Starter's Guide to Air Pollution
(This quick reference guide has been provided to give a short introduction to the basic processes of air pollution and some of the common terms used in APIS. However, APIS has provided individual pollutant (and pollutant issue) overviews where more detailed information can be obtained. In addition, each overview lists references of up to date research. Overviews are listed on the home page.)
Pollutant Sources and Emissions
An overview providing useful information on the main pollutant sources can be found here.
SO2, NO2
and NH3
The processes of emissions, chemical transformation, and deposition
are illustrated below (figure 1). Primary pollutants SO2, NO and NO2 are
oxidised in the atmosphere to form SO42-
and NO3- respectively,
while NH3 reacts with these oxidised components to form NH4+
(ammonium). These pollutants know as aerosols can travel long distances,
and together with primary pollutants can be deposited in the form of wet
or dry deposition.
Wet Deposition: Process whereby pollutants are removed from the atmosphere by precipitation. Wet deposition removes most of the aerosols containing SO42-, NO3- and NH4+, but some can be captured directly (dry deposition) at the terrestrial surface by aerodynamically rough surfaces (e.g. forests). Since rainfall efficiently removes these aerosols, parts of the country with the largest wet deposition tend to be areas of high rainfall.
Dry Deposition: Deposition of gases and aerosols directly to the Earth's surface. Includes most of the primary pollutants (SO2, NO2, NH3 and O3), and to a lesser extent aerosols.
Figure 1: Pollutant emission and deposition processes
O3
Ground-level (tropospheric) ozone is produced by photochemical reactions
involving nitrogen oxides, carbon monoxide, oxygen, and volatile organic
compounds. Ground-level ozone should not be confused with stratospheric
ozone famous for the ozone hole, although some is transferred to the troposphere.
Critical Loads:
The threshold level for the deposition of a pollutant above which harmful
indirect effects can be shown on a habitat or species, according to current
knowledge. Additional deposition above the Critical Load is termed Critical
Load Exceedance. Pollutants concerned with critical loads in the APIS
database are:
- Nitrogen deposition (kg Nitrogen ha-1 year-1)
- Acid deposition (keq ha-1 yr-1)
Critical Levels:
The threshold level for the atmospheric concentration of a pollutant above
which harmful direct effects can be shown on a habitat or species, according
to current knowledge. Pollutant air concentrations above the Critical
Level are termed Critical Level Exceedances. Pollutants that are concerned
with critical levels in the APIS database are:
- Ammonia (µg m-3)
- Sulphur Dioxide (µg m-3)
- Nitrogen Oxides (µg m-3)
- Ozone (ppb hours)
There are currently no critical loads or levels for Halogens, Heavy Metals, POPs, VOCs or Dusts, but in some cases an environmental critieria (e.g. an EQS) has been given.