Observational study of the PM2.5 and O3 superposition-composite pollution event during spring 2020 in Beijing associated with the water vapor conveyor belt in the northern hemisphere

Abstract

Unlike the heavy haze situations in previous years, the emergence of a water vapor conveyor belt flowing through North China is an important factor in the foggy-haze event during the 48-h snowfall in Beijing and its surroundings, which disrupted the customary pollution snow removal atmospheric purification.①In the spring of 2020, during a period of negative emissions increment and weak human activity impact, a special interaction between the high humidity content of large-scale continuous water vapor transmission and the weather-scale daily cycling radiation cycle change (due to daily change of sun's zenith angle), as well as the atmospheric microphysical process, provided rare meteorological conditions for the superposition-compound growth of PM2.5 and O3 pollution. The water vapor conveyor belt, through enhanced transmission of easterly wind momentum, induced a high value super-saturation signal (S) and condensation rate (fc), resulting in 48 consecutive hours of snow in Beijing, but was also conducive to fine particulates, including precursor emissions, through a microphysical humidification process, which became synchronized. ②The weather-scale process impact refers to the cloud height rise in the daytime and decrease at night. Due to the daily fluctuations in cloud height, the concentration of O3 followed a coordinated daily cycle. During the day, when the sun's zenith angle going on the top, the PM2.5 and O3 superposition composite effects occur. Therefore, even during a negative emission period, PM2.5 and O3 superposition-composite serious pollution events occur as usual, rather than the expected possible atmospheric cleaning through snow.