As required by the Clean Air Act, the Environmental Protection Agency (EPA) has set National Ambient Air Quality Standards (NAAQS) for six principal pollutants considered harmful to the environment and public health. These six “criteria pollutants” are nitrogen dioxide (NO2), sulfur dioxide (SO2), carbon monoxide (CO), particulate matter (PM), lead (Pb), and ozone (O3). We will describe each of these further below.
Ground level ozone is a key component of smog and can cause respiratory damage. The EPA regulates ground level ozone pollution in our communities, but human activities do not emit much ozone directly; instead ozone is formed through a set of chemical reactions in the air that can be generally described by the following equation:
VOC + NOX + heat + sunlight = ground level ozone (O3)1
To control ozone pollution we must control the chemicals, known as “Precursor Pollutants,” which react in the air to create it. Therefore, the TravelMatters transit calculator reports volatile organic compound (VOC) and nitrogen oxide (NOX) emissions. Ozone is not reported in the TravelMatters calculator because transit vehicles do not directly emit it.
Nitrogen Oxides (NOX)
Nitrogen oxides comprise a family of highly reactive gases that includes nitrogen dioxide (NO2), nitric oxide (NO), and nitrous oxide (N2O). NO2 is a lung irritant, a cause of acid rain, and a precursor to ground level ozone. The burning of fossil fuels, such as in motor vehicles or power plants, produces both NO and NO2. Once in the atmosphere, NO reacts to form NO2, so while the National Ambient Air Quality Standards regulate NO2 concentrations in the air, emissions regulations focus on controlling emissions of the whole NOX family. We report emissions of NOX in the TravelMatters Transit Calculator. The EPA has issued new limits on NOX emissions for manufacturers of heavy-duty diesel engines, such as those used in many transit buses, beginning in 2007.
Sulfur Dioxide (SO2)
Sulfur dioxide belongs to a family of gases called sulfur oxides (SOX). SO2 can cause respiratory and pulmonary difficulties; and, the gas can react with NOX and other chemicals in the air to form acid rain. Roughly one-third of atmospheric sulfur compounds come from human-made sources.2 Of this one-third, only 5% of annual anthropogenic SO2 emissions come from mobile sources.3 The EPA has mandated that the sulfur content of diesel fuel be reduced beginning in 2006.4
Particulate Matter (PM)
Particulate matter can be either solid particles or liquid droplets. PM is measured in micrometers, with matter less than 10 micrometers in diameter posing the greatest health risk. Particles less then 2.5 micrometers in diameter are described as being “fine” particles. These particles are easily inhaled and can become lodged in the lungs and produce respiratory illness. PM greater then 2.5 micrometers in diameter is usually the result of smoke and dust from industry and agricultural production, while particles less then 2.5 generally come from combustion of fossil fuels, such as in vehicles. The TravelMatters Transit Calculator reports PM 2.5 emissions, because this is the main component of vehicle particulate emissions.
The EPA has recently started regulating PM 2.5 in the atmosphere and is expected to designate which communities are meeting the air quality standard for this pollutant in December 2004. Communities that do not meet the standard are said to be out of “attainment” and must design a plan to reduce the concentration of PM 2.5 in the air. This may impact transit agencies with diesel-fueled vehicles. In addition, the EPA has issued new limits on PM 2.5 emissions for manufacturers of heavy-duty diesel engines, such as those used in many transit buses, beginning in 2007.
Lead is a heavy, soft metal with a metallic blue hue. When inhaled or ingested, lead particles are toxic. Lead is a cumulative poison to the central nervous system and is particularly damaging to the mental development of young children. Lead emissions in the US fell 93 percent from 1982 to 2002. Transportation used to be the primary source of atmospheric lead pollution, but because leaded gasoline was banned, transportation lead emissions from on-road vehicles have been nearly eliminated, so we do not report lead emissions in the TravelMatters Transit Calculator.
Carbon Monoxide (CO)
Carbon monoxide is a colorless, odorless gas that is poisonous in high concentrations. CO is created from incomplete combustion of fossil fuels. When there is not enough oxygen present during combustion to form carbon dioxide (CO2), CO is the result. Mobile sources account for 77% of CO emission in the US. To help control CO emissions, the regulators have begun requiring oxygenated fuels in the winter, when CO is more of a problem. The TravelMatters Transit Calculator reports the estimated CO emissions of transit agency vehicles.
Volatile Organic Compounds (VOCs)
Volatile organic compounds, which are sometimes referred to as hydrocarbons (HC), are organic chemicals that, when released into the atmosphere, participate in photochemical reactions such as the reaction to create ground level ozone. VOCs can be emitted when unburned or partially burned fossil fuels are released as exhaust. They can also be emitted when fuel evaporates, including during refueling. Mobile sources account for about a quarter of VOC emissions in the US.5 The TravelMatters Transit Calculator reports on VOC emissions from transit agency vehicles.
National Ambient Air Quality Standards
The Clean Air Act required the federal government to set maximum levels of pollution for the six criteria air pollutants. The law mandated both Primary (levels concerned with public health of sensitive populations including infants, children, and the elderly) and secondary standards that protect the general welfare. The national EPA works with state and local agencies to meet these standards. Any area not meeting the standards is called a non-attainment area. States with non-attainment areas are required to create State Implementation Plans (SIP) to meet the federal guidelines. Failure to submit a satisfactory SIP results in federal sanctions, and ultimately, federal intervention.
Resources and links:
To learn what criteria air pollutants are present in your region, click here: http://www.scorecard.org/env-releases/cap/
To find out what the levels of criteria air pollutants are today in your region, click here: http://www.epa.gov/airnow/
- US EPA. Ground Level Ozone: What is it? Where does it come from? http://www.epa.gov/air/urbanair/ozone/what.html (May 2004)
- Encyclopedia of the Atmospheric Environment. http://www.ace.mmu.ac.uk/eae/Air_Quality/Older/Air_Pollution_Chemistry.html (May 2004).
- US EPA AirTrends http://www.epa.gov/airtrends/sulfur2.html (May 2004)
- US EPA. http://www.epa.gov/otaq/regs/fuels/diesel/diesel.htm (May 2004).