Reactive nitrogen chemistry in aerosol water as a source of sulfate during haze events in China

TLDR

Fine‑particle pollution during winter haze threatens the health of more than 400 million people in the North China Plain, with sulfate being a major component that current air‑quality models fail to predict due to weak photochemistry. The study aims to identify the missing source of sulfate and particulate matter during haze events. The authors propose that aerosol water acts as a reactor where alkaline components trap SO₂, which is oxidized by NO₂ to sulfate, and the process is self‑amplifying because higher aerosol mass increases water content and sulfate production. They find that reactive nitrogen chemistry in aerosol water explains the record sulfate concentrations of up to ~300 µg m⁻³ observed during the January 2013 Beijing haze.

Abstract

Fine-particle pollution associated with winter haze threatens the health of more than 400 million people in the North China Plain. Sulfate is a major component of fine haze particles. Record sulfate concentrations of up to ~300 μg m-3 were observed during the January 2013 winter haze event in Beijing. State-of-the-art air quality models that rely on sulfate production mechanisms requiring photochemical oxidants cannot predict these high levels because of the weak photochemistry activity during haze events. We find that the missing source of sulfate and particulate matter can be explained by reactive nitrogen chemistry in aerosol water. The aerosol water serves as a reactor, where the alkaline aerosol components trap SO2, which is oxidized by NO2 to form sulfate, whereby high reaction rates are sustained by the high neutralizing capacity of the atmosphere in northern China. This mechanism is self-amplifying because higher aerosol mass concentration corresponds to higher aerosol water content, leading to faster sulfate production and more severe haze pollution.