Environmental influences on Arctic halogen chemistry: investigation of melt onset and snowpack properties

Abstract

Reactive halogen radicals (e.g. Br, Cl and their oxide forms) dominate tropospheric oxidation mechanisms during Arctic springtime (Feb. – Apr.) by depleting ozone and changing the fate of pollutants. During ozone depletion events, reactive bromine radicals rapidly oxidize mercury which gets subsequently deposited, becoming more bioavailable. During Arctic springtime, a heterogeneous surface reaction (referred to as BrO recycling) between hypobromous acid (HOBr) and bromide (Br-) rapidly increases the abundance of reactive bromine episodically up to 40 pptv peaks. However, as spring transitions to summer (May - June), elevated reactive bromine levels suddenly decrease. There are two key requirements to maintain BrO recycling including surface area and sea salt (i.e. bromide) abundance. This study investigated environmental factors that impact BrO recycling during late spring (May-June) in the Arctic, including temperature, snowpack depth and rain/snow precipitation events. Near horizon BrO was measured using Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) at Barrow, AK and above frozen Arctic sea ice. The late spring “end” to elevated reactive bromine (referred as the Seasonal End Date, SED) was objectively determined at all sites (N=12). Air temperature derived melt onset dates were determined for all sites (N=12) and occurred within two days of the SED (RMS = 1.8 days, R² = 0.989). Through these studies, we determined BrO recycling is hindered by melt onset of snowpack, ending the reactive bromine season.