Page 229 - Fundamentals of Air Pollution

192 13. Ambient Air Sampling

air pollution sources is to limit the buildup of contaminants in the atmo-
sphere so that adverse effects are not observed. This suggests that sampling
sites should be selected to measure pollutant levels close to or representative
of exposed populations of people, plants, trees, materials, structures, etc.
Generally, sites in air quality networks are near ground level, typically 3
m aboveground, and are located so as not to be unduly dominated by a
nearby source such as a roadway. Sampling sites require electrical power
and adequate protection (which may be as simple as a fence). A shelter,
such as a small building, may be necessary. Permanent sites require ade-
quate heating and air conditioning to provide a stable operating environ-
ment for the sampling and monitoring equipment.

VI. SAMPLING FOR AIR TOXICS

Public awareness of the release of chemicals into the atmosphere has gone
beyond the primary ambient pollutants (e.g., SO 2 or O 3) and governments
require air toxics management plans. One component of this process is the
characterization of the air quality via sampling.
Most of the airborne chemicals classified as "air toxics" are organic com-
pounds with physical and chemical properties ranging from those of formal-
dehyde found in the gas phase to polynuclear aromatic hydrocarbons
(PAHs) which may be absorbed on particle surfaces. This range of volatility
and reactivity represented by air toxics requires a variety of sampling tech-
niques—from grab sampling to filter techniques followed by extraction and
detailed derivatization techniques. When these compounds are present in
the atmosphere, the concentration level can be quite low, in the parts per
3
billion (ppb) to sub-ppb range for gases and the picogram/m range for
particulate components. This generally requires extended sampling times
and very sensitive analytical techniques for laboratory analysis.
Two examples of this type of ambient sampling are described. The U.S.
Environmental Protection Agency established a pilot Toxics Air Monitoring
System network for sampling ambient volatile organic compounds (VOCs)
at ppb levels in Boston, Chicago, and Houston for a two-year period (8).
3
Evacuated stainless steel canisters were used to collect air at 3 cm /min for
24 hours. The canisters were returned to a central laboratory and analyzed
by cryogenic concentration of the VOCs, separation by gas chromatogra-
phy, and mass-selective detection. This system provided information on
13 VOCs in three classes: chlorofluorocarbons, aromatics, and chlorinated
alkanes.
A second sampling program in Southern California sampled for poly chlo-
rinated dioxins and poly chlorinated dibenzofurans at seven locations (9).
Because of the semivolatile nature of these compounds, a tandem sampler
was used with a glass fiber filter to collect the particulate-associated compo-