Aircraft Measurements of Total Mercury and Monomethyl Mercury in Summertime Marine Stratus Cloudwater from Coastal California, USA

Water samples from marine stratus clouds were collected during 16 aircraft flights above the Pacific Ocean near the Central California coast during the summer of 2016. These samples were analyzed for total mercury (THg), monomethyl mercury (MMHg), and 32 other chemical species in addition to aerosol physical parameters. The mean concentrations of THg and MMHg in the cloudwater samples were 9.2 ± 6.0 ng L^-1 (2.3-33.8 ng L^-1) (N = 97) and 0.87 ± 0.66 ng L^-1 (0.17-2.9 ng L^-1) (N = 22), respectively. This corresponds to 9.5% (3-21%) MMHg as a fraction of THg. Low and high nonsea salt calcium ion (nss-Ca²⁺) concentrations in cloudwater were used to classify flights as "marine" and "continental", respectively. Mean [MMHg]_marine was significantly higher (p < 0.05) than [MMHg]_continental consistent with an ocean source of dimethyl Hg (DMHg) to the atmosphere. Mean THg in cloudwater was not significantly different between the two categories, indicating multiple emissions sources. Mean [THg]_continental was correlated with pH, CO, NO₃^-, NH₄⁺, and other trace metals, whereas [THg]_marine was correlated with MMHg and Na⁺. THg concentrations were negatively correlated with altitude, consistent with ocean and land emissions, coupled with removal at the cloud-top due to drizzle formation.