Insights on nitrate sources at Dome C (East Antarctic Plateau) from multi-year aerosol and snow records

Abstract

A B S T R A C THere we present the first multi-year record of nitrate in the atmospheric aerosol (2005Á2008) and surface snow (2006Á08) from central Antarctica. PM10 and size-segregated aerosol, together with superficial snow, have been collected all year-round at high resolution (daily for all the snow samples and for most of aerosol samples) at Dome C since the 2004/05 field season and analysed for main and trace ionic markers. The suitability of the sampling location in terms of possible contamination from the base is shown in detail. In spite of the relevance of nitrate in Antarctic atmosphere, both for better understanding the chemistry of N cycle in the plateau boundary layer and for improving the interpretation of long-term nitrate records from deep ice core records, nitrate sources in Antarctica are not well constrained yet, neither in extent nor in timing. A recurring seasonal pattern was pointed out in both aerosol and snow records, showing summer maxima and winter minima, although aerosol maxima lead the snow ones of 1Á2 months, possibly due to a higher acidity in the atmosphere in mid-summer, favouring the repartition of nitrate as nitric acid and thus its uptake by the surface snow layers. On the basis of a meteorological analysis of one major nitrate event, of data related to PSC I extent and of irradiance values, we propose that the high nitrate summer levels in aerosol and snow are likely due to a synergy of enhanced source of nitrate and/or its precursors (such as the stratospheric inputs), higher solar irradiance and higher oxidation rates in this season. Moreover, we show here a further evidence of the substantial contribution of HNO 3 /NOx re-emission from the snowpack, already shown in previous works, and which can explain a significant fraction of atmospheric nitrate, maintaining the same seasonal pattern in the snow. As concerning snow specifically, the presented data suggest that nitrate is likely to be controlled mainly by atmospheric processes, not on the daily timescale but rather on the seasonal one.

Modelling the physical multiphase interactions of HNO<sub>3</sub> between snow and air on the Antarctic Plateau (Dome C) and coast (Halley)

“…(3), the net adsorption rate is expressed as Ullerstam et al (2005b) have shown that for partial pressures of HNO 3 lower than 10 −5 Pa the ice surface is not entirely covered with HNO 3 and, therefore, undersaturated. The annual average atmospheric partial pressure of HNO 3 recorded at Dome C is ∼ 10 −6 Pa (Traversi et al, 2014) and is ∼ 10 −7 Pa at Halley ; hence, the ice surface is unlikely to be saturated with HNO 3 . A nonequilibrium kinetic approach is taken instead of an equilibrium adsorption as natural snowpacks are constantly undergoing sublimation and condensation of H 2 O, especially in the skin layer, due to temperature gradients present over a range of timescales from a fraction of seconds to days and seasons (Bartels-Rausch et al, 2014).…”

“…In particular, concentrations of sea salt and other particles that may scavenge atmospheric HNO 3 are low on the East Antarctic Plateau (Legrand et al, 2016). Hence, the main atmospheric nitrate is gaseous HNO 3 that dissolves in or adsorbs onto snow grains (Traversi et al, 2014).…”

Air–snow transfer of nitrate on the East Antarctic Plateau – Part 2: An isotopic model for the interpretation of deep ice-core records

“…Stratospheric inputs of nitrate are believed to be mostly caused by the sedimentation of polar stratospheric clouds (PSCs) in winter (Seinfeld and Pandis, 1998;Jacob, 1999). The interpretation of nitrate deep ice-core records remains elusive (e.g., Wolff et al, 2010), mainly because its deposition to the snow is not irreversible (Traversi et al, 2014, and references therein) at low-accumulation sites such as Dome C or Vostok (78…”

Potential genesis and implications of calcium nitrate in Antarctic snow

“…However, there are many uncertainties and speculations over the sources of nitrate, its seasonality, and post-depositional losses (e.g. Traversi et al, 2014;Shi et al, 2015).…”

“…Several studies have focused on its origin (e.g. Wolff, 1995;Wagenbach et al, 1998;Traversi et al, 2014), seasonal and spatial variations (e.g. Dahe et al, 1992;Lee et al, 2014;Savarino et al, 2007), air-snow interactions (e.g.…”