Seasonal Differences in the Phase States of Coastal Aerosols and the Role of Chemical Composition

KANURI ROUNDTREE, Renee Niles, Paul Tumminello, Maya Morris, Diana Tran, Rachel Tseng, Xitlali Ventura, Adam Cooper, Benjamin Carr, Jonathan Slade, University of California San Diego

     Abstract Number: 614
     Working Group: Aerosol Processes and Properties in Changing Environments in the Anthropocene

Abstract
Marine aerosols influence coastal atmospheric systems due to their ability to impact cloud formation, which is influenced by particle phase state (viscosity) and hygroscopicity. During a field campaign in La Jolla, CA, the phase states of coastal aerosols were examined across seasons (summer and winter) and diurnal cycles. The aerosol phase state was determined directly online from particle bounce using an electrical low-pressure impactor coupled to a scanning electrical mobility sizer. Findings demonstrated statistically significantly higher particle bounce during the winter compared to summer. This was attributed to a distinct, liquid-like sea spray aerosol (SSA) size mode measured during the summer field study. In addition, in the winter, there were higher levels of Chlorophyll A in the ocean and non-refractory organic carbon mass detected in the particles. This is consistent with previous studies of SSA phase state, which demonstrated increased viscosity during periods of high biological activity in a wave flume but shown for the first time in this natural coastal environment. It was suggested in that study that divalent cations such as Ca²⁺ and Mg²⁺ complex with organic anions producing semi-solid or gel-like particles. Aerosols were additionally collected on quartz-fiber filters and analyzed offline with ion chromatography. Na⁺ concentrations were utilized for tracing SSA and divalent cation enrichment. The results from the field are undergoing comparisons with laboratory studies on the phase state and hygroscopicity of synthetic SSA generated in a marine aerosol reference tank, generated systematically with varying artificial seawater composition incorporating different organic components (glucose and palmitic acid) and sea salts, including CaCl₂, MgCl₂, and NaCl. These results will be presented and are important in building an understanding of the factors impacting aerosol phase state and hygroscopicity, particularly in the context of the marine boundary layer.