Experimental rainwater divalent mercury speciation and photoreduction rates in the presence of halides and organic carbon

Abstract

Mercury (Hg) photochemical redox reactions control atmospheric Hg lifetime and therefore play an important role in global Hg cycling. Oxidation of Hg(0) to Hg(II) is currently thought to be a Br-initiated two-stage reaction with end-products HgBr2, HgBrOH, HgBrONO, HgBrOHO. Atmospheric photoreduction of these Hg(II) compounds can take place in both the gas and aqueous phase. Here we present new experimental observations on aqueous Hg(II) photoreduction rates in the presence of dissolved organic carbon and halides and compare the findings to rainfall Hg(II) photoreduction rates. The pseudo first-order, gross photoreduction rate constant, kred, for 0.5 μM Hg(II) in the presence of 0.5 mg/ L of dissolved organic carbon (DOC) is 0.23 h−1, which is similar to the mean kred (0.15 ± 0.01 h−1(σ, n = 3)) in high altitude rainfall and at the lower end of the median kred (0.41 h−1, n = 24) in continental and marine waters. Addition of bromide (Br−) to experimental Hg(II)-DOC solutions progressively inhibits Hg(II) photoreduction to reach 0.001 h−1 at total Br− of 10 mM. Halide substitution experiments give Hg(II)Xn(n-2) photoreduction rate constants of 0.016, 0.004 h−1, and < detection limit for X = Cl−, Br−, and I− respectively and reflect increasing stability of the Hg(II)-halide complex. We calculate equilibrium Hg(II) speciation in urban and high-altitude rainfall using Visual Minteq, which indicates Hg(II)-DOC to be the dominant Hg species. The ensemble of observations suggests that atmospheric gaseous HgBr2, HgCl2, HgBrNO2, HgBrHO2 forms, scavenged by aqueous aerosols and cloud droplets, are converted to Hg(II)-DOC forms in rainfall due to abundant organic carbon in aerosols and cloud water. Eventual photoreduction of Hg(II)-DOC in aqueous aerosols and clouds is, however, too slow to be relevant in global atmospheric Hg cycling.