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

SummaryRapid industrialization and urbanization during recent decades are having dramatic effects on urban soil properties and lead to large discharges of pollutants, which inevitably affect the health of the soil, ecosystems and human populations. This paper provides a systematic review of the relations between urban soil and human health. First, it summarizes the organic and inorganic pollutants in urban soil and their potential risks to human health. Second, the relations between urban greenbelt land, soil microbial diversity and human health are also explored. Third, we propose that future research should focus on the integration of assessments of health risks with exposure pathways and site characteristics. Bioavailability-based risk assessment frameworks for pollutants in urban soil can elucidate the complicated relations between urban soil, pollutant exposure and human health in cities. Finally, management of urban soil and policy should be strengthened in the future to maintain its sustainable development and utilization. More effort should be directed to understanding the relations between soil microbial diversity, green space and human health in cities.