Anthropogenic Impacts on Tropospheric Reactive Chlorine and Bromine since the Pre-industrial

Abstract

Tropospheric reactive halogens (chlorine, bromine, iodine) impact the oxidation capacity of the atmosphere, oxidize reduced gases such as methane and mercury, and have profound implications on air quality, climate, and eco-systems. Ice core halogens have been used as proxies for sea ice extent for the pre-satellite era, but are also shown to be heavily influenced by anthropogenic emissions. Both sea ice extent and anthropogenic emissions have changed drastically in the northern hemisphere since the pre-industrial. Using a wide range of Arctic ice core halogen records and a global chemical transport model with state-of-the-science halogen chemistry, my Ph.D. research investigates the magnitude and mechanisms of anthropogenic impacts on halogen trends observed in Arctic ice cores, and the implications for the tropospheric chlorine and bromine budgets. In the first part of my Ph.D. project (Chapter 2), I implemented recent advances in halogen chemistry into the GEOS-Chem model, and presented new chlorine records from six Greenland ice cores covering the pre-industrial (PI) to present-day (PD) transition. The model suggests that the PI to peak-acidity (PA, 1975) increase in non-sea-salt chlorine is mainly caused by anthropogenic emissions of acidic gases, such as SO2, NOx, and HCl, and the decrease from PA to PD is a result of reduced HCl emission from the air pollution mitigation policies in North America and Western Europe. This highlights the importance of anthropogenic impacts on ice core chlorine trends. In the second part of my Ph.D. project (Chapter 3), I implemented an empirically based framework for snowpack bromine production into GEOS-Chem to evaluate the post-depositional loss of bromine for various Arctic ice core sites for the first time. Results suggest that only the Akademii Nauk ice core in the Russian Arctic, shows good preservation of bromine in the snowpack, and all the Greenland ice cores suffer significant bromine loss during the sunlit season. Key factors influencing snowpack bromine preservation include time in the dark (latitude), snowfall burial rate, and potential surface melting. I point out the necessity to further understand snow bromine species and how they evolve over an annual cycle, through future field and laboratory efforts. Based on Chapter 3 results, I explored the ice core bromine trends in Academia Nauk in the third part of my Ph.D. project (Chapter 4), using GEOS-Chem historical simulations with additional anthropogenic bromine sources from coal combustion and leaded gasoline. Model suggests that acid-catalyzed sea-salt debromination of Br2 is the major driver for the observed increase in ice core bromine from PI to PA, consistent with the robust correlation observed between ice core bromine and acidity after 1940 (r=0.9). The simulation underestimates the observed decreases from PA to PD for both bromine and acidity, suggesting uncertainties in anthropogenic emissions of acidic gas precursors in the Russian Arctic during this time period. I showed that changes in sea ice are unlikely to explain the PA to PD decrease in ice core bromine. The results highlight that anthropogenic acidity is essential for understanding ice core bromine trends, and cannot be neglected when interpreting ice core bromine. Future work on interpretation of ice core iodine records is proposed, which rely on better understanding of iodine speciation and chemistry through field observations and model development. Large uncertainties in snowpack bromine speciation and preservation hinder the accurate model representation of snowpack bromine production, and warrants future work as well.