Abstract
Abstract. Snowpack is a multiphase (photo)chemical reactor that strongly influences the air composition in polar and snow-covered regions. Snowpack plays a special role in the nitrogen cycle, as it has been shown that nitrate undergoes numerous recycling stages (including photolysis) in the snow before being permanently buried in the ice. However, the current understanding of these physicochemical processes remains very poor. Several modelling studies have attempted to reproduce (photo)chemical reactions inside snow grains, but these have relied on strong assumptions to characterise snow reactive properties, which are not well defined. Air–snow exchange processes such as adsorption, solid-state diffusion, or co-condensation also affect snow chemical composition. Here, we present a physically based model of these processes for nitrate. Using as input a 1-year-long time series of atmospheric nitrate concentration measured at Dome C, Antarctica, our model reproduces with good agreement the nitrate measurements in the surface snow. By investigating the relative importance of the main exchange processes, this study shows that, on the one hand, the combination of bulk diffusion and co-condensation allows a good reproduction of the measurements (correlation coefficient r = 0.95), with a correct amplitude and timing of summer peak concentration of nitrate in snow. During winter, nitrate concentration in surface snow is mainly driven by thermodynamic equilibrium, whilst the peak observed in summer is explained by the kinetic process of co-condensation. On the other hand, the adsorption of nitric acid on the surface of the snow grains, constrained by an already existing parameterisation for the isotherm, fails to fit the observed variations. During winter and spring, the modelled concentration of adsorbed nitrate is respectively 2.5 and 8.3-fold higher than the measured one. A strong diurnal variation driven by the temperature cycle and a peak occurring in early spring are two other major features that do not match the measurements. This study clearly demonstrates that co-condensation is the most important process to explain nitrate incorporation in snow undergoing temperature gradient metamorphism. The parameterisation developed for this process can now be used as a foundation piece in snowpack models to predict the inter-relationship between snow physical evolution and snow nitrate chemistry.
Highlights
1.1 Nitrogen cycle and snow chemistryThe nitrogen cycle governs atmospheric oxidants budget through the photochemistry of nitrogen oxides (NOx = NO + NO2), which are strongly coupled with ozone (O3) and hydroxyl (OH) chemistry in the troposphere (Seinfeld and Pandis, 1998; Finlayson-Pitts and Pitts, 2000)
In this study we investigated the role of three processes that intervene in air–snow exchange of nitrate at Dome C (DC)
It revealed that the co-condensation of nitrate along with the condensation of water vapour flux driven by thermal gradient metamorphism is a major process that is absolutely required to explain the summer peak of nitrate measured in surface snow
Summary
A production pathway involving nitrate photolysis in snow was rapidly elucidated afterwards (Jones et al, 2000; Dibb et al, 2002; Honrath et al, 2002) These pioneering works drove numerous field campaigns (e.g. SNOW99: Honrath et al, 2000b; ISCAT2000: Davis et al, 2004; ANTCI: Eisele et al, 2008; CHABLIS: Jones et al, 2008; OPALE: Preunkert et al, 2012), as well as laboratory studies (Honrath et al, 2000a; Dubowski et al, 2001, 2002; Chu and Anastasio, 2003, 2007; Cotter et al, 2003; Zhu et al, 2010; Meusinger et al, 2014; Berhanu et al, 2014) and modelling studies (Jacobi and Hilker, 2007; Boxe and Saiz-Lopez, 2008; Liao and Tan, 2008; Bock and Jacobi, 2010; Thomas et al, 2011; Toyota et al, 2014; Erbland et al, 2015; Murray et al, 2015), in order to improve the understanding of the underlying processes responsible for the nitrogen recycling inside the snowpack. These studies focused on the nitrate photolysis in the photic zone of the snowpack and the subsequent release of NOx to the overlying atmosphere
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