Gradient measurements of gaseous elemental mercury (Hg0) in the marine boundary layer of the northwest Sea of Japan (East Sea)

Highlights: • Gaseous mercury (Hg⁰) increased with the arrival of air masses from the south. • Low concentrations were observed in air masses coming from the north. • We report diurnal Hg⁰ dynamics in the marine boundary layer of the Sea of Japan. • Abnormally high Hg⁰ is linked to three major factors, including air mass origin. Gaseous elemental mercury (Hg⁰) is a prolific and persistent contaminant in the atmosphere. Atmospheric concentrations of Hg⁰ 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 Hg⁰ concentrations were performed 2 m and 20 m above the sea surface using automatic Hg⁰ analysers RA-915M and RA-915+, respectively. Concentrations ranged from 0.3 to 25.9 ng/m³ (n = 5207) and from 0.3 to 27.8 ng/m³ (n = 4415), with medians of 1.7 and 1.6 ng/m³, respectively. Elevated Hg⁰ was observed during three episodes from 19 to 22 September, likely caused by one or more of the following factors: 1) atmospheric transport of Hg⁰ from the west and south-west (from N. Korea, China, and the Yellow Sea region); 2) Hg⁰ emission from the sea due to pollution by water from the Tumannaya River; or 3) underwater geological activities. Increased Hg⁰ 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 Hg⁰ concentrations at a height of 2 m occurred simultaneously with decreasing Hg⁰ at a height of 20 m. These diurnal variations suggest that two contrasting processes occur during the daytime in the marine boundary layer (MBL): Hg⁰ emission from the sea surface and Hg⁰ oxidation in the MBL by active halogens formed by photolysis.