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Atmospheric Pollutants

Carbon dioxide

Carbon dioxide, methane, and halocarbons, the primary greenhouse gases, were discussed in The Carbon Dioxide Problem so they will not be discussed further here. The other greenhouse gases, ozone and nitrous oxides, have other important effects described on this page.

The Environmental Protection Agency's Six Principal Pollutants.

The Environmental Protection Agency has set national air quality standards for six principal air pollutants (also called the criteria pollutants): nitrogen dioxide (NO2), ozone (O3), sulfur dioxide (SO2), particulate matter (PM), carbon monoxide (CO), and lead (Pb).

EPA National Ambient Air Quality Standards

Primary Stds.
Averaging Times
Secondary Stds.
Carbon Monoxide 9 ppm
(10 mg/m3)
8-hour None
35 ppm
(40 mg/m3)
1-hour None
Lead 1.5 µg/m3 Quarterly Average Same as Primary
Nitrogen Dioxide 0.053 ppm
(100 µg/m3)
Annual (Arithmetic Mean) Same as Primary
Particulate Matter (PM10) 50 µg/m3
Annual (Arith. Mean)
Same as Primary
150 ug/m3 24-hour  
Particulate Matter (PM2.5) 15.0 µg/m3
Annual (Arith. Mean)
Same as Primary
65 ug/m3 24-hour  
Ozone 0.08 ppm 8-hour Same as Primary
0.12 ppm 1-hour
(Applies only in limited areas)
Same as Primary
Sulfur Oxides 0.03 ppm Annual (Arith. Mean)
0.14 ppm 24-hour
3-hour 0.5 ppm
(1300 ug/m3)

Primary standards set limits to protect public health, including the health of "sensitive" populations such as asthmatics, children, and the elderly. Secondary standards set limits to protect public welfare, including protection against decreased visibility, damage to animals, crops, vegetation, and buildings. From Environment Protection Agency.

Nitrogen dioxide

Nitrogen dioxide is a reddish-brown, highly reactive gas that is formed in the ambient air through the oxidation of nitric oxide (NO). It is produced by motor vehicles (55%), power plants (22%), and other industrial, commercial, agricultural, and residential sources that burn fuels or biomass (22%). The higher the combustion temperature, the more nitrogen is oxidized. Lightning and microbial activity are natural sources of nitrogen oxides. Nitrogen oxides in the atmosphere react with water, oxygen, and oxidants to form acidic compounds. These compounds fall to the Earth in either dry form (gas and particles) or wet form (rain, snow, and fog) as acid rain.

Oxides of nitrogen can be carried great distances by winds, and they:

  1. Are one of the main ingredients involved in the formation of ground-level ozone, which can trigger serious respiratory problems.
  2. React to form nitrate particles, acid aerosols (microscopic particles in the air), as well as NO2, which also cause respiratory problems.
  3. Contribute to formation of acid rain.
  4. Contribute to nutrient overload that deteriorates water quality.
  5. Contribute to atmospheric particles, that cause visibility impairment most noticeable in national parks.
  6. React to form toxic chemicals.
  7. Contribute to global warming.
  8. Absorbs sunlight giving air a reddish brown color.

global distribution of nitrogen dioxide averager for 2004
Average density of nitrogen dioxide in the troposphere in 2004. The image makes clear just how human activities impact air quality. Click on image for a zoom. Data are from the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) instrument on Envisat. From European Space Agency GMES Service Nitrogen dioxide page.

The Environmental Protection Agency publishes maps of their Air Quality Index based on the concentration of these major air pollutants. There was no area of the country in noncompliance with EPA standards for nitrogen dioxide in March 2006.

Ozone (O3)

Ozone is Good Up High Bad Nearby. It is good when it occurs in the stratosphere, where it absorbs ultraviolet radiation (energy) from the sun. It is bad when it occurs close to the ground in the troposphere, where it is a pollutant. Tropospheric ozone irritates the respiratory system, aggravates asthma and bronchitis, and it inflames the lining of the lungs. It harms vegetation and agricultural crops, and it damages rubber and other materials.

