Environmental Chamber Studies of Mercury Reactions in the Atmosphere

Sumner, Ann Louise; Spicer, Chester W.; Satola, Jan; Mangaraj, Raj; Cowen, Kenneth A.; Landis, Matthew S.; Stevens, Robert K.; Atkeson, Thomas D.

doi:10.1007/0-387-24494-8_9

Cited by 22 publications

(31 citation statements)

References 16 publications

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“…The experimentally-determined rate constant for the oxidation of Hg 0 by NO 3 has been previously reported to be <4 × 10 −15 cm 3 molecule −1 s −1 by Sommar et al [39] and <7 × 10 −15 cm 3 molecule −1 s −1 by Sumner et al [36] at 1 atm and 298 K. However, based on the new HgO thermochemistry, Hynes et al estimated this reaction to be highly endothermic and, thus, suggested that this oxidation pathway is not viable in the atmosphere [14,49]. Furthermore, theoretical calculations by Dibble et al [47] suggest that NO 3 do not form strong bonds with Hg 0 and therefore is unlikely to initiate gas-phase Hg 0 oxidation reactions.…”

Section: Oxidation Of Hg 0 By Nomentioning

confidence: 99%

Recent Advances in Atmospheric Chemistry of Mercury

Ariya

2018

Atmosphere

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Mercury is one of the most toxic metals and has global importance due to the biomagnification and bioaccumulation of organomercury via the aquatic food web. The physical and chemical transformations of various mercury species in the atmosphere strongly influence their composition, phase, transport characteristics and deposition rate to the ground. Modeling efforts to evaluate the mercury cycling in the environment require an accurate understanding of atmospheric mercury chemistry. We focus this article on recent studies (since 2015) on improving our understanding of the atmospheric chemistry of mercury. We discuss recent advances in (i) determining the dominant atmospheric oxidant of elemental mercury (Hg 0 ); (ii) understanding the oxidation reactions of Hg 0 by halogen atoms and by nitrate radical (NO 3 ); (iii) the aqueous reduction of oxidized mercury compounds (Hg II ); and (iv) the heterogeneous reactions of Hg on atmospherically-relevant surfaces. The need for future research to improve understanding of the fate and transformation of mercury in the atmosphere is also discussed.

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Section: Oxidation Of Hg 0 By Nomentioning

confidence: 99%

Recent Advances in Atmospheric Chemistry of Mercury

Ariya

2018

Atmosphere

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“…However, in a more recent theoretical work, Cremer et al (2008) found the reaction energy of Hg 0 + OH to be comparable to the reaction energy for Hg 0 + Br, and concluded that the reaction Hg 0 + OH is possible in the atmosphere. Use of much larger reaction chamber and low reactant concentrations in more recent studies of Hg 0 + O 3 reaction suggests that the rate constants obtained previously are viable in the atmosphere and are free of surface effects (Snider et al, 2008;Sumner et al, 2005). Tossell (2006) suggest that stable oligomers of Hg oxide, HgO n , can subsist in the atmosphere.…”

Section: Model Descriptionmentioning

confidence: 95%

Evaluation of discrepancy between measured and modelled oxidized mercury species

et al. 2013

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Abstract. L. Zhang et al. (2012), in a recent report, compared model estimates with new observations of oxidized and particulate mercury species (Hg2+ and Hgp) in the Great Lakes region and found that the sum of Hg2+ and Hgp varied between a factor of 2 to 10 between measurements and model. They suggested too high emission inputs as Hg2+ and too fast oxidative conversion of Hg0 to Hg2+ and Hgp as possible causes. This study quantitatively explores measurement uncertainties in detail. These include sampling efficiency, composition of sample, interfering species and calibration errors. Model (Global/Regional Atmospheric Heavy Metals Model – GRAHM) sensitivity experiments are used to examine the consistency between various Hg measurements and speciation of Hg near emission sources to better understand the discrepancies between modelled and measured concentrations of Hg2+ and Hgp. We find that the ratio of Hg0, Hg2+ and Hgp in the emission inventories, measurements of surface air concentrations of oxidized Hg and measurements of wet deposition are currently inconsistent with each other in the vicinity of emission sources. Current speciation of Hg emissions suggests higher concentrations of Hg2+ in air and in precipitation near emission sources; however, measured air concentrations of Hg2+ and measured concentrations of Hg in precipitation are not found to be significantly elevated near emission sources compared to the remote regions. The averaged unbiased root mean square error (RMSE) between simulated and observed concentrations of Hg2+ is found to be reduced by 42% and for Hgp reduced by 40% for 21 North American sites investigated, when a ratio for Hg0 : Hg2+ : Hgp in the emissions is changed from 50 : 40 : 10 (as specified in the original inventories) to 90 : 8 : 2. Unbiased RMSE reductions near emissions sources in the eastern United States and Canada are found to be reduced by up to 58% for Hg2+. Significant improvement in the model simulated spatial distribution of wet deposition of mercury in North America is noticed with the modified Hg emission speciation. Measurement-related uncertainties leading to lower estimation of Hg2+ concentrations are 86%. Uncertainties yielding either to higher or lower Hg2+ concentrations are found to be 36%. Finally, anthropogenic emission uncertainties are 106% for Hg2+. Thus it appears that the identified uncertainties for model estimates related to mercury speciation near sources, uncertainties in measurement methodology and uncertainties in emissions can close the gap between modelled and observed estimates of oxidized mercury found in L. Zhang et al. (2012). Model sensitivity simulations show that the measured concentrations of oxidized mercury, in general, are too low to be consistent with measured wet deposition fluxes in North America. Better emission inventories (with respect to speciation), better techniques for measurements of oxidized species and knowledge of mercury reduction reactions in different environments (including in-plume) in all phases are needed for improving the mercury models.

