This is the function of micro-scale effects being interpolated over a larger area. The increased incident solar radiation during the summertime as well as more 'stagnant' air masses make ozone a summertime occurence. The presence of the subsidence inversion limits venting and causes pollution concentrations to increase. Ozone at the surface decreases rapidly at sunset. The net result is that although ozone decreases rapidly at the surface, ozone above the nocturnal inversion remains relatively unaffected.
Thunderstorms rapidly reduce ozone levels.
This is in response to the extreme vertical venting associated with the updraft and downdraft. In the event of an isolated cell in close proximity to a monitor, ozone at the affected site will suddenly drop on the order of 50-100 ppb at one location. The effect on the ozone map will be an isolated clean area within a region of higher ozone. In the event of a squall line, the effect is less isolated and rather spectacular, as what appears to be a wall of clean air will take over the region. This effect, also known as 'NOx scavenging', results from 'too much of a bad thing'.