Atmospheric mercury (Hg) cycling
in polluted coastal
atmosphere
is complicated and not fully understood. Here, we present measurements
of total gaseous mercury (TGM) monitored at a coastal mountaintop
in Hong Kong downwind of mainland China. Sharp TGM peaks during cold
front passages were frequently observed due to Asian pollution outflow
with typical TGM/CO slopes of 6.8 ± 2.2 pg m–3 ppbv–1. Contrary to the daytime maximums of other
air pollutants, TGM exhibited a distinct diurnal variation with a
midday minimum. Moreover, we observed four cases of extremely fast
TGM depletion after sunrise, during which TGM concentrations rapidly
dipped to 0.3–0.6 ng m–3 accompanied by other
pollutants on the rise. Simulated meteorological fields revealed that
morning upslope flow transporting anthropogenically polluted but TGM-depleted
air masses from the mixed layer caused morning TGM depletion at the
mountaintop location. The TGM-depleted air masses were hypothesized
to result mainly from fast photooxidation of Hg after sunrise with
minor contributions from dry deposition (5.0%) and nocturnal oxidation
(0.6%). A bromine-induced two-step oxidation mechanism involving abundant
pollutants (NO2, O3, etc.) was estimated to
play a dominant role, contributing 55%–60% of depleted TGM
and requiring 0.20–0.26 pptv Br, an amount potentially available
through sea salt aerosol debromination. Our findings suggest significant
effects of the interaction between anthropogenic pollution and marine
halogen chemistry on atmospheric Hg cycling in the coastal areas.
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