Photochemical aqueous-phase secondary organic aerosol formation promotes severe haze formation on the North China Plain
Extremely low visibility events caused by severe haze pollution under high relative humidity conditions on the North China Plain have aroused increasing public attention in recent years. Secondary organic aerosol (SOA) is a major aerosol component during such haze events, however its formation mechanism remains poorly understood. SOA can be classified into gasSOA and aqSOA. The former is formed through gas phase partitioning of low volatility VOCs (volatile organic compounds), and the latter through aqueous phase oxidation of water-soluble VOCs. Recent studies associated high contribution of SOA to aerosol mass during severe pollution episodes with aerosol aqueous-phase reactions due to prevailing high RH conditions. However, aqSOA formation pathways remain elusive, i.e. how photochemistry and aqueous-phase chemistry coordinate to promote SOA formation, the relative importance of nighttime dark aqueous-phase reactions and daytime aqueous-phase photochemistry in aqSOA formation, etc.
To tackle with these questions, Dr. Ye Kuang collaborated with Dr. Wanyun Xu from the Chinese Academy of Meteorological Sciences and Professor Yele Sun from the Chinese Academy of Sciences in an observational study in the polluted rural region of the North China Plain (NCP). Their results demonstrate that SOA exhibits significant diurnal cycles with a rapid SOA formation during daytime and relatively small variations during nighttime. Nighttime dark aqSOA formation was only observed during the fogs, which contributed only negligibly to the growth of SOA due to fog scavenging processes. Daytime SOA formation was strongest during the days with nighttime fogs and especially high daytime RH, and was proved to be mostly contributed by aqSOA formation. During the period with relatively higher RH but no nighttime fogs, both gasSOA and aqSOA contributed to daytime SOA growth, while during the low RH period, gasSOA dominantly contributed to SOA formation and SOA production rates were the lowest in comparison. These results indicate that photochemical aqSOA formation was mostly responsible for the growth of SOA during severe haze events with high relative humidity.
Simultaneous increases in ultraviolet radiation, photooxidant and aqSOA precursor levels worked together to promote the daytime photochemical aqSOA formation. Additionally, biomass burning activities in rural areas also promoted photochemical aqSOA formation by adding to the levels of photooxidants and aqSOA precursors. Their research stresses the importance of strict control on biomass burning activities, especially under high RH conditions, and highlights that laboratory and modelling studies need to focus more on photochemical aqueous-phase SOA formation.
This research is published on Environmental Science & Technology (https://pubs.acs.org/doi/full/10.1021/acs.est.9b06836). This work is supported by National Key Research and Development Program of China (Grant 2017YFC0210104,2016YFC0202300, and 2017YFC0212803), National Natural Science Foundation of China (91644218 and 41505107, 41877302 and 41805109), the National research program for key issues in air pollution control (DQGG0103), the Guangdong Innovative and Entrepreneurial Research Team Program (Research team on atmospheric environmental roles and effects of carbonaceous species: 2016ZT06N263), Guangdong Natural Science Funds for Distinguished Young Scholar (grant No. 2018B030306037), and the Basic Research Fund of CAMS (2017Z011).