The principal purpose of air quality monitoring is to acquire data for comparison to regulated standards. In the United States, standards have been promulgated by the federal government and by many states. Where possible, these standards are based on the physiological effect of the air pollutant on human health. The averaging time over which various concentration standards must be maintained differs for each pollutant. (See Chapter 4 for tabulations of the ambient air quality standards promulgated by the federal government.)
Some air quality monitoring systems determine the impact of a single source or a concentrated group of sources of emissions on the surrounding area. In this case, determining the background level, the maximum ground-level concentration in the area, and the geographical extent of the air pollutant impact is important.
When the source is isolated, e.g., a single industrial plant in a rural area, the system design is straightforward. Utilizing the meteorological records available from nearby airports or government meteorological reporting stations, environmental engineers can prepare a wind rose to estimate the principal drift direction of the air pollutant from the source. Then they can perform dispersion calculations to estimate the location of the expected point of maximum ground-level concentration. As a general rule with stacks between 50 and 350 ft tall, this point of maximum concentration is approximately 10 stack heights downwind.
The air quality monitoring system should include at least one sensor at the point of expected maximum ground-level concentration. Additional sensors should be placed not less than 100 stack heights upwind (prevailing) to provide a background reading, and at least two sensors should be placed between 100 and 200 stack heights downwind to determine the extent of the travel of the pollutants from the source. If adequate resources are available, sampling at the intersection points of a rectilinear grid with its center at the source is recommended.
With such a system for an isolated source, environmental engineers can obtain adequate data in a one-year period to determine the impact of the source on the air quality of the area. Because of the variability in climatic conditions on an annual basis, few areas require less than one year of data collection to provide adequate information. If a study is performed to determine the effect of process changes on air quality, the study may have to continue for two to five years to develop information that is statistically reliable.
Some air quality monitoring is designed for the purpose of investigating complaints concerning an unidentified source. This situation usually happens in urban areas for odor complaints. In these cases, human observers use a triangulation technique to correlate the location of the observed odor with wind direction over several days. Plotting on a map can pinpoint the offending source in most cases. While this technique is not an air pollution monitoring system in an instrumental sense, it is a useful tool in certain situations.
Air pollution research calls for a completely different approach to air quality monitoring. Here, the purpose is to define some unknown variable or combination of variables. This variable can be either a new atmospheric phenomenon or the evaluation of a new air pollution sensor. In the former case, the most important consideration is the proper operation of the instrument used. In the latter case, the most important factor is the availability of a reference determination to compare the results of the new instrument against.
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