> Progress by Xu Wanyun and others in the study of the formation of secondary organic aerosol
Organic aerosol constitutes a large fraction of particulate matter 20-90%, with Secondary Organic Aerosol SOA being a significant contributor to a heavy pollution process. Due to the complexity of organic aerosol components, the formation of SOA remains poorly understood. Volatile organic compounds VOCs, the volatility of which is reduced through photochemical oxidation, condense directly on the surface of particulate matter as SOA gasSOA, while water-soluble VOCs also generate SOA aqSOA through aqueous-phase reaction, with the latter forming AqSOA at nighttime under high RH conditions as well as at daytime through photochemical reaction. It is still inconclusive that which process dominantly contributes to SOA formation under different pollution conditions in North China Plain.
Based on the field observations made at the agro-ecological meteorological station in Gucheng, Hebei Province in autumn and winter of 2018, Xu Wanyun, associate research professor from the Chinese Academy of Meteorological Sciences CAMS, Dr. Kuang Ye from Jinan University and Sun Yele, research professor from the Institute of Atmospheric Physics of the Chinese Academy of Sciences IAP CAS, found the pattern of fast daytime growth of SOA under various RH conditions. Daytime photochemical aqSOA production dominantly contributed to SOA during the nighttime periods with fog RH, daily average 77 ± 13% and caused fast daytime growth of SOA. During high RH periods daily average 53 ± 19%, aqSOA and gasSOA jointly contributed to daytime SOA formation, while during low RH periods daily average 38 ± 19%, the gasSOA process dominantly contributed to SOA formation. Nighttime aqSOA formation was observed during a fog period, a process that contributed negligibly to the increased SOA concentration due to the wet deposition in the fog. It is shown that with the increase in the concentration of photochemical oxidants in the daytime, VOCs were rapidly oxidized into water-soluble VOCs, which contributed to aqSOA formation. In addition, biomass combustion simultaneously increases the concentration of aqSOA precursors and atmospheric oxidants, which makes an important contribution to aqSOA formation. Therefore, more strict measures should be taken for biomass combustion in the future air pollution control process, especially in high RH environments.
The study has been published in Environmental Science and Technology http://dx.doi.org/10.1021/acs.est.9b06836