Scientific Findings from the
Southern Oxidants Study (SOS)

[a pdf version of this document is available under Publications]


During the first ten years of the SOS program, the major focus in research was on regional oxidant (especially ozone) pollution and the earliest objectives were to identify ozone-related chemical and meteorological factors that were unique to the SOS region – such as the large quantities of biogenic VOC emissions, greater frequencies of air stagnation days, and abundant NOx-producing lightning strikes during summer thunderstorms. As the SOS program matured, however, the focus broadened to include both ozone and particulate matter (especially PM2.5) pollution and more emphasis on quantitative comparisons regarding ozone and PM pollutant exposures with those in other regions of the US, Canada, and Mexico.

Thus, some of the research studies within SOS were designed to:

  1. Identify and quantify many of the unique natural processes (including plant physiological and ecological processes, topographic features, and meteorological and climatological processes) that influence the formation and accumulation of ozone, other photochemical oxidants, and particulate matter (especially PM2.5) pollution in the SOS region;
  2. Identify and quantify some of the unique changes in human activities (agricultural, forestry, industrial, commercial development, and both demographic and land-use patterns) that influence ozone and PM exposures in the SOS region; and
  3. Compare and contrast these natural processes in the atmosphere and human activities in the SOS region with those in other regions of the United States, Canada, and Mexico – especially with regard to development of optimal management strategies and tactics for efficient and cost-effective control of the accumulation of ozone, PM, and/or regional haze.

From the standpoint of ozone and PM management, it is very important to recognize the differences between ozone formation and ozone accumulation. The air concentration of ozone at a given location near the ground is the net result of the following six different processes in the atmosphere above that location:

  1. The rate of ozone formation from chemical precursors emitted at or transported into that location,
  2. The rate of ozone destruction by chemical reactions in the same air parcel,
  3. The rate of vertical transport of ozone from the stratosphere (or from an ozone reservoir aloft, within the atmosphere) to ground level at that location,
  4. The rate of horizontal transport of ozone from up-wind sources,
  5. The rate of atmospheric deposition of ozone from the air to vegetation or other surfaces exposed to the air at that location, and
  6. The rate of ozone atmospheric dispersion and dilution as a result of mixing with cleaner air during advection or when the height of the planetary boundary layer rises.

In essence, the air concentration of ozone is a kind of “algebraic sum" of all six of these processes:

  1. increasing (+) if the ozone formation rate is high,
  2. decreasing (-) if the ozone destruction rate is high,
  3. increasing (+) if the rate of vertical transport of ozone is high,
  4. decreasing (-) if the rate of horizontal transport of ozone is high,
  5. decreasing (-) if the ozone deposition rate is high, and
  6. decreasing (-) if the ozone dispersion (dilution) and/or advection rates are high.

The National Ambient Air Quality Standards (NAAQS) for ozone and PM are based on maximum air concentrations that are allowed to accumulate at ground level in the atmosphere at any given urban, suburban, rural, or remote location within a certain well-defined period of time. For example, the recently promulgated "8-hour NAAQS" for ozone requires that air concentrations of ozone be maintained below an average of 80 ppm over any eight-hour period on all but two allowable days during any three-year period.