Photochemical Oxidant Transport: Mesoscale Lake Breeze and Synoptic-Scale Aspects

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Abstract
Limited ozone monitoring in the Milwaukee area by the Environmental Protection Agency in 1971 revealed surprisingly high values (19 pphm, maximum hourly average) and frequent exceedences of the 8 pphm federal standard. Since 1973 routine monitoring in southeastern Wisconsin has recorded hourly averages as high as 30 pphm, with frequent excursions beyond standard and episode alert dosage. In addition, high ozone readings at rural sites throughout Wisconsin have been noted. These data are discussed with respect to changing concepts of the photochemical oxidant phenomenon. The Wisconsin data appear consistent with the view that long-range (≳200 km) transport of oxidants (and/or their precursors) is more the rule than the exception. It appears that entire anticyclones can become polluted, with widespread ozone levels approaching 8 pphm on the western side of the system. More importantly, along the western shore of Lake Michigan, adding to this general pattern, are lake-induced mesoscale wind systems which apparently result in large amounts of ozone (or precursors) being transported from the Chicago metropolitan area into southeastern Wisconsin. On a smaller scale, a pattern of high oxidant levels paralleling the lake shore from 1 to about 8 km inland is explained in terms of fumigation of oxidants stored aloft with the upper portion of a lake breeze or other onshore flows.

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