Worth, Houston, St. Louis, are the major metropolitan areas that exceed the standard two or more times a year. Spatial extrapolation from the monitoring sites can yield estimated areas where exceedances of the standards occur. In contrast, areas exceeding the proposed 8-hour 80 ppb ozone standard extend further from the major metropolitan areas and include a large portion of the central part of the OTAG domain. In general, the atmosphere acts as a dispersing agent for air pollutants. Dispersion can cause both beneficial pollutant reduction near a source and harmful pollutant increases downwind of a source.
Dispersion takes place through horizontal transport and vertical mixing in the lower atmosphere.
Horizontal transport depends on location as well as on elevation above ground, and to some extent on time of day. Such conditions are seen in every episode chosen for OTAG modeling. The south-central and southeastern portions of the OTAG domain, which experience relatively more stagnation, can benefit from this aspect of transport. The atmospheric lifetime of ozone influences the distance over which it is transported. There is evidence that the ozone formation time and removal time in urban plumes is shorter, with a resulting lifetime of about 20 hours while in elevated, NOx-rich point source plumes, the ozone lifetime can be twice as long.