Investigating the Impact of Snowpack Photodenitrification on Polar Atmospheric Chemistry Utilizing Results From a Snowpack Radiative Transfer Model in GEOS-ChemEPA Grant Number: FP917302
Title: Investigating the Impact of Snowpack Photodenitrification on Polar Atmospheric Chemistry Utilizing Results From a Snowpack Radiative Transfer Model in GEOS-Chem
Investigators: Zatko, Maria C
Institution: University of Washington
EPA Project Officer: Michaud, Jayne
Project Period: September 1, 2011 through August 31, 2014
Project Amount: $126,000
RFA: STAR Graduate Fellowships (2011) RFA Text | Recipients Lists
Research Category: Fellowship - Clean Air , Academic Fellowships
The NOx produced through snowpack photodenitrification increases the atmospheric oxidative capacity above the snowpack. The preservation of photochemically active species, such as nitrate, is altered in ice cores when NOx is converted to nitric acid in the atmosphere and deposited back to the snowpack. This study will implement a process based representation of snowpack photodenitrification in polar regions in a global chemical transport model utilizing a parameterization for actinic flux in snowpack to investigate how photodenitrification redistributes nitrate across Antarctica and Greenland.
This study will use a snowpack radiative transfer model (Grenfell, 1991) with updated optical properties in the UV wavelength region (Warren and Brandt, 2008) to develop a simple and broadly applicable parameterization for vertical profiles of actinic flux in snowpack. The e-folding depth of actinic flux in snowpack is two to eight times higher than previously calculated. Photolysis is occurring at deeper depths in the snowpack than previously assumed and it is therefore necessary to evaluate the assumption that all photoproduced NOx will escape from the snowpack to the atmosphere. This study will evaluate this assumption by comparing the lifetime of NOx against escape processes, such as diffusion and wind pumping, to the chemical lifetime of NOx, such as conversion to nitric acid, in the snowpack. The depth below which the photo-produced NOx will not escape into the atmosphere is called the ventilation depth. This study will compute a range of NOx fluxes based on variations in ventilation depths and nitrate concentrations from snowpacks in Antarctica at South Pole, Halley and Neumayer and in Greenland at Summit. The next step is to incorporate the snowpack actinic flux parameterization and the ventilation depth methodology into a global chemical transport model (GEOS-Chem).
The updated optical properties of ice in the UV wavelength region lead to larger e-folding depths of actinic flux in snowpack than previously calculated in Antarctica (30 cm) and Greenland (15 cm). The study finds that the e-folding depth of actinic flux in the snowpack is most dependent on soot and dust concentrations, effective snow grain radius and solar zenith angle. Because photodenitrification is occurring at deeper depths in the snowpack, the study finds that it is necessary to consider the escape processes and chemical processes influencing the lifetime of NOx in the snowpack because not all the NOx produced through snowpack photodenitrification can escape to the atmosphere. Because the ventilation depth is strongly dependent on sastrugi dimensions, wind speed and halogen (BrO, IO) concentrations, it is necessary to accurately determine the vertical profile of halogen concentrations in snowpack interstitial air. Although there are large uncertainties in the ventilation depth and a wide range of observed nitrate concentrations at South Pole, Halley, Neumayer and Summit, the computed NOx fluxes agree well with observations at these polar locations.
Potential to Further Environmental / Human Health Protection
The release of NOx and OH from snowpacks influence the lifetimes of trace gases such as carbon monoxide, methane and mercury in the overlying atmosphere. Enhanced boundary layer ozone concentrations have been observed in at least one region of elevated photo-produced NOx (South Pole), and it is possible that snowpack photodenitrification is related to the high ozone concentrations recently reported in Wyoming. Gaining a better understanding of snowpack photodenitrification and its influence on the global nitrogen and oxidant budgets will help determine if snowpacks are linked to the production of ozone and other pollutants in the atmosphere.