Emissions Estimates
Emissions of Ozone Precursors in the Austin-Round Rock MSA
Since the Austin area is at risk for nonattainment for ozone standards, NOX and VOC emissions are especially important to the area.
Data Sources: TexAER, EPA NEI, Data provided by TCEQ Staff. Biogenic emissions estimates are for 1999, and are taken from the Austin Area's Early Action Compact SIP. Note that some of the changes in estimates reflect changes in methodology rather than actual increases or decreases in emissions. Mobile source estimates are derived from the MOBILE model rather than EPA's new MOVES model, which estimates NOX emissions from on-road sources to be roughly 50% higher than estimates using MOBILE.
Types of Emissions
Air pollution emissions come from a variety of sources, including highway vehicles, off-road equipment, power plants, industrial processes, and natural phenomena such as lightning and forest fires. Every three years, EPA compiles a National Emissions Inventory, and has just recently released the 2008 inventory. To learn more about the National Emissions Inventory, click here.
Criteria pollutants-those air pollutants that EPA has set National Ambient Air Quality Standards-can either be directly emitted or formed in the air through reactions or other processes.
Ozone is not directly emitted, but is rather the result of reactions of nitrogen oxides (NOX) and volatile organic compounds (VOC) in the presence of light.
Particulate matter can either be directly emitted or can be formed in the air as the result of other emissions, including NOX, sulfur dioxide (SO2), ammonia (NH3), metals, organic compounds, and dust.
SO2, NOX, carbon monoxide (CO), and lead (Pb) are all directly emitted from combustion or other industrial processes.
To learn more about criteria air pollution emissions, click here.
Hazardous Air Pollutants (HAPS) include a much longer list of compounds and metals such as mercury, hydrogen flouride, and others for which EPA sets technology-based standards. EPA compiles To learn more about toxic air pollution, click here.
Greenhouse gas emissions include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and flourinated gases. Flourinated gases can also cause stratospheric ozone depletion. To learn more about greenhouse gas emissions, click here.
What is an emissions inventory?
An emission inventory is a list of estimated emissions of criteria pollutants in a given area for a specified amount of time. These estimates are then used in conjunction with meteorological data and sophisticated computer software to perform air pollution modeling and planning.
There are five basic categories of emissions inventories:
- On-road sources: cars, trucks, buses, and motorcycles that are registered for highway and roadway use
- Non-road sources: airplanes, locomotives, boats, construction equipment, agricultural equipment, and other non-road equipment
- Point sources: large stationary facilities that emit enough pollution to be required to submit annual reports on their emissions
- Area sources: smaller stationary facilities that are not required to report their emissions
- Biogenic sources: emissions from natural processes such as vegetative emissions, lightning, and wildfires
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 MOVES, which was developed by the Environmental Protection Agency (EPA). MOVES replaces the previous MOBILE model. MOVES calculates an emissions factor for mobile sources using a set of complex mathematical equations that require several user input values such us activity hours, ages and fuel type. After an emissions factor is generated for each vehicle classes, the factor is then used in conjunction with an area's 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.
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 2009 Point Source Emissions Inventory is the most recent inventory and can be accessed through the TCEQ website.
Area 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 typically calculated and reported on a county-wide basis.
Major categories of non-point sources are:
- Stationary source fuel combustion such as industrial, commercial, and institutional boilers
- Solvent use (e.g., small surface coating operations)
- Product storage and transport distribution (e.g., gasoline)
- Agriculture (e.g., feedlots, crop burning)
- Waste management (e.g., landfills)
- Miscellaneous area sources (e.g., forest fires, wind erosion, unpaved roads)
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 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, 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.
Organic matter, particularly when it decomposes, can give off methane emissions. Methane can also bubble up from the ocean floor or come to the surface and into the atmosphere from natural reserves of gas or other fossil fuels.
Forest fires can cause NOX and PM emissions. Due to the very high temperatures it creates its immediate vicinity, lightning can also cause NOX formation.
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