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).
| Carbon Monoxide
|| 9 ppm
|| 1.5 µg/m3
|| Quarterly Average
|| Same as Primary
| Nitrogen Dioxide
|| 0.053 ppm
| Annual (Arithmetic Mean)
|| Same as Primary
|Particulate Matter (PM10)
|Annual (Arith. Mean)
|Same as Primary
| Particulate Matter (PM2.5)
|| 15.0 µg/m3
| Annual (Arith. Mean)
| Same as Primary
||Same as Primary
(Applies only in limited areas)
|Same as Primary
| Sulfur Oxides
|| 0.03 ppm
|| Annual (Arith. Mean)
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
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:
- Are one of the main ingredients involved in the formation of ground-level
ozone, which can trigger serious respiratory problems.
- React to form nitrate particles, acid aerosols (microscopic particles
in the air), as well as NO2, which also cause respiratory problems.
- Contribute to formation of acid rain.
- Contribute to nutrient overload that deteriorates water quality.
- Contribute to atmospheric particles, that cause visibility impairment
most noticeable in national parks.
- React to form toxic chemicals.
- Contribute to global warming.
- Absorbs sunlight giving air a reddish brown color.
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
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 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
Chemical reactions leading to ozone formation in cities. Click on
image for a zoom.
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.
Click on image for a zoom. From EPA
Sulfur dioxide is emitted in great quantities:
- By volcanoes,
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
- By power plants that burn fuel containing sulfur (mainly coal and
- By smelters, petroleum refineries, iron and steel mills, and by other
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
- 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 according to the EPA.
Concentrations are also decreasing in Europe, but they are increasing
in other parts of the world, especially Asia.
Sulfur dioxide readily 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.
Click on image for a zoom. From EPA.
Particulates are small particles (aerosols) suspended in the air. The
can be composed of:
- Sulfates from SO2,
- Nitrates from NO2,
- Dust (dust storms),
- Soot from diesel engines and fires (smoke), including burning of
agricultural waste, and forest fires)
- Carbon-based molecules from industry or plants (think of the Great
- 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.
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 Visibility Information
Exchange Web System. Daily maps are available from NOAA GOES
Aerosol/Smoke Product (GASP).
Click on image for a zoom. From EPA.
Distribution of PM2.5 aerosols over Europe in 2000, From LOTOS-EUROS
aerosol analysis system.
Aerosols and small particles are important for three reasons:
- They influence health.
- They influence earth's radiation (radiant energy) balance.
- They reflect sunlight. This increase the amount of sunlight
reflected back to space, cooling earth's surface.
- 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.
- They influence precipitation.
- They reduce rainfall because small cloud
droplets fall slower than larger droplets.
- Rain in hilly regions
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.
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 occurring
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 is a colorless, odorless, poisonous gas produced by
the incomplete burning of carbon 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
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 web site 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.
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
Volatile carbon-based molecules are mostly evaporated hydrocarbons,
chemical compounds that contain hydrogen and carbon. In typical urban
areas, a very significant fraction comes from cars, buses, trucks, and
non-road mobile sources such as construction vehicles and boats. EPA.
Information on air quality in Texas is available from many agencies,
and from Texas Environmental
Rosenfeld, D., J. Dai, et al. (2007). Inverse Relations Between Amounts
of Air Pollution and Orographic Precipitation. Science 315
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.
23 December, 2008