Air pollution surveys are unique in their development and conduct. A common goal is to obtain a representative sample from an unconfined volume of air in the vicinity of one or more emission sources.
Depending on the objectives of an air pollution survey, a mobile or fixed sampler can be used. Other than the obvious considerations such as accessibility and the relation ship to interfering pollutant sources, the principal factors in site selection are meteorology and topography. The controlling factor for site selection is wind movement. With some knowledge of the predominant wind direction, the environmental engineer can predict the path of pollution from the emission source to the point of ground-level impact and determine the most suitable location for an air monitoring site. The most convenient method for performing this analysis is to use the wind rose described in Section 5.5.
Besides wind direction and wind speed, other meteorological data necessary for sample correlations are temper ature, cloud cover, and lapse rate where possible. The environmental engineer uses local temperatures to estimate the contribution of home heating to the total pollutant emission rates.
The simplest case is one where one wind direction predominates over a uniform topography for an isolated plant emitting a single pollutant that remains unchanged in the atmosphere. Two monitors are used: one monitors the effects of the source and the other is placed upwind to provide background concentrations. Where wind directions vary and other emission sources are operating nearby, the environmental engineer requires additional samples to identify the concentrations attributable to the source.
Environmental engineers often use a variation of the wind rose, called a pollution rose, to determine the source of a pollutant. Instead of plotting all winds on a radial graph, they use only those days when the concentration of a pollutant is above a minimum. Figure 5.7.1 is a plot of pollution roses. Only winds carrying SO3 levels greater than 250 fg/m are plotted. The fingers of the roses point to plant three. Pollution roses can be plotted for other pollutants and are useful for pinpointing sources of atmospheric contamination.
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