Postdepositional change in snowpack nitrate from observation of year-round near-surface snow in coastal Antarctica
Postdepositional loss of nitrate in near-surface snowfall is well known, with mean levels of nitrate in ice cores of around 20 to 80 ng g−1, while nitrate in surface snow may occasionally reach over 300 ng g−1. This has been explained as reemission of nitrate (as nitric acid) during aging, via processes that are not yet clear. However, clear seasonal cycles remain in nitrate profiles from higher accumulation rate ice cores and, across Antarctica, the mean concentration of nitrate is remarkably similar despite widely varying deposition conditions, marking nitrate out as quite different to other major ice core species. This paper examines the year-round deposition of nitrate at the snow surface at a coastal Antarctic site and discusses the degree and timing of nitrate loss. At Halley Station, Antarctica, the mean concentration of near-surface snow was 96 ng g−1 over a 2.6 year daily sampling period, while the mean concentration in newly accumulated snow was 79 ng g−1. At the end of this period, a shallow core integrating the sampling period had a mean nitrate concentration of 65 ng g−1 taken over 2 full years of accumulation. Nitrate concentrations in the surface layer were in general highest during the summer period reaching 400 ng g−1 with a mean of about 150 ng g−1 in each of December, January, and February, and lowest during the winter with a mean of around 50 ng g−1 in June. Fresh snow data from Neumayer Station shows a similar seasonal signal, with a mean nitrate concentration of 77 ng g−1, while year-round aerosol data shows total nitrate (particulate and gas phase) in the air is at a minimum in April to June and reaches a maximum in late November [Wagenbach et al., this issue], slightly out of phase with snowfall nitrate. The observation of a reduction in high nitrate concentrations in new snowfall over a few days does not appear to be general, and at Halley, there is evidence of both uptake and loss of nitrate in the surface snow layer, possibly indicating an equilibrium with changing air concentrations. However, there is attenuation of the nitrate signal over the longer period, with concentrations in the ice core taken at the end of sampling never reaching values seen in the upper surface layer.
Authors: Mulvaney, R., Wagenbach, D., Wolff, E. W.