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Emissions Inventories

Learn about emissions in the CAPCOG region and emissions inventory research projects completed by CAPCOG's Air Quality Program.

What is an emissions inventory?

An emissions inventory is an accounting of the mass of emissions for a specific source or source type over a period of type within a specific geographic area. Often, emissions inventories are expressed in tons per year or tons per day at the county level or facility level.

Emissions inventories are used for a variety of air quality planning purposes, including identifying source categories to target for control strategies, identifying locations for locating ambient air quality monitors, tracking trends over time, and modeling ambient air pollution levels.

What pollutants are included in an emissions inventory?

Different pollutants are included in an emissions inventory depending on the purpose of the inventory. Emissions inventories are generally developed for three classes of emissions, based on how they are regulated:

  1. "criteria" air pollutants,
  2. hazardous air pollutants (HAPs), and
  3. greenhouse gases (GHG).

Criteria Pollutants

Criteria pollutants are air pollutants that EPA has set National Ambient Air Quality Standards (NAAQS) for, and which can either be directly emitted or formed in the air through reactions or other processes. Emissions inventories for criteria pollutants are developed for pollutants that contribute to the presence of the criteria pollutants in the atmosphere - either directly or indirectly. Criteria air pollutants include:

  • ozone (O3),
  • particulate matter (PM),
  • nitrogen dioxide (NO2),
  • sulfur dioxide (SO2),
  • carbon monoxide (CO), and
  • lead (Pb).

O3 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. Therefore, these two pollutants are included in emissions inventories for O3. Because CO also plays a minor role in photochemistry, O3 emissions inventories also include CO.

PM is divided into two types of particles - those with diameters 10 microns or less (PM10) and those with diameters of 2.5 microns or less (PM2.5). PM can either be directly emitted or can be formed in the air as the result of other emissions, including NOX, SO2, VOC, ammonia (NH3), metals, and dust.

SO2, NOX (which includes both NO and NO2), CO, and Pb are all directly emitted from combustion or other industrial processes.

> Discover more about the six "criteria" air pollutants.

Hazardous Air Pollutants

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.

> Learn more about HAPS.

Greenhouse Gases

As a result of a 2007 Supreme Court ruling in Massachussets v. EPA and subsequent regulatory actions taken by EPA, GHG are now regulated under the Clean Air Act. GHGs include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and flourinated gases. Flourinated gases can also cause stratospheric ozone depletion.

> Learn more about GHG.

What is the most recent emissions inventory for CAPCOG counties?

Every three years, states must submit a complete inventory of emissions that contribute to "criteria" air pollution (ozone, particulate matter, nitrogen dioxide, sulfur dioxide, carbon monoxide, and lead). These "national emissions inventories" (NEI) include county-level emissions estimates of all emissions sources, and are expressed in tons per year. The most recent NEI available is for 2011. The total emissions from the NEI for each county in the CAPCOG region, including from biogenic and natural sources, is shown in the table below.

2011 NEI Criteria Pollutant Emissions by County (tons per year)
County NOX PM10 PM2.5 VOC CO NH3 SO2 Pb
Bastrop* 5,594.12 32,436.92 11,675.72 36,944.88 117,037.31 2,476.23 1,138.44 0.04
Blanco 1,288.52 5,035.72 767.78 9,713.83 7,122.48 284.41 31.02 0.00
Burnet 2,581.87 12,555.38 1,960.42 15,915.44 17,805.96 592.30 82.96 0.12
Caldwell 3,200.76 9,389.95 2,460.15 16,790.29 25,310.02 928.40 490.37 0.13
Fayette 11,766.04 9,197.94 1,966.77 19,110.89 20,586.37 1,712.04 5,455.07 0.49
Hays 8,023.10 26,381.53 3,416.38 12,988.86 26,754.99 342.75 1,276.29 0.01
Lee 2,312.53 6,932.88 1,046.96 16,356.91 8,142.31 989.89 45.46 0.01
Llano 1,987.14 4,700.33 692.59 13,198.94 8,986.53 330.80 21.97 0.08
Travis 19,330.30 47,050.85 6,906.58 39,419.86 108,411.29 827.87 837.06 0.31
Williamson 8,347.92 32,076.96 4,237.14 25,502.02 40,094.30 1,452.27 157.04 0.34
Total 64,432.32 185,758.46 35,130.47 205,941.91 380,251.55 9,936.96 9,535.67 1.55

*Note that since this year included the large Bastrop wildfires that occured in September and October 2011, the emissions inventory for Bastrop County for this year is disproportionately influenced by emissions from wildfires. In fact, the 2011 wildfires accounted for 28% of Bastrop County NOX emissions, 33% of PM10 emissions, 77% of PM2.5 emissions, 66% of VOC emissions, 90% of CO emissions, 68% of NH3 emissions, and 72% of SO2 emissions in 2011.

The 2011 NEI also includes inventories of HAPS and GHGs at the county level and, for specific facilities, at the facility level too.

> Review 2011 NEI data.

How are emissions sources classified?

There are five basic categories of emissions inventories:

  • On-road sources include cars, trucks, buses, and motorcycles that are registered for highway and roadway use.
  • Non-road sources include any mobile sources (defined as moving at least once a year) that are not registered for on-road use, including airplanes, locomotives, boats, construction equipment, drill rigs, and agricultural equipment, among others.
  • Point sources are large stationary facilities such as power plants and factories that emit enough pollution to be required to submit annual reports on their emissions.
  • Area sources are smaller stationary or difuse sources that are not required to report their emissions and sources, including small boilers, asphalt, gasoline refueling, etc.
  • Natural and biogenic sources are sources of emissions that are the result of natural processes, including vegetation, soil, lightning, and wildfires.

What kinds of emissions inventories are relevant to ozone?

Since the pollutant that the CAPCOG region is closest to violating federal standards for is ozone, emissions inventories for ozone are particularly important for regional air quality planning.

Since ozone levels peak during summertime, and the standard is based on daily eight-hour ozone averages, emissions inventories for ozone are typically developed for typical summer days, and expressed in tons per day.

Since weekday emissions are typically higher than weekend emissions, NOX, VOC, and CO emissions inventories for ozone planning are usually developed to represent typical summer season weekdays.

For photochemical modeling, speciated emissions inventories of NOX, VOC, and CO are developed for every hour of a photochemical modeling episode for each photochemical modeling grid cell.

What kind of emissions inventory research does CAPCOG conduct?

CAPCOG's Air Quality Program conducts a variety of emissions inventory research in support of air quality planning. Among the types of emissions research CAPCOG's Air Quality Program conducts are the following:

  • Review and analyze emissions inventory data developed by TCEQ, EPA, and other agencies relevant to local air quality planning
  • Identify opportunities for improving the emissions estimates and the spatial or temporal allocation of emissions
  • Conduct surveys, data collection, literature review, and other analysis to improve estimates of:
    • Activity data
    • Emissions rates
    • Spatial distribution of emissions
    • Temporal distribution of emissions
    • Emission reductions due to control strategies

CAPCOG's emissions inventory research reports are available in the "Technical Reports" section. Raw data is available upon request.

> Review CAPCOG emissions inventory research reports.


> Contact Andrew Hoekzema, CAPCOG Regional Services director.