Observations show increased nitrate in coastal cloud droplet residuals from large-particle activation and N 2 O 5 hydrolysis
Abigail S. Williams, Lynn M. Russell, Sanghee Han, Veronica Z. Berta, Jeramy L. Dedrick, Teresa Campos, Lauren T. Robinson, Rachel Y.-W. Chang, Ryan N. Farley, Allison C. Aiken, Cynthia H. Twohy, Michael J. Wheeler, Jeremy J. B. Wentzell, John LiggioClouds change the chemical composition of aerosols, yet observations characterizing these processes are quite limited. Ground-based in-cloud measurements in coastal Southern California show that the highest nitrate to sulfate ratio (2.5 ± 1.7) in droplet residuals is near cloud base where cloud liquid water content and droplet number concentration are below the observed 20th percentile. The number size distribution of nitrate-rich particles peaks at a larger diameter than sulfate-rich particles, supporting growth to droplets of nitrate-rich particles at lower supersaturations than for sulfate-rich particles. Nitrate mass concentrations are up to three times higher in-cloud compared to pre-cloud and correlated with statistically significant reductions in ozone concentrations, consistent with contributions from in-cloud nitrate formation by N 2 O 5 hydrolysis. This study provides quantitative relationships of cloud properties to nitrate production in droplet residuals and associated ozone reduction, illustrating the impacts of aqueous reactions in clouds.