Observations show increased nitrate in coastal cloud droplet residuals from large-particle activation and N(2)O(5) hydrolysis.

Nitrate levels in coastal cloud droplet residuals are up to three times higher in-cloud than pre-cloud, with the highest nitrate to sulfate ratio (2.5 ± 1.7) near cloud base.

• Why it matters: Understanding how clouds alter aerosol chemistry is crucial for assessing their impact on air quality and climate, yet observational data on these processes are limited.
• What they did: Ground-based in-cloud measurements in coastal Southern California analyzed aerosol composition, size distribution, and ozone levels, focusing on nitrate and sulfate in droplet residuals.
• The result: Findings show nitrate-rich particles grow at lower supersaturations, and increased nitrate correlates with significant ozone reductions, highlighting in-cloud nitrate formation via N2O5 hydrolysis.