Characteristics of heavy aerosol pollution during the 2012–2013 winter in Beijing, China

A comprehensive measurement was carried out to analyze the heavy haze events during 2012–2013 winter in Beijing. The measured variables include some important meteorological parameters, such wind directions, wind speeds, relative humidity (RH), planetary boundary layer (PBL), solar radiation, and visibility. The aerosol composition and concentrations (including particular matters (PM2.5), nitrate (NO3), sulfate (SO4), ammonium (NH4)) as well as their gas-phase precursors (including nitrogen oxides (NOx) and sulfur dioxide (SO2)) were analyzed during the period between Nov. 16, 2012 and Jan. 15, 2013. The results show that the hourly mean concentrations of PM2.5 often exceeded 200 μg/m3, with a maximum concentration of 600 μg/m3 on Jan. 13, 2013. The relative humidity was increased during the haze events, indicating that both aerosol concentrations and RH had important effect on the reduction of visibility, causing the occurrence of the haze events. Because the wind speeds were generally low (less than 1 m/s) during the haze event, the vertical dispersion and the PBL heights were very important factors for causing the strong variability of aerosol concentrations. This study also finds that under the lower visibility condition, the conversion from the gas-phase of NOx and SO2 to the particle phase of NO3 and SO4 were higher than the values under the higher visibility condition. Because the lower visibility condition was corresponding to the lower photochemical activity than the higher visibility condition, the higher conversion from gas phase to particle phase in the lower visibility condition indicated that there was important heterogeneous formation of NO3 and SO4 during the heavy haze events.