The Atmospheric System

As discussed in Chapter 1, much of our understanding of the chemistry of our atmosphere is based on early studies of air pollution; these are often treated in the context of an overall "system." This approach starts with the various sources of anthropogenic and natural emissions and tracks the resulting pollutants through their atmospheric transport, transformations, and ambient concentrations—on local, regional, and global scales—to their ultimate chemical and physical fates, including their impacts on our health and environment.

Figure 2.1 is a simplified diagram illustrating the major elements. Primary pollutants are defined as those emitted directly into the air, e.g., S02, NO, CO, Pb, organics [including HAPS (hazardous air pollutants)], and combustion-generated particulate matter (PM). Sources may be anthropogenic, biogenic, geogenic, or some combination thereof. Once in the atmosphere, they are subjected to dispersion and transport, i.e., meteorology, and simultaneously to chemical and physical transformations into gaseous and particulate secondary pollutants; the latter are defined as those formed from reactions of the primary pollutants in air. Both primary and secondary pollutants are removed at the earth's surface via wet or dry deposition and, in the processes of transport, transformation, and deposition, can impact a variety of receptors, for example, humans, animals, aquatic ecosystems, forests and agricultural crops, and materials.

From a detailed knowledge of the emissions, topography, meteorology, chemistry, and deposition processes, one can develop mathematical models that predict the concentrations of primary and secondary pollutants as a function of time at various locations. Depending on the particular model, these may describe pollutant concentrations over a variety of scales:

• In a plume from a specific point source (plume models)

• In an air basin from a combination of diverse mobile and stationary sources (airshed models)

• Over a large geographical area downwind from a group of sources (long-range transport and regional models)

• Over the entire earth (global models)

To test these models, their predictions must be compared to the observed concentrations of various species; model inputs are adjusted to obtain acceptable agreement between the observed and predicted values. These models can then be used, in combination with the documented impacts on receptors, to develop health and/or environmental risk assessments and various control strategy options.

Finally, through legislative and administrative action, health-protective and cost-effective risk-management decisions can be made, and regulatory actions implemented, that directly affect the starting point of our atmospheric system, that is, the primary emissions and their sources.

To place the remainder of this book on atmospheric chemistry in perspective, the various components of our "atmospheric system" are treated briefly next.

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