions
- Emissions from Point Sources
- Emissions from Non-Point 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 ).
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 Transportation.
The HPMS Program produces VMT estimates for various roadway types categorized into two major population areas—urban and rural.
Other freeways and expressways
Other principal arterial
Other principal arterial
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.
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)
Lawn and garden
Airport support vehicles
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:
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:
Light industrial and commercial sources:
Agriculture and miscellaneous area sources:
Stationary source fuel combustion:
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.