Atmospheric Dispersion, Transformation, and Deposition

This section is adapted from USEPA, Basic Air Pollution Meteorology, U.S. Environmental Protection Agency, Washington, DC, 2005 (www.epa.gov/apti). 

Weather conditions have a significant impact on air quality and air pollution, both favorable and unfavorable. On hot, sun-filled days, when the weather is calm with stagnating high-pressure cells, air quality suffers because of the buildup of pollutants at ground level. When local weather conditions include cool, windy, stormy weather with turbulent low-pressure cells and cold fronts, these conditions allow the upward mixing and dispersal of air pollutants. 

Chemically, weather can also affect pollution levels. Winds and turbulence mix pollutants together in a sort of giant chemical broth in the atmosphere. Energy from the sun, moisture in the clouds, and the proximity of highly reactive chemicals may cause chemical reactions, which lead to the formation of secondary pollutants. Many of these secondary pollutants may be more dangerous than the original pollutants. 

The importance of the state of the atmosphere and the effects of stability cannot be overstated. The ease with which pollutants can disperse vertically into the atmosphere is mainly determined by the rate of change of air temperature with height (altitude) therefore, air stability is a primary factor in determining where pollutants will travel and how long they will remain aloft. Stable air discourages the dispersion and dilution of pollutants conversely, in unstable air conditions, rapid vertical mixing takes place, encouraging pollutant dispersal, which increases air quality. 

Think of air inside a balloon as an analogy for the air parcel. This theoretically infinitesimal parcel is a relatively well-defined body of air (a constant number of molecules) that acts as a whole. Self-contained, it does not readily mix with the surrounding air. The exchange of heat between the parcel and its surroundings is minimal, and the temperature within the parcel is generally uniform. 

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