The purpose of this chapter is to understand the major factors that affect the dispersion of plumes (or pollutants). Meteorological conditions are the most important factors that drive the behavior of plumes. There are other factors that are also introduced: understanding of microscale, mesoscale, and macroscale air motions that affect pollutant dispersion; understanding of the nature of turbulence, the forces that cause it, and its role in dispersing pollutants; better knowledge of temperature changes with height (lapse rates) and the relationship to atmospheric stability and pollutant dispersion; understanding of how lapse rate conditions affect plume behavior; knowledge of the relationship between mixing height and pollutant dispersion; understanding of how surface-based inversions form in river valleys at night; becoming familiar with the concepts of an urban plume and long-range transport with the processes that affect pollutant lifetimes and deposition phenomena; understanding of the characteristics of subsidence inversions and atmospheric conditions that give rise to them; understanding of how stratosphere–troposphere exchange occurs; and awareness of the importance of meteorological information for industrial siting, episode planning, and air quality modeling.
The meteorological factors such as wind speed and direction can affect the speed and direction of plumes, and ambient temperature is related to atmospheric stability and the nature of turbulence. Vertical meteorological conditions such as lapse rate, mixing height, and stratosphere–troposphere exchange are reviewed as well. The characteristics of pollutants also play an important role in dispersion. The pollutant lifetimes, their chemical and physical reactions with other compounds, and wet and dry deposition in the atmosphere will affect the horizontal range which the plume reaches. The terrain is another crucial element affecting the dispersion of plumes. For instance, river valleys can trap plumes at night due to the daily occurrence of surface-based inversions. Plumes in urban areas also easily accumulate at the surface level due to subsidence inversions in winter and cause serious air pollution. In addition to dispersion behavior of plumes at microscale and mesoscale, macroscale air motion like transboundary or long-range transport can also affect regional or even global air quality.