Ozone is the major component of smog. It is produced from nitrogen oxides and volatile carbon-based compounds when there is intense solar radiation (energy), above all in the spring and summer. See The Physics and Chemistry of of Ozone.
For more information: see the Environmental Protection Agency's page

ozone sources and cause
From Environmental Protection Agency.

ozone and VOX and NO2
Atmospheric concentration of ozone depends on the concentration of both volatile organic carbon VOC and nitrous oxides NOx. Notice that ozone concentration (the contours) decrease as VOC increases if NOX concentrations are high, but that it increases if VOX concentrations are low. Thus for some cities, decreasing NOx emissions leads to higher ozone concentrations.
From National Academies Press (1991) Rethinking the Ozone Problem in Urban and Regional Air Pollution.

The US National Weather Service publishes maps of ozone concentration for the lower 48 states.

nonattainment ozone 8 hour
From EPA

Sulfur dioxide

Sulfur dioxide is emitted in great quantities:

  1. By volcanos,
    sulfur dioxibe emitted from kilauea volcano in hawaii in 2008
    Sulfur dioxide cloud emitted from Kilauea Volcano observed on 26 April 2008 by sensors on NASA's Aura satellite.
    From NASA Goddard Space Flight Center Science Page.

  2. By power plants that burn fuel containing sulfur (mainly coal and oil),
  3. By smelters, petroleum refineries, iron and steel mills, and by other industrial processes.
    sulfur dioxide emissions from Peruvian copper smelters
    Sulfur dioxide emissions from copper smelters in Peru as observed by sensors on NASA's Aura satellite. Click on image for a zoom.
    From Goddard Space Flight Center Science page on Top Ten Smelters.

  4. Additionally, plankton emit dimethylsulfide (DMS), which is oxidized to form SO2.

Fuel combustion, largely from coal-fired power plants, accounts for most of the total SO2 emissions in the USA. But, thanks to the Clean Air Act, the national average of ambient SO2 concentrations have decreased 52 percent from 1982 to 2001 accorning to the EPA. Concentrations are also decreasing in Europe, but they are increasing in other parts of the world, especially Asia.

Sulfer dioxide readilly dissolves in water or water vapor to form acid, which makes the rain acidic. It is oxidized by OH· in gas phase, or in cloud droplets to form sulfate aerosol (particles), which makes the air hazy and causes dry deposition of acid.

sulfer dioxide noncompliance map USA
From EPA.


Particulates are small particles (aerosols) suspended in the air. The can be composed of:

  1. Sulfates from SO2,
  2. Nitrates from NO2,
  3. Dust (dust storms),
  4. Soot from diesel engines and fires (smoke), including burning of agricultural waste, and forest fires)
  5. Carbon-based molecules from industry or plants (think of the Great Smoky Mountains),
  6. Sea salt (from breaking ocean waves).

Particulate matter is divided into fine particles less than 2.5 micrometers in diameter PM2.5, and coarse particles between 2.5 and 10 micrometers in diameter PM10. The smaller particles are most harmful because they can infiltrate deep into lungs. Smoke irritates eyes and lungs.

fine aerosol map 2004
Distribution of fine aerosols in the continental USA averaged over 2004. Notice the high densities in the east due mostly to industrial activity, especially emissions from power plants.
From Visiblity Information Exchange Web System. Daily maps are available from NOAA GOES Aerosol/Smoke Product (GASP).

map marticulate matter noncompliant USA
From EPA.

map particulates noncompliant USA
From EPA.

map of small aerosols over europe

Distribution of PM2.5 aerosols over Europe in 2000, From LOTOS-EUROS aerosol analysis system.

Aerosols and small particles are important for three reasons:

  1. They influence health.
  2. They influence earth's radiation (radiant energy) balance.
    1. They reflect sunlight. This increase the amount of sunlight reflected back to space, cooling earth's surface.
    2. They cause clouds to last longer. Because clouds reflect sunlight, this too increases the amount of sunlight reflected back to space, further cooling earth's surface.
  3. They influence precipitation.
    1. They reduce rainfall because small cloud droplets fall slower than larger droplets.
    2. Rain in hilly regins can be reduced by 30–50% during hazy conditions, with visibility less than 8 km at hilltops (Rosenfeld et al. 2007).

