Highly Oxidized Molecules Make a Significant Contribution to Enhanced Aromatic-Derived Secondary Organic Aerosol under a Humid Environment

Enhanced mass concentrations of aromatic-derived secondary organic aerosol (SOA) are frequently observed during humid-haze events. However, the influencing mechanism of relative humidity (RH) in aromatic-derived SOA formation remains incompletely understood. Here, the RH dependence of SOA formation in the presence of NOx was explored by a series of chamber experiments for toluene (TOL) and 1,3,5-trimethylbenzene (TMB) photooxidation. The yield of TOL SOA and TMB SOA increased by 221% and 52% with increasing RH from similar to 8% to similar to 70%, respectively. Analytical results from a high-resolution mass spectrometer showed that SOA constituents with high oxygen content (O/C > 0.6) were more abundant in SOA formed in the similar to 70% RH experiment. The elevated yields and O/C of SOA could be attributed to the promoted formation and particle-phase diffusivity of highly oxidized molecules. In addition, in comparison with TMB, TOL could produce more unsaturated aldehydes, which are oxidized into carboxylic acids with high O/C, leading to a more sensitive response of TOL SOA formation to the change in RH. Our work provides mechanistic insights into RH roles in aromatic SOA formation and is helpful for a better understanding of humid-haze events.