Urban ozone trends in Europe and the USA (2000–2021)

Trends in urban maximum daily 8 h average ozone concentrations, alongside annual ozone episodic and exposure metrics, across Europe and the United States of America were explored between 2000–2021. Using surface monitoring site data from the TOAR-II and European Environment Agency databases, piecewise quantile regression (PQR) analysis was performed on 353 time series (204 European, 149 USA). The PQR analysis permitted 2 break points over the period to balance the intent to describe changes over a large time period, while still capturing the abrupt changes that can occur in urban atmospheres. We found that there were many sites across Europe with high certainty increasing trends in the 5th and 50th quantiles, whereas the majority of high certainty trends in the 95th quantile were found to be decreasing. A similar pattern was observed across the USA, with 5th quantile trends increasing and 95th quantile trends decreasing, though a small but increasing number of sites showed a return to increasing trend at τ = 95. To group trends, hierarchical clustering with dynamic time warping was employed and these groups used to guide analysis. Clustering was typically regional across Europe and the USA, and increasing trends were identified across southern and the central alpine regions of Europe, and in California and the Intermountain West of the USA. Recent high certainty increasing trends in the Intermountain West may be related to warmer summers and increased wildfire events in the region, highlighting the need to monitor changing ozone trends with climate change, to assess human exposure risk to elevated levels of ozone.