posted on 2020-03-10, 15:05authored byYe Kuang, Yao He, Wanyun Xu, Bin Yuan, Gen Zhang, Zhiqiang Ma, Caihong Wu, Chaomin Wang, Sihang Wang, Shenyang Zhang, Jiangchuan Tao, Nan Ma, Hang Su, Yafang Cheng, Min Shao, Yele Sun
Secondary
organic aerosol (SOA) constitutes a large fraction of
organic aerosol worldwide, however, the formation mechanisms in polluted
environments remain poorly understood. Here we observed fast daytime
growth of oxygenated organic aerosol (OOA) (with formation rates up
to 10 μg m–3 h–1) during
low relative humidity (RH, daytime average 38 ± 19%), high RH
(53 ± 19%), and fog periods (77 ± 13%, fog occurring during
nighttime with RH reaching 100%). Evidence showed that photochemical
aqueous-phase SOA (aqSOA) formation dominantly contributed to daytime
OOA formation during the periods with nighttime fog, while both photochemical
aqSOA and gas-phase SOA (gasSOA) formation were important during other
periods with the former contributing more under high RH and the latter
under low RH conditions, respectively. Compared to daytime photochemical
aqSOA production, dark aqSOA formation was only observed during the
fog period and contributed negligibly to the increase in OOA concentrations
due to fog scavenging processes. The rapid daytime aging, as indicated
by the rapid decrease in m,p-xylene/ethylbenzene
ratios, promoted the daytime formation of precursors for aqSOA formation,
e.g., carbonyls such as methylglyoxal. Photooxidants related to aqSOA
formation such as OH radical and H2O2 also bear
fast daytime growth features even under low solar radiative conditions.
The simultaneous increases in ultraviolet radiation, photooxidant,
and aqSOA precursor levels worked together to promote the daytime
photochemical aqSOA formation. We also found that biomass burning
emissions can promote photochemical aqSOA formation by adding to the
levels of aqueous-phase photooxidants and aqSOA precursors. Therefore,
future mitigation of air pollution in a polluted environment would
benefit from stricter control on biomass burning especially under
high RH conditions.