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Regional Services

Emissions Inventory

• Emission Source Categories
• Austin-Round Rock MSA 2005 Emissions
• Austin-Round Rock MSA Emission Inventory Trends (1999 - 2015)

 

Emission Source Categories

An emission inventory is a list of estimated emissions of criteria pollutants in a given area for a specified amount of time.  Emission inventories are typically divided into the following source categories:

Mobile Sources

Onroad Mobile

Emissions from onroad mobile sources result from combustion and evaporation of fuels used in cars, trucks and buses traveling on public roadways. Emissions are estimated using a sophisticated model called MOBILE, which was developed by the Environmental Protection Agency (EPA). MOBILE calculates an emissions factor for mobile sources using a set of complex mathematical equations that require several user input values such us vehicle speed, ages and fuel type. After an emissions factor is generated for each vehicle classes, the factor is then used in conjunction with the vehicle miles traveled (VMT) estimates, which were developed with the area specific travel demand model in conjunction 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.

Nonroad Mobile

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. Most other non-road mobile equipment emissions are estimated using EPA's NONROAD computer model.

 Area (Nonpoint) Sources

Area sources do not individually produce sufficient emissions to qualify or to be reported as an individual point source, but collectively the emissions from all the small sources of the same type in an area may be significant and are reported as a category. 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 EPA emission factors to calculate emissions. Emissions are calculated and reported on a county-wide basis.

Major categories of non-point 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)

Point Sources

 

Point sources include industrial and non-industrial stationary equipment or processes that can be identified by name and location and are considered significant sources of air pollution emissions. Examples of point sources include industrial and commercial boilers, electric utility boilers, turbine engines, wood and pulp processers, paper mills, industrial surface coating facilities, refinery and chemical processing operations, and petroleum storage tanks. A major source emits hundred tons per year of at least one criteria pollutant, and must be inventoried and reported. Insignificant point sources are included by category in the area source inventory.

The most accurate method for determining the amount of emissions produced by a facility is with continuous emissions monitoring. However, this is expensive and not always practicable. Emissions are estimated using a variety of methods. Standard methods of estimating point source emissions are available from the EPA in the Compilation of Air Pollutant Emission Factors, Volume I, also known as AP-42. Other methods include stack testing, equipment vendor test data; material balances such as for surface coating, or TCEQ-approved permit factors. If no other method is available, then the best engineering judgment must be used.

Facilities report point source emissions to the TCEQ for the calendar year and the data are stored in the Point Source Database. Data are available for use by TCEQ staff, the EPA, state and federal legislators, air pollution researchers, public interest groups and the general public.  The 2007 Point Source Emissions Inventory is the most recent inventory and can be accessed through the TCEQ website.

Biogenic Emissions

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 TNRCC 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, eucalyptus, and poplars. Some VOCs are easily detected by their aroma. Pines, sycamores, and eucalyptus 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.

 

 

Austin-Round Rock MSA Emissions by Source

Austin-Round Rock MSA Emission Inventory Trends


A-RR MSA Total NOx and VOC Emissions (tons per day)

Source
Type
 19991 20021
 20051
 20062
 20073
 20153
NOx VOC NOx VOC NOx VOC NOx VOC NOx VOC NOx VOC
Nonroad Mobile 28.4 22.5 33.3 28.0 19.9 14.7 - - 22.6 20.8 14.4 17.1
Onroad Mobile 97.9 50.2 91.3 47.9 76.9 35.4 69.4 33.9 66.7 32.7 26.1 21.1
Area 7.8 90.6 10.7 109.2 10.9 88.6 - - 13.0 94.5 14.5 91.9
Point 32.6 2.4 20.1 2.4 19.8 2.2 20.9 2.2 18.5 2.1 18.8 1.9
TOTAL 166.7 165.7 155.3 187.4 127.5 140.9 - - 120.9 150.3 74.0 131.9

Notes
1 - 1999, 2002, 2005 Data Sources: TexAER, EPA NEI
2 - 2006 Data Source: Onroad Mobile (TTI Report "2005 and 2006 On-Road Mobile Source EIs for the Five-County Austin Near Nonattainment Area Revised Final",  August 2008), Point Source (TCEQ Point Source Database)
3 - 2007 and 2015 Data Sources: Nonroad mobile (NMIM), Nonpoint and Point (interpolated), Onroad Mobile (TTI Report "Austin EAC Region On-Road Mobile Source Emissions Inventories: 2007, 2015, and 2030",  February 2007)

 

 

 

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