Atmospheric mercury speciation and deposition in the Bay St. François wetlands

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Atmospheric mercury speciation and deposition are critical to understanding the fate of mercury in the environment. The importance of atmospheric mercury speciation and deposition was investigated during an intensive field campaign in the Bay St. François wetlands in late summer 2002. Measured gaseous elemental mercury (GEM), reactive gaseous mercury (RGM), and particulate mercury (PM: 0.1–2.5 μm) concentrations were 0.85–2.16 ng/m3 (average 1.38 ng/m3), 0–22 pg/m3 (average 3.63 pg/m3), and 0.5–18 pg/m3 (average 6.44 pg/m3), respectively. PM and RGM represented <1% of the total mercury. We reported the first synchronized automated high‐time‐resolution mercury species fluxes. The average GEM flux was 32.1 ng/m2/h (positive sign means volatilization, and negative sign indicates deposition) with a maximum deposition value of 110 ng/m2/h and a maximum volatilization value of 278 ng/m2/h. The average RGM flux was −2.6 ng/m2/h with a maximum deposition value of 25.6 ng/m2/h and a maximum evasion value of 0.6 ng/m2/h. The average PM flux was −0.4 ng/m2/h with a maximum deposition value of 8.7 ng/m2/h and a maximum resuspension value of 32.7 ng/m2/h. RGM contributed over 90% to the total dry deposition of mercury (GEM, RGM, and PM). GEM evasion exceeded deposition of GEM, RGM, and PM, thus suggesting other sources of mercury to the mercury budget such as wet deposition through precipitation or soil. The median dry deposition velocities calculated from flux and concentration measurements were 0.19 cm/s (GEM) < 2.1 cm/s (PM) < 7.6 cm/s (RGM). The dry deposition velocity of PM is large and should be viewed as upper limits only. Although RGM should be rapidly depositing species, as predicted by its solubility, little RGM flux was observed at night. Time series of canopy wetness and RGM fluxes suggested that RGM fluxes are not driven by RGM solubility (in water) since the canopy was dry when larger RGM deposition periods were observed during daytime. It is suggested that the vegetation might directly uptake RGM and release elemental gaseous mercury during daytime. Our experimental data showed that photochemically reactive air masses could produce RGM, which is rapidly removed by dry deposition to the surfaces. Lifetime of RGM over vegetation canopy is on the order of a few hours.