Multiphase Reactive Bromine Chemistry during Late Spring in the Arctic: Measurements of Gases, Particles, and Snow

Bromine radicals (Br center dot) cause ozone depletion and mercury deposition in the Arctic atmospheric boundary layer, following Polar sunrise. These Br radicals are primarily formed by the photolysis of molecular bromine (Br-2), which is photochemically produced in the snowpack. Recently, it was shown that bromine monoxide (BrO center dot), formed from the reaction of Br center dot with ozone, is episodically present until the onset of snowmelt in late Arctic spring. To examine the drivers of this late spring shutdown of reactive bromine chemistry, the gases Br-2, HOBr, BrO, and BrCl were continuously monitored using chemical ionization mass spectrometry during the spring (March-May 2016) near Utqiagvik, Alaska. On May 10th, all four reactive bromine species fell below levels of detection at the same time that air temperature increased above 0 degrees C, surface albedo decreased, and snowmelt onset was observed. Prior to the cessation of atmospheric bromine chemistry, local surface snow samples in early May became significantly enriched in bromide, likely due to the slowdown of reactive bromine recycling with continued deposition but decreased emissions from the snowpack. Particulate bromide concentrations were not sufficient to explain the quantities of reactive bromine gases observed and decreased upon snowmelt. Low wind speeds during the weeks preceding the cessation of reactive bromine chemistry point to the lack of a contribution to bromine chemistry from blowing snow. Together, these results further highlight the significance of the surface snowpack in multiphase bromine recycling with important implications as the melt season arrives earlier due to climate change.