Glossary of Terms

On this website and in the reports and analyses provided here, certain terms appear frequently and their meaning is helpful in understanding the data or the analyses provided. These terms are included here for your easy reference.

Pollutant concentrations – Concentrations of most gaseous pollutants are expressed in units denoting their “mixing ratio” in air; i.e., the ratio of the number molecules of the pollutant to the total number of molecules per unit volume of air. Because concentrations for all gases other than molecular oxygen, nitrogen, and argon are very low, the mixing ratios are usually scaled to express a concentration in terms of “parts per million” (ppm) or “parts per billion” (ppb). Sometimes the units are explicitly expressed as ppm-volume (ppmV) or ppb-volume (ppbV) where 1 ppmV indicates that one molecule in one million molecules of ambient air is the compound of interest and 1 ppbV indicates that one molecule in one billion molecules of ambient air is the compound of interest. In general, air pollution standards and health effects screening levels are expressed in ppmV or ppbV units. Because hydrocarbon species may have a chemical reactivity related to the number of carbon atoms in the molecule, mixing ratios for these species are often expressed in ppb-carbon (ppbV times the number of carbon atoms in the molecule), to reflect the ratio of carbon atoms in that species to the total number of molecules in the volume. This is relevant to our measurement of auto-GC species, which are reported in ppbC units. For the purpose of relating hydrocarbons to health effects, some reports note hydrocarbon concentrations in converted ppbV units. Concentrations for particulate matter are generally expressed in gravimetric concentrations. The most common is micrograms per cubic meter of air, abbreviated using the Latin letter for “micro” as μg/m3. Pollutant concentration measurements are time-stamped based on the start time of the sample, in Central Standard Time (CST), with sample duration noted.

Auto-GC– The automated gas chromatograph collects a sample for 40 minutes, and then automatically analyzes the sample for a target list of 46 hydrocarbon species. These include benzene and 1,3-butadiene, which are air toxics, various species that have relatively low odor thresholds, and a range of gasoline and vehicle exhaust components.

Air Monitoring Comparison Values (AMCV) – The TCEQ uses AMCVs and effects screening levels (ESLs) in assessing ambient data. A valuable online page that explains AMCVs and ELSs is at About Air Monitoring Comparison Values (AMCVs) (accessed November 2019). The following text is an excerpt from this website.

AMCVs and ESLs are screening levels for ambient air set to protect human health and welfare.

AMCVs are screening levels used in TCEQ’s evaluation of ambient air monitoring data to assess the potential for measured concentrations of specific chemicals to cause health or welfare effects. Health-based AMCVs are safe levels at which exposure is unlikely to result in adverse health effects. Long-term

AMCVs are similar to the USEPA’s inhalation reference concentrations.

ESLs are screening levels used in the TCEQ’s air permitting process to establish maximum emission rates that are written into enforceable air permits. Health-based ESLs are set 70 percent lower than the safe level, or AMCV. This additional buffer allows TCEQ to take into account exposure to chemicals from multiple sources in air permit reviews.

Rationale for Differences between ESLs and AMCVs – A very specific difference between the permitting program and monitoring program is that permits are applied to one company or facility at a time, whereas monitors may collect data on emissions from several companies or facilities or other source types (e.g., motor vehicles). Thus, the protective ESL for permitting is set lower than the AMCV in anticipation that more than one permitted emission source may contribute to monitored concentrations.

National Ambient Air Quality Standards (NAAQS) – U.S. Environmental Protection Agency (EPA) has established a set of standards for several air pollutions described in the Federal Clean Air Act. NAAQS are defined in terms of levels of concentrations and particular forms. For example, the NAAQS for particulate matter with size at or less than 2.5 microns (PM2.5) has a level of 15 micrograms per cubic meter averaged over 24-hours, and a form of the annual average based on four quarterly averages, averaged over three years. Individual concentrations measured above the level of the NAAQS are called exceedances. The number calculated from a monitoring site’s data to compare to the level of the standard is called the site’s design value, and the highest design value in the area for a year is the regional design value used to assess overall NAAQS compliance. A monitor or a region that does not comply with a NAAQS is said to be noncompliant. At some point after a monitor or region has been in noncompliance, the U.S. EPA may choose to label the region as nonattainment. A nonattainment designation triggers requirements under the Federal Clean Air Act for the development of a plan to bring the region back into compliance.

  • A more detailed description of NAAQS can be found on the EPA’s Website at Criteria Air Pollutants (accessed November 2019).
  • Three pollutants measured by this project and regulated by a NAAQS are sulfur dioxide (SO2), nitrogen dioxide (NO2), and fine particulate matter (PM2.5). The SO2 NAAQS is 75 ppb averaged over one hour, with a form of the three-year average of the annual 99th percentiles of the daily maximum one-hour averages not to exceed 75 ppb. There is also a secondary SO2 standard of 0.500 ppm (500 ppb) over three hours, not to be exceeded more than once in any one year.
  • For NO2, the NAAQS is 100 ppb averaged over one hour, with a form of the three-year average of the annual 98th percentiles of the daily maximum one-hour averages not to exceed 100 ppb.
  • For PM2.5, the NAAQS is 35 μg/m3 averaged over a calendar day, with a form of the three-year average of the annual 98th percentiles of the daily maximum one day averages not to exceed 35 μg/m3. In addition, the annual average PM2.5 concentration must stay below 12 μg/m3 after averaging the annual value over three years.

Elevated Concentrations – In the event that measured pollutant concentrations are above a set threshold they are referred to as “elevated concentrations.” The values for these thresholds are summarized by pollutant below. This does not imply the occurrence of a violation of a health-based standard. A discussion of “elevated concentrations” by pollutant type follows:

  • Any pollutants that we measure have concentrations that rise and fall each hour based on the activity of pollution sources in the area (including motor vehicles, law mowers, local businesses, industries, etc.) and based on the changes in local wind direction and wind speed, and also based on instrument accuracy. By observing concentrations over some time, we begin to note the typical behavior in terms of the range of concentrations measured by time of day or day of week. Thus, we can begin to characterize the concentrations and note the rare cases when values that are much greater than the expected range of values are measured. One means of doing this is by tracking the time series average or mean concentration over time, as well as the average difference between each measurement and the mean concentration. This latter calculation is referred to as the standard deviation.
  • For SO2, NO2, PM2.5 any measured concentration greater than the level of the NAAQS is considered “elevated.” Note that the concentrations of a pollutant need not persist long enough to constitute an exceedance of the standard to be regarded as elevated. In addition, any closely spaced values that are unusually high, say, 3 standard deviations greater than the long-run average concentration for a period of one hour or more may be considered “elevated” because of their unusual appearance, as opposed to possible health consequence.
  • For benzene and other air toxics in auto-GC or canister measurements, any concentration above the AMCV is considered “elevated.” Note that 40-minute auto-GC measurements for both are compared with their respective short-term AMCV.
  • Some hydrocarbon species measured in the auto-GC data generally appear in the air in very low concentrations close to the method detection level. Similar to the case above with SO2 and PM2.5, any values that are statistically significantly (at 0.01 level) greater than the long-run average concentration at a given time or annual quarter will be considered “elevated” because of their unusual appearance, as opposed to an indication of possible health consequence.