Change point analysis of seasonal ozone trends and distribution across the United States

We present a comprehensive regional analysis of trends and variability in daily maximum 8-hour average ozone across the contiguous United States over 1990-2023. At the first stage, we evaluate the trends based on various seasonal percentiles at all available monitoring sites. Secondly, the overall regional trends (Western and Eastern USA) are derived at various seasonal percentiles. Results show that consistent and strong negative trends can be found in the eastern USA at the 95th and 50th percentiles in spring, summer and fall since the early 2000s, while winter trends are increasing. The similar seasonal trends are found in the Western USA, but with weaker magnitudes of trends. Throughout the analysis implications of the correlations between heatwave frequency/intensity and ozone variability are discussed.