Observational Evidence for Involvement of Nitrate Radicals in Nighttime Oxidation of Mercury

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.