Aqueous Phase Photochemical Transformations of Phenolic SOA: Effects of Organic Concentration and Ionic Strength on SOA Chemistry

CHRISTOPHER NIEDEK, Wenqing Jiang, Yuejun Zhou, Lan Ma, Cort Anastasio, Qi Zhang, University of California, Davis

     Abstract Number: 481
     Working Group: Aerosol Chemistry

Abstract
Understanding the formation mechanisms and properties of secondary organic aerosols (SOA) in atmospheric aqueous phases is critical for predicting and mitigating air pollution on a regional scale as well as global climate change. Biomass burning can generate significant amounts of phenolic species and previous studies have demonstrated their potentially large contribution to SOA mass via aqueous phase transformations. However, most of the studies have been performed under relatively dilute, cloud/fog water conditions. Concentrations of organics as well as ionic strength can vary greatly from cloud/fog waters to aerosol liquid water (ALW) conditions. The effects of differing concentrations of phenolics, photosensitizers, and ionic strength on SOA formation remains understudied.

This work is focused on characterizing aqueous phase SOA (aqSOA) formed from triplet excited states of organic compounds (3C*) oxidation of guaiacyl acetone (GA) under varying initial organic concentrations and ionic strengths using a short-pathlength photoreactor and a high-resolution aerosol mass spectrometer (HR-AMS). Our new photoreactor design allows for a variable light pathlength which makes exploring ALW-type conditions, where high concentrations of absorbing species can make light absorption a practical concern, more accessible. Previous work has shown that high ionic strength conditions have little effect on GA decay rates, but changes to aqSOA chemistry and kinetics were not previously explored. The mass yields, bulk composition, and photochemical evolution of the aqSOA were studied under a range of initial [GA], [3C*], and ionic strengths. High ionic strength conditions have little effect on the GA decay rate, but it can increase the formation rates of key aqSOA tracers like oligomers. This work suggests that ionic strengths typically found in ALW can modify formation rates of key brown carbon species (e.g. phenolic oligomers), suggesting the influence of phenols on the radiative properties of clouds indicated by previous studies may be greater in particles and ALW.