Characteristics of aerosol size distributions and chemical compositions during wintertime pollution episodes in Beijing

To characterize the features of particle pollution, continuous measurements of particle number size distributions and chemical compositions were performed at an urban site in Beijing in January 2013. The particle number and volume concentration from 14nm to 1000nm were (37.4±15.3)×103cm−3 and (85.2±65.6)μm3cm−3, respectively. N-Ait (Aitken mode) particles dominated the number concentration, whereas N-Acc (accumulation mode) particles dominated the volume concentration. Submicron particles were generally characterized by a high content of organics and SO42−, and a low level of NO3− and Cl−. Two types of pollution episodes were observed, characterized by the “explosive growth” (EXP) and “sustained growth” (SUS) of PM2.5. Fine particles greater than 100nm dominated the volume concentration during the ends of these pollution episodes, shifting the maximum of the number size distribution from 60nm to greater than 100nm in a few hours (EXP) or a few days (SUS). Secondary transformation is the main reason for the pollution episodes; SO42−, NO3− and NH4+ (SNA) accounted for approximately 42% (EXP) and greater than 60% (SUS) of the N-Acc particle mass increase. The size distributions of particulate organics and SNA varied on timescales of hours to days, the characteristics of which changed from bimodal to unimodal during the evolution of haze episodes. The accumulation mode (peaking at approximately 500–700nm) was dominated by organics that appeared to be internally mixed with nitrate or sulfate. The sulfate was most likely formed via heterogeneous reactions, because the SOR was constant under dry conditions (RH<50%) and began to increase when RH>50%, suggesting an important contribution from heterogeneous reactions with abundant aerosol water under wet conditions. Finally, the correlations between [NO3−]/[SO42−] and [NH4+]/[SO42−] suggest that the homogenous reaction between HNO3 and NH3 dominated the formation of nitrate under conditions of lower aerosol acidity. Therefore, controlling the precursors of SNA will effectively help to reduce the fine particulate pollution during winter in Beijing.