Incorporation of snow nitrate photochemistry into a global chemical transport model: Impact on boundary layer chemistry and ice core records in Antarctica and Greenland

Wednesday, 17 December 2014
Maria Christine Zatko, Becky Alexander and Lei Geng, University of Washington Seattle Campus, Seattle, WA, United States
The formation and cycling of reactive nitrogen in the atmosphere has important implications for air quality and the oxidation capacity of the atmosphere. Snowpack NO3- photolysis provides a source of oxidants (e.g. OH, O3) and oxidant precursors (e.g. NOx) to the overlying boundary layer in remote, snow-covered regions. We have developed a parameterization for calculating the depth dependence of actinic flux in snowpack, and have incorporated it into a global chemical transport model, GEOS-Chem. We use the model to estimate the flux and redistribution of nitrogen in Antarctic and Greenland snowpacks due to snow NO3- photolysis, and examine the implications for ice core NO3- concentration and isotope observations. The modeled potential flux of NOx from Antarctic snow (3-8x108 molec cm-2 s-1) and e-folding depths of ultraviolet actinic flux (26-80 cm) are comparable to observations. Antarctic boundary layer total nitrate (HNO3+NO3-), NOx, and OH concentrations are increased by an order of magnitude and boundary layer O3 concentrations are roughly doubled when snowpack photodenitrification is included in GEOS-Chem. The model-calculated nitrogen recycling factor (NRF) suggests that NO3- is recycled multiple times per year over the majority of the Antarctic continent. NO3- remains in the snow photolytic zone for at least 0.65 to 24 years, and depends on the snow accumulation rate. GEOS-Chem is also used to compare primary deposition of NO3- to the snow surface originating from long-range transport (FPRI), the re-deposition of photo-produced NOx to the snow surface (FPD), and the upward flux of NOx from the snow (FNOx) to examine nitrogen recycling and redistribution across Antarctica. FPD is larger than FPRI across much of Antarctica, suggesting large-scale redistribution of nitrogen across the entire continent. The utility of nitrogen isotopes measurements in Antarctic ice cores in assessing the degree of post-depositional processing across the Antarctic continent is also investigated.

Preliminary results of the flux and redistribution of nitrogen in Greenland snowpacks due to snow NO3- photolysis and the implications for ice core NO3- concentration and isotope records in Greenland will also be presented.