There is mounting evidence that land surface properties and atmospheric aerosol particles have a profound impact on earth's thermodynamic and radiative energy budgets by affecting precipitation processes. Large concentrations of sub-micron aerosols act as cloud condensation nuclei (CCN) that produce large concentrations of small cloud drops which are slow to coalesce into raindrops. The decrease in cloud drop size also delays the formation of ice in the cloud to lower temperatures. This can lead to suppression of precipitation in shallow and short-lived clouds, as during winter over topographic barriers, and subsequently to decreased water resources in semi-arid regions. Manifestations of these processes are observed in all clouds, from the shallow marine stratocumulus, through the modest rain clouds over land in the extra-tropics, to the deepest tropical convective clouds over the Amazon and equatorial oceans. From iLEAPS Newsletter, #2, May 2006.
ship aerosols
This image, made from data collected by the MODIS instrument on the Terra satellite shows the relationship between aerosols and cloud formation. It was collected off the coast of california on 26 June 2003 at 19:40 GMT. The colors correspond to size of cloud drops. The long blue tracks are clouds composed of very small drops condensing on aerosols from ship exhaust. Notice that when ship tracks cross naturally occuring clouds (orange to black areas), they change the size of the cloud drops in these clouds. From From iLEAPS Newsletter, #2, May 2006.

Carbon monoxide

Carbon monoxide is a colorless, oderless, poisonous gas produced by the incomplete burning of cabon in fuels, mainly by cars and trucks and also by forest fires and burning of agricultural waste. In the USA, about 77% of the pollutant comes from transportation sources. In cities, as much as 95 percent of all CO emissions may come from automobile exhaust. according to the EPA. It is a deadly poison which binds to hemoglobin molecules in blood, reducing the amount of oxygen carried to body tissues. Emissions and concentrations of carbon monoxide have dropped 65% and 41% respectively from 1982 to 2002 in the USA

carbon monoxide global map
Map of average concentration of carbon monoxide in the spring. From Measurements Of Pollution In The Troposphere MOPITT instrument on the Terra satellite. Note the high concentrations in the northern hemisphere associated with industrial activity, and high concentrations in Africa associated with agricultural burning. Click on the image to go to a NASA website showing a 5.6 MByte animation of the movement of carbon monoxide around the world in 2000 from the Total Emissions Spectrometer on the Aura satellite.

map of US showing areas in violation of EPS standards for carbon monoxide emissions
From EPA


In the past, automotive sources were the major contributor of lead emissions to the atmosphere. As a result of EPA’s regulatory efforts to reduce the content of lead in gasoline, however, the contribution of air emissions of lead from the transportation sector, and particularly the automotive sector, has greatly declined over the past two decades. Today, industrial processes, primarily metals processing, are the major source of lead emissions to the atmosphere. The highest air concentrations of lead are usually found in the vicinity of smelters and battery manufacturers. From EPA

It appears that lead is becoming less and less of an air pollutant. "The 2002 average air quality concentration for lead is 94 percent lower than in 1983. Emissions of lead decreased 93 percent over the 21-year period 1982–2002." Only two areas, in Montana and Missouri were in noncompliance with EPA standards in March 2006.

Volatile Carbon-Based Molecules (Volatile Organic Carbon)

Volatile carbon-based molecules are mostly evaoprated hydrocarbons, chemical compounds that contain hydrogen and carbon. In typical urban areas, a very significant fraction comes from cars, buses, trucks, and nonroad EPA.

Texas Information

Information on air quality in Texas is available from many agencies, and from Texas Environmental Profiles.


Rosenfeld, D., J. Dai, et al. (2007). Inverse Relations Between Amounts of Air Pollution and Orographic Precipitation. Science 315 (5817): 1396-1398.
Particulate air pollution has been suggested as the cause of the recently observed decreasing trends of 10 to 25% in the ratio between hilly and upwind lowland precipitation, downwind of urban and industrial areas. We quantified the dependence of this ratio of the orographic-precipitation enhancement factor on the amounts of aerosols composed mostly of pollution in the free troposphere, based on measurements at Mt. Hua near Xi'an, in central China. The hilly precipitation can be decreased by 30 to 50% during hazy conditions, with visibility of less than 8 kilometers at the mountaintop. This trend shows the role of air pollution in the loss of significant water resources in hilly areas, which is a major problem in China and many other areas of the world.

Revised on: 23 December, 2008

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