Photodecomposition of Methylmercury in Atmospheric Waters

Abstract

Experiments were conducted to empirically examine net changes in methylmercury concentration of atmospheric waters as function of irradiance. Methods were developed to allow experiments to be conducted at atmospherically relevant concentrations using trace metal clean techniques, over a range of aqueous matrices. Rain water was collected at Devil's Lake State Park, WI, and simulated cloud water was created by water extraction of particulate matter collected at the same site. These waters were spiked with methyl mercury chloride and mercuric chloride and exposed to sunlight on the roof of a building. Experiments were conducted during typical summer conditions with respect to temperature, sunlight intensity and sunlight duration. For all cases, exposure to sunlight resulted in net loss of methylmercury: -0.022 ± 0.002 1/hr in rainwater at a total UVB flux of 8 kWhrs/m 2 ; -0.008 ± 0.001 1/hr in simulated cloud water at a total UVB flux of 5.5 kWhrs/m 2 . For dark cases, no statistically significant formation in methylmercury from inorganic mercury was detected. Furthermore, laboratory experiments to form methylmercury from mercuric-acetate complexes did not give detectable yields. Given the results of this study, and the results of studies cited in this article, it is unlikely that homogeneous MeHg formation is fast enough to lead to the net formation of MeHg in atmospheric waters exposed to sunlight.

Total- and monomethyl-mercury and major ions in coastal California fog water: Results from two years of sampling on land and at sea

“…12 hours, an upper limit for the transport time between BML and PPW 40 km inland based on inspection of HYSPLIT back trajectories using the EDAS 40 km meteorological fields (Stein et al, 2015), this leaves 1.7 ng L -1 of MMHg in the fog water at PPW, greater than the observed mean MMHg which was 0.6 ng L -1 . Dark demethylation rates were essentially zero in rain water and simulated cloud water (Bittrich et al, 2011) suggesting that demethylation in the coastal California fog should be even slower than what was estimated using photo-demethylation rates since fog moves inland during the night. Thus, demethylation of MMHg appears to be too slow to account for the large differences seen between the coastal sites at BML and HSU and the inland site of PPW.…”

“…Once in the fog droplet, MMHg could undergo photo-demethylation to inorganic Hg as has been observed in sea water (Monperrus et al, 2007;Black et al, 2009), rain water and simulated fog water (Bittrich et al, 2011). We wondered if this could account for the coastal-inland MMHg concentration gradient that was observed (Figure 4).…”

“…We wondered if this could account for the coastal-inland MMHg concentration gradient that was observed (Figure 4). Based on a previously published photo-demethylation rate constant in rain water (-0.022 hr -1 ) (Bittrich et al, 2011), and an initial MMHg concentration of 2.2 ng L -1 , which was the mean value at the coastal site BML, the initial loss rate of MMHg in fog water was -0.048 ng L -1 h -1 . After Figure 6 MMHg vs. HgT from fog water samples from all terrestrial sites 2014-2015.…”

“…Some research indicated that an abiotic mechanism involving the reaction between divalent Hg and the acetate ion or similar organic ligand could form MMHg (Gardfeldt et al, 2003;Hammerschmidt et al, 2007). Another study indicated that the photodecomposition of MMHg in rain water is likely faster than any abiotic MMHg formation reaction involving acetate (Bittrich et al, 2011).…”

Total and monomethyl mercury in fog water from the central California coast

“…The second hypothesis of MMHg formation in atmospheric water is an abiotic mechanism involving reactions between Hg(II) compounds and the acetate ion [ Gardfeldt et al , 2003; Hammerschmidt et al , 2007]. However, recent work has called this mechanism into question [ Bittrich et al , 2011b] as being too slow to compete with photo‐demethylation in rain water.…”