Atmospheric mercury (Hg) cycling in the coastal areas outside polar regions and Dead Sea could be much complicated due to intensive sea-land interactions mixing continental air pollutants and marine halogens, but remains unclear currently. In this study, we conducted a field measurement of total gaseous mercury (TGM) at a mountain site in Hong Kong surrounded by the Pearl River Delta region and the South China Sea. Contrary to the daytime maximums of other air pollutants, TGM was found to behave a distinct diurnal variation with a midday minimum. Moreover, we observed extremely fast TGM depletion events (MDEs) after sunrise when TGM concentrations rapidly dipped to 0.3–0.6 ng m-3 while other pollutants kept rising, which have not yet been reported in low-latitudes. With the aid of simulated wind field and up-to-date chemical mechanism, a combination of physical and chemical processes was examined to cause the fast MDEs, i.e., upslope flow transporting TGM-depleted airmasses rich in air pollutants to the mountaintop location from mixing layer. Bromine-induced two-step photooxidation of Hg involving substantial anthropogenic pollutants such as NO2 and O3 was estimated to dominate the TGM depletion after sunrise, contributing 56–62% of depleted TGM; while rapid uptake of oxidized Hg into sea-salt aerosols likely led to the ultimate formation of TGM-depleted airmasses. The needed Br of 0.21–0.27 pptv was further discussed to be potentially available through sea-salt aerosol debromination. These findings highlight the significant anthropogenic influence on the geochemical cycling and fate of Hg in addition to marine halogen chemistry in the coastal atmosphere, where the enhanced oxidation and deposition of atmospheric Hg are expected to be widespread.

Fast atmospheric mercury depletion events observed at a mountaintop in coastal Hong Kong

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