Gradient measurements of gaseous elemental mercury (Hg) in the marine boundary layer of the northwest Sea of Japan (East Sea).

Gaseous elemental mercury (Hg) is a prolific and persistent contaminant in the atmosphere. Atmospheric concentrations of Hg were determined from 17 September to 7 October 2015 in the northwest Sea of Japan aboard the Russian research vessel Professor Gagarinsky. Simultaneous measurements of Hg concentrations were performed 2 m and 20 m above the sea surface using automatic Hg analysers RA-915M and RA-915+, respectively. Concentrations ranged from 0.3 to 25.9 ng/m (n = 5207) and from 0.3 to 27.8 ng/m (n = 4415), with medians of 1.7 and 1.6 ng/m, respectively. Elevated Hg was observed during three episodes from 19 to 22 September, likely caused by one or more of the following factors: 1) atmospheric transport of Hg from the west and south-west (from N. Korea, China, and the Yellow Sea region); 2) Hg emission from the sea due to pollution by water from the Tumannaya River; or 3) underwater geological activities. Increased Hg concentration was observed during periods when air masses flowed from the south, and low concentrations were observed when air masses came from the north. A daytime increase of Hg concentrations at a height of 2 m occurred simultaneously with decreasing Hg at a height of 20 m. These diurnal variations suggest that two contrasting processes occur during the daytime in the marine boundary layer (MBL): Hg emission from the sea surface and Hg oxidation in the MBL by active halogens formed by photolysis.