Surface ozone trend variability across the United States and the impact of heatwaves (1990–2023)

Abstract. This study conducts a comprehensive trend assessment of surface ozone observations across the conterminous USA over 1990–2023. A changepoint detection algorithm is applied to evaluate seasonal trends at various percentiles. Based on the results obtained from regional-scale analysis, we found that highly consistent and robust negative trends of extreme values occurred in spring, summer and fall since the 2000s across the eastern USA. A less strong, but similar picture is found in the western USA, while increasing winter trends are commonly observed in the Southwest and Midwest. The impact of a potential climate penalty that might offset some of the improvement in the ozone extremes is also investigated based on various heatwave metrics. By comparing threshold exceedances, we found that the exceedance probabilities during heatwaves are higher than normal conditions, but the differences have decreased over time because the effectiveness of emission controls led to a great reduction of ozone extremes for both heatwave and normal conditions. When the increasing heatwave trends are accounted for, we find evidence that the decrease of exceedance trends have likely halted during heatwave events at 20 %–40 % of sites. By identifying monitoring sites with (1) reliably decreasing ozone exceedances and (2) reliably increasing co-occurrences of ozone exceedances and heatwave events, we can show that several sites in California have been impacted by the ozone climate penalty (1995–2022). These findings are limited by the availability of long-term continuous ozone records, which are sparsely distributed across the USA and typically less than 30 years in length.