Global Perspective of Drought Impacts on Ozone Pollution Episodes.

Ozone (O 3 ) pollution threatens global public health and damages ecosystem productivity. Droughts modulate surface O 3 through meteorological processes and vegetation feedbacks. Unraveling these influences is difficult with traditional chemical transport models. Here, using an atmospheric chemistry-vegetation coupled model in combination with a suite of existing measurements, we investigate the drought impacts on global surface O 3 and explore the main driving processes. Relative to the mean state, accelerated photochemical rates dominate the surface O 3 enhancement during droughts except for eastern U.S. and western Europe, where reduced stomatal uptakes make comparable contributions. During 1990-2012, the simulated frequency of O 3 pollution episodes in western Europe decreases greatly with a negative trend of -5.5 ± 6.6 days per decade following the reductions in anthropogenic emissions if meteorology is fixed. However, such decreased trend is weakened to -2.1 ± 3.8 days per decade, which is closer to the observed trend of -2.9 ± 1.1 days per decade when year-to-year meteorology is applied because increased droughts alone offset 43% of the effects from air pollution control. Our results highlight that more stringent controls of O 3 precursors are necessary to mitigate the higher risks of O 3 pollution episodes by more droughts in a warming world.