Chemical Composition Of Ice Surfaces: Implications For Springtime Bromine Chemistry

Abstract

Reactive bromine chemistry is responsible for events of almost total tropospheric O3 destruction and the deposition of mercury during the Arctic spring. The source of the majority of the atmospheric bromine loading is salts from seawater, but many questions remain on the mechanism by which salts are transported and chemically activated to reactive species. Specifically, the role of snow and ice surfaces in exchanging bromine with the atmosphere needed investigation. Therefore, we undertook a detailed study of the ionic composition of selected ice surfaces near Barrow, Alaska and tracked modifications with respect to Cl- and Na+ (sea-salt tracers) in approximately 1,400 samples. We developed data analysis tools to observe modifications and related these methods to the traditional enrichment factor and the non-sea-salt abundance methods. Surface snow was highly modified in Br- composition by atmospheric exchanges that both add and remove bromine, providing evidence for snow's involvement in reactive bromine chemistry. Calcium was enriched by dust input. Sulfate in surface snow was fractionated at the source by mirabilite (Na2SO 4 • 6H2O) precipitation and enriched by Arctic haze inputs. Frost flowers are vapor-grown ice crystals that wick brine and may be involved in sea-salt aerosol production and production of reactive halogen species. Detailed examination of frost flower growth and chemical composition shows that they are sites of mirabilite precipitation and separation, which can lead to sulfate-depleted aerosol particles, but show no sign of direct reactive bromine production. By simultaneously studying snow, ice, aerosol particles, and gas-phase bromine species, we made a mass balance of bromine in various reservoirs. This mass balance points away from frost flowers and towards snow as the dominant source of reactive bromine. This work develops a mechanistic picture of how reactive bromine chemistry depends upon snow and sea ice that is needed to make meaningful predictions of how the recent changes to the Arctic sea ice cover will affect air pollution chemistry.