Trends in Hourly Ozone, Daily Ozone, and Ozone Health Metrics across the United States, the European Union, and China

Over the past 25 years, there have been dramatic changes in ozone precursor emission levels. Emissions in the US and EU have decreased substantially, especially in the last 15 years, and Chinese emissions are estimated to have peaked around 2010. These emissions changes, along with changes in meteorology have led to spatial and temporal shifts in ozone. Here we present a comprehensive set of work that examines how ozone has changed in these regions. We start by examining changes in 8-hr daily maximum ozone across the United states since 1998 and show that the direction and magnitude of change varies with degree of urbanization and season. This analysis shows a tendency for ozone to have decreased in summer, in rural locations, and at the high end of the ozone distribution (e.g. 75th percentile and 95th percentile concentrations) and a tendency for ozone to have increased in winter, in urban locations and at the low end of the ozone distribution (e.g. 5th percentile and 25th percentile concentrations). The US analysis is supplemented by modeling simulations that show how mid-range and high-end ozone is predicted to respond to large reductions in NOx emissions in several urban areas. Finally, a comprehensive evaluation compares US, EU, and Chinese trends since 1990 in the hourly ozone distribution as well as in key ozone health metrics. This work highlights the fact that a compression of the ozone distribution observed at many sites in the US and the EU (i.e. decreases in peak hourly ozone concentrations and increases in low-end ozone concentrations) can lead either to increases or decreases in key health indices depending on which health metric is used. In contrast, Chinese measurements show that the entire ozone distribution has shifted upwards and associated ozone health metrics have also increased significantly.