Synoptic Analysis of Meteorological Conditions Associated with Elevated Near-Surface Ozone Concentrations in Illinois

Atmospheric ozone (O₃) in the stratosphere is beneficial to humans, as it prevents harmful ultraviolet radiation from penetrating to the surface. However, near-surface O₃, produced by chemical reactions involving naturally-occurring gases and pollutants, results in poor health outcomes for humans, particularly respiratory and cardiovascular issues. Despite the serious health risks posed by O₃ and its relevance for urban areas in the Midwest, few studies have examined the relationship between ground level O₃ and meteorological conditions in Illinois. We examine synoptic-scale atmospheric circulation features and corresponding surface weather conditions associated with elevated or extreme O₃ occurrences at nine locations throughout Illinois between 1980 and 2015. We find that over 98% of all extreme O₃ days occur between May and September, and they are significantly associated with dry tropical, dry moderate, and moist tropical air masses. These air masses correspond with elevated air temperature, solar radiation, and reduced humidity, consistent with previous studies connecting O₃ concentrations to weather. The similarity in meteorological conditions between all nine stations suggests the occurrence of extreme O₃ days is generally attributable to large-scale atmospheric processes. Significant, positive trends in the frequencies of dry moderate, dry tropical, and moist moderate air masses in Illinois suggests possible increased exposure to extreme O₃ conditions in the future.