Meteorological and chemical causes of heavy pollution in winter in Hohhot, Inner Mongolia Plateau

Abstract

In January 2020, severe and persistent haze events occurred in the plateau city of Hohhot, which was one of the regions with the worst air quality in China. The monthly average concentration of PM1, PM2.5 and PM10 came to 74 μg·m−3, 106 μg·m−3 and 130 μg·m−3, respectively. Coal burning for heating, pollutant transport, and stable atmosphere led to frequent haze events. Local temperature inversion and regional neutral atmospheric stability provided good meteorological conditions for the accumulation of particulate matter. The southerly and southeasterly winds carried in primary and secondary pollutants, as well as large amounts of water vapor, which was an important reason for the haze events this month. The large increase of fine particles was mainly caused by the massive generation of sulfate and nitrate aerosols, with monthly average concentrations of 26 μg·m−3 and 20 μg·m−3, respectively. During the day, SO2 was rapidly converted to SO42− through photochemical and heterogeneous reactions under the effect of sufficient NO2 and high humidity. Meanwhile, the conversion of NOx to NO3− was also enhanced. During the night, suspended fine particles generated during the day accumulated, and the increased humidity strengthened the oxidation of S(IV) to SO42− again and NO2 and N2O5 to NO3− under the effect of NO2 and HONO. The sulfur oxidation rate (SOR) and nitrogen oxidation rate (NOR) rose from ~0.20 and ~0.10 in the cleaning period to ~0.60 and ~0.30 in the pollution period, respectively. In addition, the setting off of fireworks, firecrackers and the traditional bonfires caused a steep rise in PM2.5 to ~1120 μg·m−3 at midnight on the Chinese Spring Festival. A strong dry north wind broke the static and neutral structure of the atmosphere and completely remove the pollutants from the region.