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

Abstract. This paper outlines a comprehensive trend assessment of surface ozone observations across the conterminous USA over 1990–2023. A change point detection algorithm is applied to evaluate seasonal trends at various percentiles. We found that highly consistent and robust negative trends in extreme values have 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 Southwestern and Midwestern regions of the country. The impact of a potential climate penalty that might offset some of the improvement in the ozone extremes is also investigated based on various heat wave metrics. By comparing ozone threshold exceedances, we found that the exceedance probabilities during heat waves are higher than those under normal conditions; moreover, the differences have decreased over time, as the effectiveness of emission controls has led to a great reduction in ozone extremes under both heat wave and normal conditions. When the increasing heat wave trends are accounted for, we find evidence that decreases in exceedances during heat waves have likely halted at 20 %–40 % of sites, depending on heat wave definitions. By identifying monitoring sites with (1) reliably decreasing ozone exceedances and (2) reliably increasing co-occurrences of ozone exceedances and heat wave 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.