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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 (NO<sub>3</sub>) 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<sup>–3</sup> and 27 pg m<sup>–3</sup> respectively), in contrast to early morning and evening RGM levels
which dropped to low levels (9 and 13 pg m<sup>–3</sup>). During
daytime, the presence of RGM was increased when solar radiation exceeded
200 W m<sup>–2</sup>, 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<sup>–3</sup>) compared to nighttime
levels in other urban regions. A strong correlation was observed between
nighttime RGM concentrations and nitrate radical concentration (<i>R</i><sup>2</sup> averaging 0.47), while correlations to other
variables were weak (e.g., RH; <i>R</i><sup>2</sup> = 0.35)
or absent (e.g., ozone, wind speed and direction, pollution tracers
such as CO or SO<sub>2</sub>). Detailed analyses suggest that advection
processes or tropospheric influences were unlikely to explain the
strong nighttime correlations between NO<sub>3</sub> and RGM, although
these processes may contribute to these relationships. Our observations
suggest that NO<sub>3</sub> radicals may play a role in RGM formation,
possibly due to a direct chemical involvement in GEM oxidation. Since
physical data, however, suggest that NO<sub>3</sub> unlikely initiates
GEM oxidation, NO<sub>3</sub> may play a secondary role in GEM oxidation
through the addition to an unstable HgÂ(I) radical species