Investigating the impact of snowpack photodenitrification on Antarctic atmospheric chemistry utilizing results from a snowpack radiative transfer model in a global chemical transport model
We utilize a global chemical transport model, GEOS-Chem, coupled to a snowpack radiative transfer model to calculate the flux of NOx from the Antarctic snowpack and examine the implications for the chemistry of the overlying atmosphere and the redistribution of nitrate across the Antarctic continent. The photolysis of nitrate (NO3-) in snowpack is a source of NOx to the overlying atmosphere, with implications for the oxidizing capacity of polar atmospheres. This process also has implications for the preservation of nitrate in ice cores as the NOx created during NO3- photolysis can escape into the atmosphere, convert to HNO3, and redeposit to the snowpack as NO3-. Sensitivity studies involving perturbations centered around a set of parameters determined from surface-based observations in the clean air sector near South Pole Station are performed to determine the importance of wavelength, depth in snowpack, ice grain radius, solar zenith angle, soot concentration, dust concentration, ice grain density, and cloud fraction on actinic flux within the snowpack. Soot concentration, solar incidence angle, and radiation-equivalent mean ice grain radius affect the actinic flux in snowpack most significantly. Based upon observations during an Antarctic traverse from Dome C to Dumont d'Urville (Brandt and Warren, 2008), a fixed grain radius of 100 Ám for continental Antarctica ice grains is used in this study. We compare the lifetime of NOx in the firn against NOx sinks (NO2 + OH) and ventilation to the atmosphere to determine a depth in the snowpack over which to integrate. Preliminary results of the flux of NOx to the atmosphere, and the resulting nitrate redistribution will be shown and compared to atmospheric and ice core observations of NOx and NO3-. This study is the first processed-based global modeling study of photodenitrification.