Aircraft measurements of total mercury and monomethyl mercury in summertime marine stratus cloud water 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 16 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 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.