The processes that form, transport and recycle the reactive forms of nitrogen in the atmosphere are important in global air quality, atmospheric nutrient deposition, and for controlling the long term oxidation capacity of the atmosphere. The NOx gases (NOx = NO + NO2) are also important greenhouse gases. The ultimate goal being pursued in this modeling study is the understanding of past (polar) atmospheric composition, and through a better accounting of the variability of contemporary atmospheric reactive nitrogen compounds as may be discerned from measurements of nitrate concentrations retained in snow, firn and ice cores on the Antarctic continent. The approach will use a inverse chemical transfer model (adjoint GEOS-Chem) to test the sensitivity of nitrate deposition to Antarctic ice from various possible sources. By integrating a snowpack radiative transfer subroutine into the chemical transport model, comparisons of calculated with observed fluxes of snowpack NOx at some observed inland and coastal Antarctic sites are to be used to parameterize photdenitrification rates. An additional question to be investigated with the developed modeling tools is what fraction of the nitrate concentration undergoing photolysis at the snow surface is recycled back to the atmosphere, or else preserved in the accreting ice.
The major goals of the project were to 1) investigate the primary sources of nitrate to Antarctica and 2) to estimate the degree of recycling and preservation of that nitrate under conditions of rapid loss via photolysis. An adjoint model was used to calculate the sensitivity of primary nitrate deposition in Antarctica to various NOx sources and other processes related to transport of nitrate in the atmosphere. Background concentrations of surface level nitrate in May through July show significant impacts from mid-latitude emissions from both natural and anthropogenic sources of ammonia and NOx. By tracking the sensitivity of Antarctic surface nitrate with respect to chemical reaction rate constants, the main species and governing chemical reactions involved in these long-range influences were identified, demonstrating how gas-phase NOx reservoir species formed above the boundary layer at southern mid-latitudes contribute to surface-level nitrate concentrations in Antarctica. For the first time, and adjoint model was used to explore the relative influence of surface emissions versus stratospheric production and loss rates. Peak concentrations of total nitrate in the model in August were found to be influenced by stratospheric HNO3 and NOx production in conjunction with photolysis, and to some extent enhanced concentrations owing to the polar vortex. Overall, the seasonality of Antarctic total nitrate concentrations depends upon post-depositional process and stratospheric influences, governing up to 70% of the surface nitrate concentrations. A snowpack radiative transfer model and a forward chemical transport model was used to calculate the photolysis and transport of the primary nitrate, and to examine the impacts of photolysis on the nitrate budget in Antarctica. Using measurements of optical properties from snow collected in Antarctica and Greenland combined with a snow radiative transfer model, we have evaluated the sensitivity of UV actinic flux in the snowpack to the physical and chemical properties of snow. The UV light penetration depth is most sensitive to snow grain radius and the concentration of absorbing impurities, particularly brown carbon. The latter is the largest source of uncertainty for calculating photolysis frequencies in the snow, and highlights the need for additional observations of the concentration of non-black carbon absorbers in the snow. Using this information, we developed a simple and broadly applicable parameterization to calculate photolysis frequencies in snowpack that can be used in large scale models of the atmosphere, one that can easily incorporate new information (such as additional information about brown carbon impurities) when it becomes available. We are currently implementing this parameterization into the GEOS-Chem model, which is a global 3D chemical transport model. Resulting GEOS-Chem model calculations will be used to estimate the impact of snow photodenitrification on polar boundary layer photochemistry, on the Antarctic and Greenland nitrogen budgets, and on the preservation of nitrate in polar ice cores. These results will assist in the interpretation of nitrate concentration and isotope measurements from ice cores. We measured the concentration and isotopic composition (δ15N, Δ17O) of nitrate from a Summit, Greenland snowpit covering three years of snow accumulation. A nitrate concentration maximum is found in the spring of 2005 which occurred at the same time as the stratospheric ozone layer was significantly weakened (total column zone of 290 DU compared to the long term springtime average of 390 ± 50 (1σ) DU from 1970 to 2006). Isotopic measurements of nitrate combined with photochemical calculations suggest that the presence of the spring nitrate peak is due to enhanced local nitrate production. The enhanced local nitrate production rate arises primarily from an enhanced local production rate of OH, with a possible significant contribution from an enhanced local NOx source from the photolysis of snowpack nitrate. The locally enhanced OH and NOx production rate is due to the weakened stratospheric ozone layer which causes elevated surface UV-B radiation and thus enhanced photolysis rates of OH and NOx precursors. In a shallow ice core covering the past 235 years, 19 such spring nitrate concentration maxima are seen after the 1950s. These spring nitrate concentration maxima coincide with the years with low spring total column ozone (< 390 DU). The interannual variability of total column ozone over the Arctic is largely controlled by the Brewer-Dobson circulation (BDC), while halogen-motivated chemical destruction reduces ozone in the more recent past. Therefore, it is likely that dynamic control via the BDC on total column ozone combined with recent chemical ozone destruction in the stratosphere determines the presence of the spring nitrate maxima, under the condition of elevated local NOx abundance at Summit after the 1950s resulting from anthropogenic activities.
