Influence of Sea Ice‐Derived Halogens on Atmospheric HO_x as Observed in Springtime Coastal Antarctica

Abstract

We present first observations of OH and (HO2 + RO2) carried out in Antarctica outside the summer season. Measurements were made over 23 days in spring at the coastal Antarctic station Halley. Increases in concentrations were evident during the measurement period due to rapidly increasing solar irradiance, and clear diurnal cycles were present throughout. There were also notable differences in air mass composition depending on wind direction. Air masses that had traversed the sea ice zone had both higher concentrations of OH and a larger OH:(HO2 + RO2) ratio. We use steady state kinetic arguments and a 0‐D box model to probe the chemical drivers. We find that differences in bromine chemistry, previously measured at Halley, are sufficient to account for the observed differences in OH concentration as well as the ratio. There is some evidence also that chlorine chemistry is influencing concentrations of RO2.

The importance of alkyl nitrates and sea ice emissions to atmospheric NO<sub>x</sub> sources and cycling in the summertime Southern Ocean marine boundary layer

“…6A). There is evidence that sea ice can lead to enhanced peroxy radical production (Brough et al, 2019 Cl atoms are much more reactive with hydrocarbons than OH (Monks, 2005) and can enhance hydrocarbon oxidation even when present at low concentrations. Brough et al (2019) suggest that air masses that traversed the sea ice zone contained photolabile chlorine compounds that built up at night until photolysis occurred during the next day (Brough, et al, 2019).…”

“…However, unusually low δ 18 O-NO3values observed at the midlatitudes and in the Weddell Sea indicate the increased importance of peroxy radicals (and decreased importance of O3) in NO oxidation to NO2. At the mid-latitudes peroxy radicals (RO2) may derive from RONO2 photolysis in the MBL, while In the Weddell Sea, sea ice appears to play an important role in the formation of this oxidant via its influence on chlorine chemistry in the MBL (Brough, et al, 2019). This implies that snow covered sea ice is not only a source of NOx but also other species that have the potential to change the composition of the atmosphere above the ice and impact NOx oxidation chemistry.…”

“…During summer, high levels of photochemistry result in the emission of reactive gases from sea ice and snow cover in the Antarctic. As a result, highly elevated concentrations of hydrogen oxide radicals (HOx = OH + peroxy radicals), halogens, nitrous acid (HONO), and NOx have been observed during spring and summer in the polar regions (Brough et al, 2019). Furthermore, photochemical production of NOx within the surface snow of Antarctica and subsequent oxidation in the overlying atmosphere represents a significant NO3source to the Antarctic troposphere (Jones, et al, 2000;Jones, et al, 2001).…”

Reply on RC1

“…6A). There is evidence that sea ice can lead to enhanced peroxy radical production (Brough et al, 2019). In that work, increased HO2 + RO2 concentrations were observed during spring at a coastal Antarctic site when air masses arrived from across a sea ice covered zone.…”

“…During summer, high levels of photochemistry result in the emission of reactive gases from sea ice and snow cover in the Antarctic. As a result, highly elevated concentrations of hydrogen oxide radicals (HOx = OH + peroxy radicals), halogens, nitrous acid (HONO), and NOx have been observed during spring and summer in the polar regions (Brough et al, 2019).…”