Sulfate aerosols exert profound impacts on human and ecosystem health, weather, and climate, but their formation mechanism remains uncertain. Atmospheric models consistently underpredict sulfate levels under diverse environmental conditions. From atmospheric measurements in two Chinese megacities and complementary laboratory experiments, we show that the aqueous oxidation of SO 2 by NO 2 is key to efficient sulfate formation but is only feasible under two atmospheric conditions: on fine aerosols with high relative humidity and NH 3 neutralization or under cloud conditions. Under polluted environments, this SO 2 oxidation process leads to large sulfate production rates and promotes formation of nitrate and organic matter on aqueous particles, exacerbating severe haze development. Effective haze mitigation is achievable by intervening in the sulfate formation process with enforced NH 3 and NO 2 control measures. In addition to explaining the polluted episodes currently occurring in China and during the 1952 London Fog, this sulfate production mechanism is widespread, and our results suggest a way to tackle this growing problem in China and much of the developing world.sulfate aerosol | severe haze | pollution | human health | climate

Tropospheric ozone (O 3 ) is a trace gas playing important roles in atmospheric chemistry, air quality and climate change. In contrast to North America and Europe, longterm measurements of surface O 3 are very limited in China. We compile available O 3 observations at Mt. Tai -the highest mountain over the North China Plain -during 2003-2015 and analyze the decadal change of O 3 and its sources. A linear regression analysis shows that summertime O 3 measured at Mt. Tai has increased significantly by 1.7 ppbv yr −1 for June and 2.

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