Photochemical Aqueous-Phase Reactions Induce Rapid Daytime Formation of Oxygenated Organic Aerosol on the North China Plain

Abstract

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 H 2 O 2 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.

Secondary aerosol formation alters CCN activity in the North China Plain

“…In summary, in high RH events, the enhancement of CCN activity was lighter but at broader particle size range than that in event 3. As reported by Kuang et al (2020b), the SA formation in event 1 and 2 under high RH conditions occurred in both gaseous phase and aqueous phase, and formed SA mainly at larger particle size up to 1um, while SA formation in event 3 under low RH conditions dominantly occurred in gasphase and formed SA at particle size smaller than 300nm. At particle size smaller than 200nm where https://doi.org/10.5194/acp-2020-939 Preprint.…”

“…As the decrease of mass concentration of PM2.5 PA was much smaller than the increase of PM2.5 SA mass concentration, the increase of NR-PM2.5 mass concentration to can be expected to be similar to the increase of PM2.5 SA mass concentration. As reported by Kuang et al (2020b), SA during daytime were mainly formed at larger particle sizes characterized, as there were increase of Vconc in particle size range of 400nm to 1um. In detail, there were significant increases of particle number concentration (Nconc) in particle size range of 150nm to 1000 nm ( Fig.…”

“…In following discussions, high RH conditions correspond to the periods before 4 th Dec. whose daily maximum and minimum RH are generally higher than 75% and 50%, respectively, especially from 25 th Nov to 27 th Nov (Event 1) and from 30 th Nov to 2 nd Dec (Event 2) with fog events during these days (blue shaded areas), and low RH conditions correspond to the periods after 4 th Dec whose daily maximum and minimum RH are generally lower than 70% and 30%, especially from 9 th Dec to 11 th Dec (Event 3). As reported in Kuang et al (2020b), these high RH conditions promote SA formation in aerosol liquid water content and lead to a persistent increase of NCCN, mass concentration of NR-PM2.5 and especially mass concentration of PM2.5 SA during event 1 and 2. Atmospheric RH after 4 th Dec are generally lower, with daily maximum lower than 70% and the dominating SA formation were in gas-phase, that generate much less SA than aqueous-phase formation (Kuang et al, 2020b).…”

“…As reported in Kuang et al (2020b), these high RH conditions promote SA formation in aerosol liquid water content and lead to a persistent increase of NCCN, mass concentration of NR-PM2.5 and especially mass concentration of PM2.5 SA during event 1 and 2. Atmospheric RH after 4 th Dec are generally lower, with daily maximum lower than 70% and the dominating SA formation were in gas-phase, that generate much less SA than aqueous-phase formation (Kuang et al, 2020b). Thus, the persistent increases of NCCN and PM2.5 from 7 th Dec to 11 th Dec (Event 3) are much weaker than those in events 1 and 2.…”

“…During the same time, PA mass decreased by 50% and the variation of NR mass was small. Under low RH condition, SA formation mainly contributed mass enhancements for smaller particle size as there were enhances of Vconc at particle size range of 150nm to 300nm rather than larger than 300nm (Kuang et al, 2020b). In detail, there were increase of Vconc in the range of 150nm to 300nm (Fig.…”