Certain general observations related to sampling ambient air must be recognized. For example, the quantity of a substance contained in a volume of air is often extremely small; therefore, the sample size for the analytical method must be adequate. Even heavily polluted air is not likely to contain more than a few milligrams per cubic meter of most contaminants; and frequently, the amount present is best measured in micrograms, or even nanograms, per cubic meter.
For example, the air quality standard for particulates is 75 ¡g/M3. A cubic meter of air, or 35.3 cu ft, is a large volume for many sampling devices, and a considerable sampling period is required to draw such a quantity of air through the sampler. When atmospheric mercury analyses are made, the environmental engineer must realize that background levels are likely to be as low as several nano-grams per cubic meter. In general, most substances are of concern at quite low levels in ambient air.
In addition to the problems from low concentrations of the substances being sampled, the reactivity of some substances causes other problems, resulting in changes after collection and necessitating special measures to minimize such changes.
Whenever a substance is removed from a volume of air by sampling procedures, the substance is altered, and the analysis can be less informative or even misleading. Ideally, environmental engineers should perform analyses of the unchanged atmosphere using direct-reading devices that give accurate information concerning the chemical and physical state of contaminants as well as concentration information. Such instruments exist for some substances, and many more are being developed. However, conventional air sampling methods are still used in many instances and will continue to be required for some time.
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