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.
Thesis (M.S.) University of Alaska Fairbanks, 2016
Table of Contents
Chapter 1: Introduction -- 1.1 Motivation -- 1.2 Arctic tropospheric chemistry -- 1.3 Bromine explosion mechanism -- 1.3.1 Saline surfaces -- 1.3.2 Snow surface area -- 1.4 Melt onset -- 1.5 Ion pulse -- 1.6 Gas-phase bromine seasonal behavior -- 1.7 Multi-AXis differential optical absorption spectroscopy (MAX-DOAS) -- 1.8 Thesis structure -- 1.9 References -- 1.10 Figures -- Chapter 2: Late spring snow melt onset hinders bromine monoxide heterogeneous recycling in the arctic -- 2.1 Introduction -- 2.2 Data sources and methods -- 2.2.1 Bromine monoxide measurement sites -- 2.2.2 Snow/ rain precipitation observations and snow depth measurements -- 2.2.3 Objective determination of seasonal end and recurrence events -- 2.2.4 Objective determination of melt onset dates using surface air temperature -- Results -- 2.3.1 Seasonal end date determination and sensitivity to threshold parameters -- 2.3.2 Melt onset determination and correlation to the seasonal end date -- 2.3.3 Snow depth changes at seasonal end and recurrence events -- 2.3.4 Rain/snow precipitation at seasonal end and recurrence events -- 2.4 Discussion -- 2.4.1 Seasonal end date is the melt onset date -- 2.4.2 Decaying snowpack and rain precipitation prevents BrO recycling -- 2.4.3 Below freezing temperatures and new surface area re-initialize BrO recycling -- 2.5 Conclusion -- 2.6 Acknowledgements -- 2.7 References -- 2.8 Figures -- Chapter 3: Conclusions and outlook -- 3.1 Conclusions -- 3.2 Future Outlook -- 3.3 References.
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