Effect of relative humidity on SOA formation from aromatic hydrocarbons: Implications from the evolution of gas- and particle -phase species

Relative humidity (RH) plays a significant role in secondary organic aerosol (SOA) formation, but the mechanisms remain uncertain. Using a 30 m3 indoor smog chamber, the influences of RH on SOA formation from two conventional anthropogenic aromatics (toluene and m-xylene) were investigated from the perspective of both the gas- and particle- phases based on the analysis of multi-generation gas-phase products and the chemical composition of SOA, which clearly distinguishes from many previous works mainly focused on the particle-phase. Compared Lo experiments with RH of 2.0%, SOA yields increased by 11.1%133A% and 4.0%-64.5% with higher RH (30.0%-90.0%) for toluene and m-xylene, respectively. The maximum SOA concentialion always appeared at 50.0% RH, which is consistent with the change trend of SOA concenfration with RH in the summertime field observation. The most plausible reason is Thal the highest gas-phase OH concentialion was observed al 50.0% RH, when the. increases in gas-phase OH formation and OH uptake Lo aerosols and chamber walls with increasing RH reached a balance. The maximum OH concentration Lion was accompanied by a notable decay of second-generation products and formation of third-generation products al 50.0% RH. With further increasing RH, more second-generation products with insufficient oxidation degree will be partitioned into the aerosol phase, and the aqueous-phase oxidation process will also be promoted due to the enhanced uptake of OH. These processes concurrently caused the O/C and oxidation state of carbon (OSc) to first increase and then slightly decrease. This work revealed the complex influence of RH on SOA formation from aromatic VOCs through affecting the OH concentration, partitioning of advanced gas-phase oxidation products as well as aqueous-phase oxidation processes. Quantitative studies to elucidate the role of RH in the partitioning of oxidation products should be conducted to further clarify the mechanism of the influence of RH on SOA formation. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

This research revealed the complex influence of relative humidity (RH) on secondary organic aerosol (SOA) formation from aromatic VOCs, which include affecting the OH concentration, partitioning of advanced gas-phase oxidation products, and aqueous-phase oxidation processes. Further quantitative studies are needed to elucidate the role of RH in the partitioning of oxidation products and clarify the mechanism of the influence of RH on SOA formation.