posted on 2020-03-16, 17:04authored byStephen
M. McNamara, Natasha M. Garner, Siyuan Wang, Angela R. W. Raso, Sham Thanekar, Anna J. Barget, Jose D. Fuentes, Paul B. Shepson, Kerri A. Pratt
Bromine and chlorine
chemistry in the Arctic atmospheric boundary
layer has significant impacts on tropospheric ozone depletion and
the fates of atmospheric pollutants, including mercury and hydrocarbons.
Bromine chloride (BrCl) produces bromine and chlorine radicals upon
photolysis and links these two halogen cycles. However, because of
the limited number of BrCl measurements, the relative importance of
its production and removal pathways are uncertain. Here we report
BrCl observations near Utqiaġvik, AK, during March–May
2016 using chemical ionization mass spectrometry as part of the Photochemical
Halogen and Ozone Experiment: Mass Exchange in the Lower Troposphere
(PHOXMELT). Two distinct BrCl diel regimes were identified, with daytime
BrCl primarily observed in March and nighttime BrCl observed in April
and May, demonstrating a dependence on photochemistry. The dominant
BrCl production mechanisms for these regimes were explored using a
zero-dimensional numerical model constrained to a suite of halogen
measurements. Multiphase reactions on the snowpack surface, mainly
via Cl2 + Br–(aq) and HOBr + Cl–(aq), are predicted to be the largest contributors to near-surface
BrCl production. Average net snowpack fluxes of 1.9 × 108 and 2.2 × 108 BrCl molecules cm–2 s–1 for two case periods in March and May are
needed to explain the observations. The findings in this work highlight
coupled bromine and chlorine chemistry and important halogen activation
pathways in the springtime Arctic boundary layer.