Reproducing Arctic springtime tropospheric ozone and mercury depletion events in an outdoor mesocosm sea ice facility

Zhiyuan Gao,Feiyue Wang,Alfonso Saiz-Lopez,Nicolas-Xavier Geilfus

doi:10.5194/acp-22-1811-2022

Abstract

Abstract. The episodic buildup of gas-phase reactive bromine species over sea ice and snowpack in the springtime Arctic plays an important role in boundary layer processes, causing annual concurrent depletion of ozone and gaseous elemental mercury (GEM) during polar sunrise. Extensive studies have shown that these phenomena, known as bromine explosion events (BEEs), ozone depletion events (ODEs), and mercury depletion events (MDEs) are all triggered by reactive bromine species that are photochemically activated from bromide via multi-phase reactions under freezing air temperatures. However, major knowledge gaps exist in both fundamental cryo-photochemical processes causing these events and meteorological conditions that may affect their timing and magnitude. Here, we report an outdoor mesocosm study in which we successfully reproduced ODEs and MDEs at the Sea-ice Environmental Research Facility (SERF) in Winnipeg, Canada. By monitoring ozone and GEM concentrations inside large acrylic tubes over bromide-enriched artificial seawater during sea ice freeze-and-melt cycles, we observed mid-day photochemical ozone and GEM loss in winter in the in-tube boundary layer air immediately above the sea ice surface in a pattern that is characteristic of BEE-induced ODEs and MDEs in the Arctic. The importance of UV radiation and the presence of a condensed phase (experimental sea ice or snow) in causing such reactions were demonstrated by comparing ozone and GEM concentrations between the UV-transmitting and UV-blocking acrylic tubes under different air temperatures. The ability of reproducing BEE-induced photochemical phenomena in a mesocosm in a non-polar region provides a new approach to systematically studying the cryo-photochemical processes and meteorological conditions leading to BEEs, ODEs, and MDEs in the Arctic, their role in biogeochemical cycles across the ocean–sea ice–atmosphere interface, and their sensitivities to climate change.

Highlights

IntroductionEvery year during springtime in the Arctic, a series of episodic photochemical events is observed concurrently in the boundary layer air, including bromine explosion events (BEEs), ozone depletion events (ODEs), and mercury depletion events (MDEs) (Barrie et al, 1988; Barrie and Platt, 1997; Bottenheim et al, 1986; Oltmans et al, 1989; Oltmans and Komhyr, 1986; Platt and Hausmann, 1994; Schroeder et al, 1998; Steffen et al, 2005)
While there is a general consensus on the reaction schemes involved in bromine explosion events (BEEs), ozone depletion events (ODEs) and mercury depletion events (MDEs) (Fig. 1), major uncertainties exist with respect to the fundamental cryophotochemical processes causing these events and meteorological conditions that may affect their timing and magnitude
We show that the Arctic springtime ODEs and MDEs can be reproduced in an outdoor mesocosm sea ice facility located in an urban area far away from the Arctic

Summary

Introduction

Every year during springtime in the Arctic, a series of episodic photochemical events is observed concurrently in the boundary layer air, including bromine explosion events (BEEs), ozone depletion events (ODEs), and mercury depletion events (MDEs) (Barrie et al, 1988; Barrie and Platt, 1997; Bottenheim et al, 1986; Oltmans et al, 1989; Oltmans and Komhyr, 1986; Platt and Hausmann, 1994; Schroeder et al, 1998; Steffen et al, 2005). Subsequent studies have shown that these events are triggered by the cycling of photolytically activated halogen species (especially bromine species such as Br, BrO, HOBr) over sea ice or snowpack, which rapidly react with ozone and gaseous elemental mercury (GEM) in the boundary layer air (Abbatt et al, 2012; Bognar et al, 2020; Pratt et al, 2013; Saiz-Lopez and von Glasow, 2012; Simpson et al, 2007b; Wang et al, 2019) Atmospheric and sea state conditions, such as air mass origin, sea ice and boundary layer dynamics, and blowing-snow events, may affect the timing and magnitude of BEEs, ODEs, and MDEs (Bognar et al, 2020; Moore et al, 2014; Thomas et al, 2011; Zhao et al, 2016)

Methods

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Discussion

Conclusion