Unexpectedly Efficient Aging of Organic Aerosols Mediated by Autoxidation

Multiphase oxidative aging is a ubiquitous process for atmospheric organic aerosols (OA). But its kinetics was often found to be slow in previous laboratory studies where high hydroxyl radical concentrations ([center dot OH]) were used. In this study, we performed heterogeneous oxidation experiments of several model OA systems under varied aging timescales and gas-phase [center dot OH]. Our results suggest that OA heterogeneous oxidation may be 2-3 orders of magnitude faster when [center dot OH] is decreased from typical laboratory flow tube conditions to atmospheric levels. Direct laboratory mass spectrometry measurements coupled with kinetic simulations suggest that an intermolecular autoxidation mechanism mediated by particle-phase peroxy radicals greatly accelerates OA oxidation, with enhanced formation of organic hydroperoxides, alcohols, and fragmentation products. With autoxida-tion, we estimate that the OA oxidation timescale in the atmosphere may be from less than a day to several days. Thus, OA oxidative aging can have greater atmospheric impacts than previously expected. Furthermore, our findings reveal the nature of heterogeneous aerosol oxidation chemistry in the atmosphere and help improve the understanding and prediction of atmospheric OA aging and composition evolution.

Automated summary

In this study, the oxidation kinetics of atmospheric organic aerosols (OA) were investigated under various aging timescales and gas-phase hydroxyl radical concentrations. The results show that OA heterogeneous oxidation is 2-3 orders of magnitude faster at atmospheric hydroxyl radical levels compared to typical laboratory conditions. The enhanced oxidation of OA is attributed to an intermolecular autoxidation mechanism mediated by particle-phase peroxy radicals, leading to the formation of organic hydroperoxides, alcohols, and fragmentation products. These findings highlight the significant atmospheric impacts of OA oxidative aging and provide insight into the chemistry of heterogeneous aerosol oxidation in the atmosphere.