Article pubs.acs.org/est
Observational Evidence for Involvement of Nitrate Radicals in Nighttime Oxidation of Mercury Mordechai Peleg,*,† Eran Tas,‡ Daniel Obrist,§ Valeri Matveev,† Christopher Moore,§ Maor Gabay,‡ and Menachem Luria† †
Institute of Earth Sciences, Edmud Safra Campus, Givat Ram, The Hebrew University of Jerusalem, Jerusalem 91904, Israel The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, P.O. Box 12, Rehovot 76100, Israel § Desert Research Institute, Division of Atmospheric Sciences, 2215 Raggio Parkway, Reno, Nevada 89512, United States ‡
S Supporting Information *
ABSTRACT: In the atmosphere, reactive forms of mercury species can be produced by oxidation of the dominant gaseous elemental mercury (GEM). The oxidation of GEM is an important driver for deposition, but oxidation pathways currently are poorly constrained and likely differ among regions. In this study, continuous measurements of atmospheric nitrate radical (NO3) concentrations and mercury speciation (i.e., elemental and reactive, oxidized forms) were performed during a six week period in the urban air shed of Jerusalem, Israel during summer 2012, to investigate the potential nighttime contribution of nitrate radicals to oxidized mercury formation. Average nighttime concentrations of reactive gaseous mercury (RGM) were almost equivalent to daytime levels (25 pg m−3 and 27 pg m−3 respectively), in contrast to early morning and evening RGM levels which dropped to low levels (9 and 13 pg m−3). During daytime, the presence of RGM was increased when solar radiation exceeded 200 W m−2, suggesting a photochemical process for daytime RGM formation. Ozone concentrations were largely unrelated to daytime RGM. Nighttime RGM concentrations were relatively high (with a maximum of 97 pg m−3) compared to nighttime levels in other urban regions. A strong correlation was observed between nighttime RGM concentrations and nitrate radical concentration (R2 averaging 0.47), while correlations to other variables were weak (e.g., RH; R2 = 0.35) or absent (e.g., ozone, wind speed and direction, pollution tracers such as CO or SO2). Detailed analyses suggest that advection processes or tropospheric influences were unlikely to explain the strong nighttime correlations between NO3 and RGM, although these processes may contribute to these relationships. Our observations suggest that NO3 radicals may play a role in RGM formation, possibly due to a direct chemical involvement in GEM oxidation. Since physical data, however, suggest that NO3 unlikely initiates GEM oxidation, NO3 may play a secondary role in GEM oxidation through the addition to an unstable Hg(I) radical species.
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INTRODUCTION
GEM oxidation is likely to be an important mechanism leading to Hg deposition, the mechanisms are not completely understood and still remain under debate.10−13 Several atmospheric constituents have been proposed as contributing to the oxidation of GEM, including ozone (O3), OH, halogens (particularly bromine [Br]), and to a lesser extent the nitrate radical (NO3).2,12,14−19 Uncertainties exist regarding the contribution of the various chemical species participating in the oxidation reactions.20,21 Originally, gasphase OH and O3 were suggested as being the main oxidants active in atmospheric Hg models, but these oxidants have been suggested as being too slow to be of atmospheric relevance.9,19
Mercury (Hg) is an important neurotoxic pollutant that has a complex global biogeochemical cycle.1 There are three operationally defined species of Hg measured in the atmosphere: gaseous elemental mercury (GEM), reactive gaseous Hg (RGM), and particulate bound (
