Effect of Condensed-Phase Photochemistry on the Viscosity of Atmospheric Secondary Organic Aerosol Particles

Vahe Baboomian, Giuseppe Crescenzo, Yuanzhou Huang, Fabian Mahrt, Manabu Shiraiwa, A. K. Bertram, SERGEY NIZKORODOV, University of California, Irvine

     Abstract Number: 151
     Working Group: Aerosol Chemistry

Abstract
Information on the viscosity of secondary organic aerosol (SOA) particles is needed to predict their role in air quality and climate. Solar ultraviolet (UV) radiation is known to change the chemical composition and mass concentration of SOA. Nevertheless, the effect of solar UV radiation on the viscosity of SOA particles is unknown, leading to uncertainties when predicting their impacts on air quality and climate. This presentation discusses our measurements of the viscosity of SOA aged by exposure to 305 nm UV radiation for 12 days. This is equivalent to a UV exposure of 6–14 days at midlatitudes in summer, and captures typical tropospheric lifetimes of SOA particles. SOA particles were generated by the ozonolysis of d-limonene and α-pinene, two abundant biogenic volatile organic compounds in the atmosphere. Our results indicate that the viscosity of SOA particles can be as much as five orders of magnitudes larger for aged compared to unaged SOA throughout most of the troposphere. We show that this increase in viscosity is consistent with an increase in average molecular mass of the organic molecules constituting the SOA material, as determined by high-resolution mass spectrometry. The increased viscosity likely leads to increased abundance of highly viscous, glassy SOA particles that can possibly act as ice nucleation particles and impact ice cloud formation and climate in the upper troposphere. In addition, our results show that the mixing times of organic molecules within a SOA particle of 200 nm diameter can increase by as much as five orders of magnitude due to UV-aging, with implications for predicting of growth, evaporation, and the size distribution of SOA. Overall, our results, underscore the need to consider the effects of UV-aging when predicting the properties of SOA and their environmental impacts.