Characteristic features of the winter haze in northern China include stagnant meteorological conditions with low mixing heights, high relative humidity (RH), large emissions of primary air pollutants, and fast production of secondary inorganic aerosols, especially sulfate (see section M1) ( 1– 5). Persistent haze shrouding Beijing and the North China Plain (NCP) during cold winter periods threatens the health of ~400 million people living in a region of ~300,000 km 2. 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. The aerosol water serves as a reactor, where the alkaline aerosol components trap SO 2, which is oxidized by NO 2 to form sulfate, whereby high reaction rates are sustained by the high neutralizing capacity of the atmosphere in northern China. We find that the missing source of sulfate and particulate matter can be explained by reactive nitrogen chemistry in aerosol water. 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. Record sulfate concentrations of up to ~300 μg m −3 were observed during the January 2013 winter haze event in Beijing. Sulfate is a major component of fine haze particles.

Fine-particle pollution associated with winter haze threatens the health of more than 400 million people in the North China Plain.

Nitrogens reactivity