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Linking North American Summer Ozone Pollution Episodes to Subseasonal Atmospheric Variability

Ozone concentrations in the planetary boundary layer (PBL) are positively correlated with surface air temperature due to shared influences, including incident solar radiation and PBL stagnancy, as well as the temperature-sensitive emission of ozone precursor compounds. While previous studies have linked heat waves in North America to modes of subseasonal atmospheric variability, such analyses have not been applied to summertime ozone pollution episodes. This study investigates a possible link between subseasonal atmospheric variability in reanalysis data and summertime ozone pollution episodes identified in almost thirty years of in-situ ozone measurements from the United States Air Quality System (AQS) and the Canadian National Air Pollution Surveillance Program (NAPS). AQS and NAPS stations are grouped into regions likely to experience simultaneous extreme ozone concentrations using statistical clustering methods. Composite meteorological patterns are calculated for ozone episodes in each of these regions. The same analysis is applied to heat waves identified in AQS temperature records for comparison. Local meteorological features during typical ozone episodes include extreme temperatures and reduced cloud cover related to anomalous synoptic-scale anticyclonic circulation aloft. These anticyclonic anomalies are typically embedded in wave trains extending from the North Pacific to North Atlantic. Sea surface temperature anomalies with subseasonal persistence may play a role in the formation of these wave trains. These long-lived circulation patterns may provide a means to increase air quality prediction lead-times and to estimate the frequency of ozone pollution episodes under climate change.