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“…Sillman et al (2007) reported higher (60-248 pg m −3 ) Hg(II) concentrations at 3 km altitude than near the surface in aircraft flights off the Florida coast. Swartzendruber et al (2009) found a large variability in Hg(II) concentrations (0-500 pg m −3 ) during five flights over the Pacific Northwest below 5 km altitude, with higher concentrations in free-tropospheric air with low aerosol concentrations. Lyman and Jaffe (2012) found enhanced Hg(II) concentrations of ∼ 450 pg m −3 and depleted total mercury (THg, THg = Hg(0)+Hg(II)) concentrations (< 1 ng m −3 ) in a stratospheric intrusion, suggesting rapid oxidation of Hg(0) and loss of Hg(II) above the tropopause.…”

Section: Introductionmentioning

confidence: 93%

Origin of oxidized mercury in the summertime free troposphere over the southeastern US

et al. 2016

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Abstract. We collected mercury observations as part of the Nitrogen, Oxidants, Mercury, and Aerosol Distributions, Sources, and Sinks (NOMADSS) aircraft campaign over the southeastern US between 1 June and 15 July 2013. We use the GEOS-Chem chemical transport model to interpret these observations and place new constraints on bromine radical initiated mercury oxidation chemistry in the free troposphere. We find that the model reproduces the observed mean concentration of total atmospheric mercury (THg) (observations: 1.49 ± 0.16 ng m−3, model: 1.51 ± 0.08 ng m−3), as well as the vertical profile of THg. The majority (65 %) of observations of oxidized mercury (Hg(II)) were below the instrument's detection limit (detection limit per flight: 58–228 pg m−3), consistent with model-calculated Hg(II) concentrations of 0–196 pg m−3. However, for observations above the detection limit we find that modeled Hg(II) concentrations are a factor of 3 too low (observations: 212 ± 112 pg m−3, model: 67 ± 44 pg m−3). The highest Hg(II) concentrations, 300–680 pg m−3, were observed in dry (RH  <  35 %) and clean air masses during two flights over Texas at 5–7 km altitude and off the North Carolina coast at 1–3 km. The GEOS-Chem model, back trajectories and observed chemical tracers for these air masses indicate subsidence and transport from the upper and middle troposphere of the subtropical anticyclones, where fast oxidation of elemental mercury (Hg(0)) to Hg(II) and lack of Hg(II) removal lead to efficient accumulation of Hg(II). We hypothesize that the most likely explanation for the model bias is a systematic underestimate of the Hg(0) + Br reaction rate. We find that sensitivity simulations with tripled bromine radical concentrations or a faster oxidation rate constant for Hg(0) + Br, result in 1.5–2 times higher modeled Hg(II) concentrations and improved agreement with the observations. The modeled tropospheric lifetime of Hg(0) against oxidation to Hg(II) decreases from 5 months in the base simulation to 2.8–1.2 months in our sensitivity simulations. In order to maintain the modeled global burden of THg, we need to increase the in-cloud reduction of Hg(II), thus leading to faster chemical cycling between Hg(0) and Hg(II). Observations and model results for the NOMADSS campaign suggest that the subtropical anticyclones are significant global sources of Hg(II).

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Environmental Chamber Studies of Mercury Reactions in the Atmosphere

Cited by 22 publications

References 16 publications

Recent Advances in Atmospheric Chemistry of Mercury

Recent Advances in Atmospheric Chemistry of Mercury

Evaluation of discrepancy between measured and modelled oxidized mercury species

Origin of oxidized mercury in the summertime free troposphere over the southeastern US

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