Examining the chemistry of atmospheric mercury using aircraft-based measurements and a global chemical transport model
Shah, Viral Pinakin
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Mercury is a potent neurotoxin that accumulates in fish and enters the human food chain. The largest source of mercury to the oceans is from the deposition of atmospheric mercury. Oxidized mercury species (Hg(II)) in the atmosphere are highly soluble and deposit efficiently; thus strongly influence the amount and the spatial distribution of mercury deposition. Currently, the distribution of oxidized mercury species in the atmosphere, and their source from in situ oxidation of elemental mercury (Hg(0)) by reaction with bromine atoms remain poorly understood. The Nitrogen, Oxidants, Mercury, and Aerosol Distribution, Sources, and Sinks (NOMADSS) aircraft experiment was designed with the aim of addressing these limitations. Speciated mercury concentrations were measured over the Southeastern US between June 1 and July 15, 2013 using a novel instrument developed at the University of Washington-Bothell. In this thesis, I use the GEOS-Chem chemical transport model to interpret the NOMADSS mercury measurements, and place new constraints on mercury chemistry in the free troposphere. The mean (and standard deviation) of the observed concentration of total atmospheric mercury (THg) was 1.49 ± 0.16 ng/m3, while that of Hg(II) was 111 ± 106 pg/m3. 60% of the Hg(II) observations were below the instrument’s limit of detection. High concentrations of Hg(II) (300-680 pg/m3) were observed in clean, dry air masses originating in the upper troposphere of the subtropical anticyclones, where fast production and lack of deposition lead to accumulation of Hg(II). The mean THg concentrations modeled by GEOS-Chem are found to be in close agreement with the observations. In contrast, the modeled Hg(II) concentrations are found to be low by a factor of three relative to the observations, indicating a faster than expected oxidation rate.I perform two sensitivity simulations with GEOS-Chem: (i) by increasing bromine radical concentrations by a factor of three, and (ii) by using a faster oxidation rate constant. The model underestimate of observed Hg(II) concentrations in the clean, dry air masses is reduced from a factor of 3 to factors of 2 and 1.6 in the two sensitivity simulations. The modeled lifetime of Hg(0) against oxidation to Hg(II) decreases from 4.8 months in the base simulation to 2.8 months and 1 month, respectively, in the sensitivity simulations. The model predicts that the 500 hPa summertime Hg(II) concentrations are three to five times higher in the subtropical anticyclones than elsewhere, highlighting the role of these regions as significant reservoirs for the global long-range transport of Hg(II). The model results imply that the faster oxidation of Hg(0) leads to a significant increase in the importance of reduction of Hg(II) in the cycling of mercury in the atmosphere.
- Atmospheric sciences