The lake breeze–ground‐level ozone connection in eastern Wisconsin: a climatological perspective

Abstract

AbstractThe Lake Michigan Air Quality Region (LMAQR) experiences exceedances of the 1 h health standard for ozone numerous times each summer. Previous short‐term investigations have revealed that the lake breeze circulation is connected with very high levels of ozone in eastern Wisconsin (EWI). Findings from one of the more recent short‐term research efforts have led to the development of a generalized conceptual model that details the role that the lake breeze circulation plays in transporting ozone‐rich air of the Lake Michigan conduction layer onshore to EWI. Short‐term studies, however, are limited by the small number of cases examined. To understand the ozone–lake breeze relationship from a climatological perspective, we analysed the spatial and temporal pattern of 1 h ozone exceedances in EWI during the months of May through to September, over the period 1985–99. Further, we used Laird et al.'s recently developed technique for discriminating lake breeze days to determine which exceedance‐days over the period of our climatology occurred in association with lake breezes.Our results show: (1) a decrease in both magnitude and frequency of exceedances of the 1 h ozone standard in EWI with increasing distance from the lakeshore; (2) a positive correlation between average onset time of the initial exceedance‐hour and a site's distance from Lake Michigan; (3) a very high percentage of initial exceedance‐hours occurring in association with southeasterly surface air flow; and (4) exceedances occurring in association with lake breezes 82.1% of the time at near‐shore sites.Collectively, our findings provide strong evidence that the lake breeze circulation is closely associated with the distribution of ozone in EWI in both space and time. Moreover, our results show that the lake breeze circulation is directly associated with a significant proportion of the exceedances of the 1 h ozone standard in EWI–particularly at near‐shore sites. Thus, mandated reductions in regional‐scale NOx may be sufficient to allow ‘inland’ locations in EWI to meet the new 8 h, 80 ppb Environmental Protection Agency health standard. However, further reductions in volatile organic compounds and NOx within the major metropolitan centres of the LMAQR will likely be necessary if ‘shoreline’ locations in EWI are to meet the 1 h standard, and the stricter 8 h standard in the future. Copyright © 2002 Royal Meteorological Society.