Inorganic Nitrate Enhances Cloud Activation of Less Hygroscopic Aerosols at a Coastal Site in California

Abstract Anthropogenic aerosols influence marine cloud properties in coastal regions, but their impacts remain poorly understood due to the complexity of sources and aerosol‐cloud interactions. To investigate how cloud processing affects aerosol composition, in situ measurements of aerosol properties were performed at Mt. Soledad in La Jolla, CA (251 m asl, 2 km from shore) as part of the Eastern Pacific Cloud Aerosol Precipitation Experiment Partitioning Thrust by Los Alamos National Laboratory from October to December 2023. A ground‐based counterflow virtual impactor coupled with high‐resolution aerosol mass spectrometry was used to directly measure the composition of cloud droplet residuals, which were compared to out of cloud aerosols. Cloud residuals showed a large enhancement of inorganic nitrate and chloride fractions, increasing by up to a factor of 2 and 5 times relative to the out‐of‐cloud periods, respectively, with nitrate mass fraction increasing with cloud liquid water content. Through positive matrix factorization analysis, we identified a factor associated with aqueous‐phase reactions in clouds. The organic material within cloud residuals was not highly oxidized, weakly hygroscopic and showed a similar size distribution to the inorganic species, suggesting that internally mixed inorganic species related to anthropogenic emissions and aged sea salt control cloud condensation nuclei activity in this location. The increase in hygroscopic inorganic components and aerosol size during cloud events could increase the ability of particles to activate into droplets. These results help constrain the role of different aerosol sources and cloud processing in cloud‐forming potential at a coastal site.