A Comparison of Process-Scale Modeling and Measurements of Atmosphere-Snowpack Exchange of Nitrogen Oxides at Summit, Greenland

Abstract:

Snowpack over glacial ice is a reservoir for reactive nitrogen gases. Previous studies indicate nitrogen oxides (NO_x) are generated in snowpack interstitial air through photolysis of nitrate (NO₃^-). Gradients in NO_x mixing ratios between snowpack interstitial air and the overlying atmosphere regulate NO_x surface exchange, which affects the Arctic ozone budget and climate. To better understand the dynamics of cryosphere-atmosphere exchange of NO_x in the Arctic, we use a 1-D process-scale model to evaluate measurements of NO_xin and above the snowpack during March-May 2009 at Summit, Greenland. The model is based upon the processes previously presented in the snowpack chemistry and physics model, MISTRA-SNOW, which represents snow grains as spheres with surfaces uniformly coated by an aqueous phase.

Modeled profiles of NO, NO₂, and O₃ up to ~ 2 meters deep into the snowpack for March-May 2009 have been compared to measured profiles and will be presented. During the March-May time period at Summit, low irradiances are observed during March, diurnal irradiance profiles are observed during April, and the sun never sets in May. The model results suggest a key chemical pathway for the formation of NO₂ during “nighttime” that was previously unexplained. In addition, modeled 24-hour NO_x fluxes are compared to measured NO_x fluxes from a MET tower at Summit. Modeled fluxes of NO_x in April 2009 are the same order of magnitude as the measurements; however, modeled fluxes of NO_x deviate up to one order of magnitude from measurements in May 2009. A detailed analysis of the modeled/measured flux comparison will be presented.