The impact of molecular iodine photochemistry in the Arctic

Reactive halogen chemistry has been shown to instigate or be active during ozone depletion events in the polar regions. It has long been hypothesized that iodine may be an important contributor to ozone depletion in the atmosphere near ice/snow-covered polar surfaces. This has been supported in recent years by measurements of IO and I2 in marine environments and around Antarctica. However, the detection of inorganic iodine compounds (I2, IO, HOI) in the Arctic has been very sparse, and molecular iodine (I2) measurements have not previously been reported. Here we present measurements of I2 in the interstitial snowpack air, conducted at Barrow, Alaska in January and February of 2014. Using chemical ionization mass spectrometry, I2 was measured at up to 35ppt in the snowpack interstitial air when the top of the snowpack was artificially illuminated. During the daylight hours, I2 concentrations were highest in the late afternoon and were significantly higher on clear days than on overcast. Together these observations suggest that, like Br2 and Cl2, I2 has a photochemical production mechanism. While I- can be directly oxidized by O3, this may not be the dominant initiation pathway for sunlit conditions. A zero dimensional photochemical model shows that, because of coupling between XO radicals, proper simulation of ozone depletion events in the arctic will require understanding and inclusion of detailed iodine chemistry.