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

Abstract:

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. NO_x) 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 NO_x 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^-), NO_x, 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 (F_PRI), the re-deposition of photo-produced NO_x to the snow surface (F_PD), and the upward flux of NO_x from the snow (F_NOx) to examine nitrogen recycling and redistribution across Antarctica. F_PD is larger than F_PRI 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.