Modeling the Sources and Chemistry of Polar Tropospheric Halogens (Cl, Br, and I) Using the CAM‐Chem Global Chemistry‐Climate Model

Abstract

Current chemistry climate models do not include polar emissions and chemistry of halogens.This work presents the first implementation of an interactive polar module into the very short-lived (VSL) halogen version of the Community Atmosphere Model with Chemistry (CAM-Chem) model. The polar module includes photochemical release of molecular bromine, chlorine, and interhalogens from the sea-ice surface, and brine diffusion of iodine biologically produced underneath and within porous sea-ice. It also includes heterogeneous recycling of inorganic halogen reservoirs deposited over fresh sea-ice surfaces and snow-covered regions. The polar emission of chlorine, bromine, and iodine reach approximately 32, 250, and 39 Gg/year for Antarctica and 33, 271, and 4 Gg/year for the Arctic, respectively, with a marked seasonal cycle mainly driven by sunlight and sea-ice coverage. Model results are validated against polar boundary layer measurements of ClO, BrO, and IO, and satellite BrO and IO columns. This validation includes satellite observations of IO over inner Antarctica for which an iodine "leapfrog" mechanism is proposed to transport active iodine from coastal source regions to the interior of the continent. The modeled chlorine and bromine polar sources represent up to 45% and 80% of the global biogenic VSL Cl and VSL Br emissions, respectively, while the Antarctic sea-ice iodine flux is~10 times larger than that from the Southern Ocean. We present the first estimate of the contribution of polar halogen emissions to the global tropospheric halogen budget. CAM-Chem includes now a complete representation of halogen sources and chemistry from pole-to-pole and from the Earth's surface up to the stratopause. Key Points:• The contribution of polar sea-ice sources to the global tropospheric halogen budget has been computed using the CAM-Chem model • The annual halogen flux from the sea-ice surface represents between 45% and 80% of the global chloro-and bromo-carbon oceanic VSL emissions • In Antarctica, the annual iodine sea-ice flux is~10 times larger than the oceanic inorganic iodine sourceSupporting Information:• Supporting Information S1

Pan-Arctic surface ozone: modelling vs. measurements

“…Ozone-sonde data from three adjacent high Arctic Canadian sites (Resolute, Eureka, and Alert), satellite BrO trop , and back-trajectory model output clearly indicate a large ODE (and BEE) in association with a stormy system, the event of which is successfully captured by the two models, further confirming that ODEs and BEEs can be transported over long distances. Although our global models cannot be able to reproduce small-scale ODEs, the success of the models in capturing large-scale ODEs (and BEEs) gives additional evidence from a chemistry side to the proposed mechanism of SSA production and reactive-bromine release from blowing snow on sea ice (Yang et al, 2008(Yang et al, , 2019Frey et al, 2020). Note that the success of the blowingsnow mechanism does not necessarily rule out other possibilities, including the proposed candidates of reactive bromine from snowpack, open leads, frost flowers, sea ice surface, etc.…”

“…All parameters applied in this study for the Arctic SSA simulation are directly taken from our recent SSA modelling work by Yang et al (2019), including a 3.5 times Antarctic snow salinity for the Arctic. The Antarctic Weddell Sea cruise data (Frey et al, 2020) are a probability of surface snow salinity, which is different to the constant salinity value (= 0.3 psu, practical salinity unit) used in Legrand et al 2016, Zhao et al (2017), and Rhodes et al (2017). The trebled snow salinity assumption is taken from Yang et al (2008) to reflect the likelihood that Arctic snow is more saline than in the Antarctic due to reduced precipitation.…”

“…The relative higher salinity in the Arctic is partly related to less precipitation as already mentioned. For instance, the depth of snowpack on sea ice near Barrow, Alaska, is in a range of 10-40 cm (Krnavek et al (2012), while in the Weddell Sea, the mean snow depth over first-year ice (FYI) is 20.9 and 50.0 cm over multi-year ice (MYI) (Frey et al, 2020).…”

“…Recent winter cruise data from the Weddell Sea, Antarctica, confirm that the sea ice surface is a large source of sea salt aerosol (Frey et al, 2020;Yang et al, 2019). Like the open-ocean-sourced sea spray, the sea-ice-sourced SSA is also a large reservoir of various chemical compounds, including inorganic halogens.…”

Long-term time series of Arctic tropospheric BrO derived from UV–VIS satellite remote sensing and its relation to first-year sea ice

“…Yang et al (2020) used both a chemistry transport and a chemistry climate model to model tropospheric BrO and compared the outputs with satellite columns and ground-based measurements. Fernandez et al (2019) provided 4 years of polar spring comparisons between GOME-2A instrument columns and model runs, for the Arctic and Antarctic, having implemented polar halogen chemistry into the CAM-Chem model.…”