Molecular Halogens Above the Arctic Snowpack: Emissions, Diurnal Variations, and Recycling Mechanisms

Abstract

Elevated levels of reactive bromine and chlorine species in the springtime Arctic boundary layer contribute to ozone depletion and mercury oxidation, as well as reactions with volatile organic compounds. Recent laboratory and field studies have revealed that snowpack photochemistry leads to Br2 and Cl2 production, the mechanisms of which remain poorly understood. In this work, we use a photochemical box model, with a simplified snow module, to examine the halogen chemistry occurring during the March 2012 Bromine, Ozone, and Mercury Experiment (BROMEX) near Utqiaġvik (Barrow), Alaska. Elevated daytime Br2 levels (e.g., 6–30 parts per trillion (ppt) at around local noon) reported in previous studies and in this work may be explained by Br + BrNO2/BrONO2 reactions under conditions of depleted O3 (<~10 ppb) and background NO2 (10–100 ppt). Even at low background NOx levels at Utqiaġvik, ClONO2 is predicted to be important in the production of Cl2 via multiphase reaction with Cl−. In the late afternoon, photolysis alone cannot explain the rapid decrease of Cl2 observed in the Arctic boundary layer. Heterogeneous reactions of Cl2 on aerosol particles and surface snowpack are suggested to play a key role in atmospheric Cl2 removal and possible BrCl production. Given the importance of the snowpack in the multiphase chemistry of the Arctic boundary layer, future measurements should focus on vertically resolved measurements of NOx and reactive halogens, as well as simultaneous particulate and snow halide measurements, to further evaluate and isolate the halogen production and vertical propagation mechanisms through one‐dimensional modeling.

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

“…Note that the polar module includes the initial sea‐ice flux of bromine and chlorine in the form of Br 2 ( ), BrCl ( ), and Cl 2 ( , see Table ). Cl 2 emission also takes place in the polar atmosphere (Liao et al, ) and can be rapidly recycled to and from BrCl at the sea‐ice surface and/or snow (Foster et al, ; Wang & Pratt, ). Thus, once the F 0 value was adjusted for Br 2 and Cl 2 , we compute the final F ICE flux by introducing a halogen‐species interconversion factor ( R XY ) representing the fraction of bromine and chlorine atoms being released from the sea‐ice surface as Br 2 , Cl 2 , and BrCl (see Table ).…”

“…Both ice and snow have been recognized as important sources of reactive halogen species to the polar atmosphere. The photochemical heterogeneous production of reactive bromine and chlorine occurring at the sea‐ice and snowpack surface have been suggested as a very efficient mechanism to release the enhanced halide content prevailing over ice‐covered regions (Abbatt et al, ; Dominé & Shepson, ; Foster et al, ; Pratt et al, ; Wang & Pratt, ; Yang et al, ). Additionally, the biologically induced release of inorganic iodine (and possibly bromine) from underneath the sea‐ice followed by brine diffusion (Saiz‐Lopez, Boxe, et al, ) can represent the dominant source of iodine (and a nonnegligible source of bromine) to the polar MBL.…”

Chlorine nitrate in the atmosphere

“…ACE-FTS is a solar occultation instrument on the Canadian SciSat Earth observation satellite, launched in August 2003. ClONO 2 measurements have been published by Wolff et al (2008), Mahieu et al (2005), Nassar et al (2006), Dufour et al (2006), Santee et al (2008), Jones et al (2011), Waymark et al (2013), and Sheese et al (2016). ACE-FTS is still operational at the time of this writing.…”

Chlorine Nitrate in the Atmosphere

“…Most investigations focus on these events as a source of NO x (e.g. Jackman et al, 1990) but Solomon and Crutzen (1981) ClONO 2 is predicted to be important in the springtime Arctic boundary layer ozone chemistry (Wang and Pratt, 2017). Via multiphase reaction it contributes to the generation of Cl 2 .…”