Differences in aerosol and cloud properties along the central California coast when winds change from northerly to southerly

Abstract

Wind reversals resulting in southerly flow along the California coast are not well understood in terms of how aerosol and cloud characteristics change. This gap is addressed using airborne field measurements enhanced with data from spaceborne remote sensing (Moderate Resolution Imaging Spectroradiometer), surface stations (Interagency Monitoring of Protected Visual Environments), and models (Navy Aerosol Analysis and Prediction System and Coupled Ocean–Atmosphere Mesoscale Prediction System), with a focus on submicron and supermicron aerosol, as well as cloud microphysical variables: cloud droplet number concentration (Nd), cloud optical thickness (COT), and cloud droplet effective radius (re). Southerly flow coincided with higher values of submicron aerosol concentration (Na) and mass concentrations of species representative of fine-aerosol pollution (NO3- and nss-SO42-) as well as shipping and continental emissions (V, oxalate, NH4+, Ni, OC, and EC). Supermicron Na did not change; however, heightened levels of acidic species in southerly flow coincided with reduced Cl− : Na+, suggestive of Cl− depletion in salt particles. Clouds responded correspondingly in southerly flow, with more acidic cloud water and higher levels of similar species as in the aerosol phase (e.g., NO3-, nss-SO42-, NH4+, V), along with elevated values of Nd and COT and reduced re during campaigns with similar cloud liquid water paths. Case study flights help to visualize offshore pollution gradients and highlight the sensitivity of the results to the presence of widespread smoke coverage including how associated plumes have enhanced supermicron Na. These results have implications for aerosol–cloud interactions during wind reversals and have relevance for weather, public welfare, and aviation.