The role of snow in controlling halogen chemistry and boundary layer
oxidation during Arctic spring: A 1D modelling case study
Abstract
Reactive chlorine and bromine species emitted from snow and aerosols can
significantly alter the oxidative capacity of the polar boundary layer.
However, halogen production mechanisms from snow remain highly
uncertain, making it difficult for most models to include descriptions
of halogen snow emissions and to understand the impact on atmospheric
chemistry. We investigate the influence of Arctic halogen emissions from
snow on boundary layer oxidation processes using a one-dimensional
atmospheric chemistry and transport model (PACT-1D). To understand the
combined impact of snow emissions and boundary layer dynamics on
atmospheric chemistry, we model \ch{Cl2} and
\ch{Br2} primary emissions from snow and include
heterogeneous recycling of halogens on both snow and aerosols. We focus
on a two-day case study from the 2009 Ocean-Atmosphere-Sea Ice-Snowpack
(OASIS) campaign at Utqia\.gvik, Alaska. The model
reproduces both the diurnal cycle and high quantity of
\ch{Cl2} observed, along with the measured
concentrations of \ch{Br2}, \ch{BrO},
and \ch{HOBr}. Due to the combined effects of
emissions, recycling, vertical mixing, and atmospheric chemistry,
reactive chlorine is confined to the lowest 15 m of the atmosphere,
while bromine impacts chemistry up to the boundary layer height. Upon
including halogen emissions and recycling, the concentration of
\ch{HO_x} (\ch{HO_x} =
\ch{OH}+\ch{HO2}) at the surface
increases by as much as a factor of 30 at mid-day. The change in
\ch{HO_x} due to halogen chemistry, as well as
chlorine atoms derived from snow emissions, significantly reduce
volatile organic compound (VOC) lifetimes within a shallow layer near
the surface.