title: Observational Evidence for Involvement of Nitrate Radicals in Nighttime Oxidation of Mercury.
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 (R(2) averaging 0.47), while correlations to other variables were weak (e.g., RH; R(2) = 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.
authors.affiliation: Institute of Earth Sciences, Edmud Safra Campus, Givat Ram, The Hebrew University of Jerusalem , Jerusalem 91904, Israel.
authors.forename: Mordechai
authors.initials: M
authors.lastname: Peleg
authors.affiliation: The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem , P.O. Box 12, Rehovot 76100, Israel.
authors.forename: Eran
authors.initials: E
authors.lastname: Tas
authors.affiliation: Desert Research Institute, Division of Atmospheric Sciences, 2215 Raggio Parkway, Reno, Nevada 89512, United States.
authors.forename: Daniel
authors.initials: D
authors.lastname: Obrist
authors.affiliation: Institute of Earth Sciences, Edmud Safra Campus, Givat Ram, The Hebrew University of Jerusalem , Jerusalem 91904, Israel.
authors.forename: Valeri
authors.initials: V
authors.lastname: Matveev
authors.affiliation: Desert Research Institute, Division of Atmospheric Sciences, 2215 Raggio Parkway, Reno, Nevada 89512, United States.
authors.forename: Christopher
authors.initials: C
authors.lastname: Moore
authors.affiliation: The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem , P.O. Box 12, Rehovot 76100, Israel.
authors.forename: Maor
authors.initials: M
authors.lastname: Gabay
authors.affiliation: Institute of Earth Sciences, Edmud Safra Campus, Givat Ram, The Hebrew University of Jerusalem , Jerusalem 91904, Israel.
authors.forename: Menachem
authors.initials: M
authors.lastname: Luria
mesh_terms: D000393:Air Pollutants / Q000032:analysis / Q000737:chemistry*; D001272:Atmosphere / Q000032:analysis; D002947:Cities; D004784:Environmental Monitoring; D005609:Free Radicals / Q000737:chemistry; D007557:Israel; D008628:Mercury / Q000032:analysis* / Q000737:chemistry*; D009566:Nitrates / Q000032:analysis / Q000737:chemistry*; D010084:Oxidation-Reduction; D010126:Ozone / Q000032:analysis; D012621:Seasons; D014919:Wind
chemical_list: D000393:Air Pollutants; D005609:Free Radicals; D009566:Nitrates; D010126:Ozone; D008628:Mercury