Interactions of bromine, chlorine, and iodine photochemistry during ozone depletions in Barrow, Alaska

Abstract

Abstract. The springtime depletion of tropospheric ozone in the Arctic is known to be caused by active halogen photochemistry resulting from halogen atom precursors emitted from snow, ice, or aerosol surfaces. The role of bromine in driving ozone depletion events (ODEs) has been generally accepted, but much less is known about the role of chlorine radicals in ozone depletion chemistry. While the potential impact of iodine in the High Arctic is more uncertain, there have been indications of active iodine chemistry through observed enhancements in filterable iodide, probable detection of tropospheric IO, and recently, observation of snowpack photochemical production of I 2 . Despite decades of research, significant uncertainty remains regarding the chemical mechanisms associated with the bromine-catalyzed depletion of ozone, as well as the complex interactions that occur in the polar boundary layer due to halogen chemistry. To investigate this, we developed a zero-dimensional photochemical model, constrained with measurements from the 2009 OA-SIS field campaign in Barrow, Alaska. We simulated a 7-day period during late March that included a full ozone depletion event lasting 3 days and subsequent ozone recovery to study the interactions of halogen radicals under these different conditions. In addition, the effects of iodine added to our Base Model were investigated. While bromine atoms were primarily responsible for ODEs, chlorine and iodine were found to enhance the depletion rates and iodine was found to be more efficient per atom at depleting ozone than Br. The interaction between chlorine and bromine is complex, as the presence of chlorine can increase the recycling and production of Br atoms, while also increasing reactive bromine sinks under certain conditions. Chlorine chemistry was also found to have significant impacts on both HO 2 and RO 2 , with organic compounds serving as the primary reaction partner for Cl atoms. The results of this work highlight the need for future studies on the production mechanisms of Br 2 and Cl 2 , as well as on the potential impact of iodine in the High Arctic.

Bromine atom production and chain propagation during springtime Arctic ozone depletion events in Barrow, Alaska

“…A detailed description of the model can be found in Thompson et al (2015). We will describe the model only briefly here.…”

“…Table 1 contains an abbreviated list of the reactions included in the model, showing only those reactions that are central to the production, propagation, and termination of bromine radical chemistry, which is the focus of this study. A complete list of reactions can be found in Thompson et al (2015).…”

“…Here, we present results from our study using a zerodimensional model constrained with time-varying measurements of molecular halogens, HOBr, O 3 , CO, NO, NO 2 , and VOCs (volatile organic compounds) from the 2009 OASIS campaign in Barrow, Alaska. This work builds on the analysis presented in Thompson et al (2015) using the same model framework. By constraining our model with observations, we were able to conduct an in-depth study of the halogen atom recycling occurring under varying conditions that were observed during the campaign.…”

Horizontal and vertical structure of reactive bromine events probed by bromine monoxide MAX-DOAS

“…At 100 µm 2 cm −3 and thermal velocity of HOBr at 253 K (−20 • C), c = 255 m s −1 , and thus k het = 0.0038 s −1 , corresponding to an ∼ 4 min HOBr lifetime. Thompson et al (2015) indicate the photolysis rate J (HOBr) = 0.0023 s −1 for springtime Barrow (Utqiaġvik) conditions, so this surface area density results in a heterogeneous reactivity rate that competes with HOBr photolysis. Photolysis of HOBr cycles reactive bromine and destroys ozone but does not increase the reactive bromine pool.…”

Reactive bromine in the low troposphere of Antarctica. Estimations at two research sites

“…These events were firstly observed in the Arctic 25 region by correlating the detection of filterable bromine with ODEs (e.g., Barrie et al, 1988) and, later on, by observing very high amount of BrO (tens of pmol mol -1 ) in the boundary layer just after the polar sunrise (e.g., Hausmann and Platt, 1994;Tuckermann et al, 1997). Since then, several studies have tried to determine the chemical sources, sinks and pathways of these compounds (e.g., Simpson et al, 2007 and2015). In particular, the main BrO source reactions involve: In pristine environments (i.e., very low nitrogen oxide), along with photodissociation (in polar spring JBrO ~ 3·10 -2 s -1 , e.g., Thompson et al, 2015), the BrO sink reactions associated with the catalytic ODEs are:…”