Atmospheric nitric oxide and ozone at the WAIS Divide deep coring site: a discussion of local sources and transport in West Antarctica

Abstract

Abstract. First measurements of atmospheric nitric oxide (NO) along with observations of ozone (O3), hydroperoxides (H2O2 and MHP) and snow nitrate (NO3−), on the West Antarctic Ice Sheet (WAIS) were carried out at the WAIS Divide deep ice-coring site between 10 December 2008 and 11 January 2009. Average ±1σ mixing ratios of NO were 19 ± 31 pptv and confirmed prior model estimates for the summer boundary layer above WAIS. Mean ±1σ mixing ratios of O3 of 14 ± 4 ppbv were in the range of previous measurements from overland traverses across WAIS during summer, while average ±1σ concentrations of H2O2 and MHP revealed slightly higher levels with mixing ratios of 743 ± 362 and 519 ± 238 pptv, respectively. An upper limit for daily average NO2 emission fluxes of 4.6–6.6 × 108 molecule cm−2 s−1 was estimated based on photolysis of measured NO3− in surface snowpack. Assuming rapid and complete mixing into the overlying atmosphere, and steady state of NOx, these snow emissions are equivalent to an average (range) production of atmospheric NO2 of 7 (2–53) pptv h−1 for a typical atmospheric boundary-layer depth of 130 (490–20) m. This indicates that local emissions from the snowpack are a significant source of short-lived nitrogen oxides above the inner WAIS. The origins of the air masses reaching WAIS Divide during this campaign were investigated with a 4-day back-trajectory analysis every 4 h. The resulting 168 back trajectories revealed that in 73% of all runs air originated from East Antarctic Plateau regions below 2500 m (41%), coastal Antarctica (17%) and inner WAIS (15%). For these air sources O3 levels were on average 13 ± 3 ppbv. The remaining 27% are katabatic outflows from the East Antarctic Plateau above 2500 m. When near-surface air from the East Antarctic Plateau reaches WAIS Divide through a rapid transport of less than 3 days, O3 levels are on average 19 ± 4 ppbv with maximum mixing ratios of 30 ppbv. Episodes of elevated ozone at WAIS Divide are therefore linked to air mass export off the East Antarctic Plateau, demonstrating that outflows from the highly oxidizing summer atmospheric boundary layer in the interior of the continent can episodically raise the mixing ratios of long-lived atmospheric chemical species such as O3 and enhance the oxidative capacity of the atmosphere above WAIS.