The formation and cycling of reactive nitrogen in the atmosphere has important implications for air quality, the oxidation capacity of the atmosphere, and atmospheric nutrient deposition.  Combustion of fossil fuel, biofuel, and biomass and lightning converts or “fixes” inert nitrogen gas (N₂) into a highly reactive form (NO_x = NO + NO₂).  Other sources of NO_x to the atmosphere include microbial processes in soils and transport from the stratosphere.  Anthropogenic activities currently dominate NO_x sources to the troposphere.  The formation of nitrate (gas-phase HNO₃ and particulate NO₃^-) is the main sink of NO_x in the atmosphere.  Nitrate is water soluble and is lost from the atmosphere through wet and dry deposition to the Earth’s surface.

Due to its importance for atmospheric chemistry, there is interest in determining past variability in atmospheric NO_x concentrations from nitrate concentration measurements in ice cores.  However, the interpretation of these records in terms of past variability in atmospheric NO_x has been difficult due in large part to postdepositional processing of snowpack nitrate.  When nitrate is deposited onto snow and ice surfaces it can undergo subsequent re-emission to the atmosphere as HNO₃ (evaporation) and NO_x (photolysis), with potentially profound implications for the interpretation of chemical signals recorded in polar ice cores.  Photolysis of snowpack nitrate provides a local NO_x source and effectively increases the lifetime of NO_x in snow covered regions.

We are implementing the process of “photodenitrification” into the GEOS-Chem model in order to investigate its implications for nitrogen cycling in Antarctica and for the ice core record of nitrate.

People:

Becky Alexander (lead P.I.), Thomas Grenfell (senior researcher), and Maria Zatko (graduate student) (UW)

Funding:

NSF ANT 0944537

Publications so far:

Zatko, M.C., T.C. Grenfell, B. Alexander, S.J. Doherty, J.L. Thomas and X. Yang, The influence of snow grain size and impurities on the vertical profiles of actinic flux and associated NO_x emissions on the Antarctic and Greenland ice sheets, Atmos. Chem. Phys. Discuss. 12, 15743-15799, doi:10.5194/acpd-12-15743-2012 (2012).