Intellectual merit of the proposed activity. Variability in the burden of reactive nitrogen compounds (primarily NOx = NO + NO2) is a first-order research question in atmospheric chemistry. Because of their central role in the tropospheric ozone (O3) cycle, these compounds largely determine the lifetimes of volatile organic compounds, methane and other natural and anthropogenic trace gases. Little is known about the magnitude of natural variability from sources such as biomass burning, soils, and lightning records of nitric acid or nitrate (NO3-) Concentrations from ice cores can extend our knowledge of natural NOx variability. However, nitrate deposition rates are only indirectly related to atmospheric NOx mixing ratios, and interpretation of concentration data from ice cores is complicated by post-depositional changes. Recent work by the Principal Investigator's group and others suggests that the triple-isotopic composition (15N/14N, 18O/16O and 17O/16O) of deposited nitrate can be related directly to NO2/NO ratios and OH and O3 photochemistry, and can possibly be used to infer NOx mixing ratios in the past. Results also show that there is potential to diagnose changes in NOx source. Their analytical technique, using bacterial denitrification to convert NO3 - to N2O for mass spectrometric measurement, permits the analysis of isotope ratios in solutions with very low (~1 M) nitrate concentrations, necessary for the low atmospheric and wet-deposition concentrations typical of remote regions. Before they can fully utilize the potential of nitrate-isotope measurements in ice core research, they will need more complete knowledge of the various factors controlling NOx and HNO3 isotope ratios. Both modeling studies and laboratory and field-based measurements will be necessary. This project will address the field-based aspect of the problem, with emphasis on obtaining data from Summit Greenland, where the deep GISP2 and GRIP ice cores were drilled. The goal is to better characterize the isotopic variability in NOy (= NOx + HNO3, HONO, etc.) in air and snow. The Principal Investigator will collect and analyze samples of fresh snowfall, aged snow surfaces, and buried snow in snow pits over the course of several summer and winter field seasons to examine variability in nitrate isotope concentrations due both to diurnal, seasonal, and interannual changes in source, and to depositional and post-depositional processes. The staff from Summit will collect fresh snowfall and surface snow daily. These snow and firn measurements will be complemented by analysis of isotopes in atmospheric NOy, using a combination of air sampling techniques (mist chambers for HNO3 and HONO, triethanolamine scavengers for NOx and PAN, and aerosol filters for particulate NO3). The atmospheric sampling will be conducted in parallel with routine meteorological and atmospheric sampling programs that are ongoing at Summit to allow for a comprehensive record of both isotope variability and relevant mixing ratios of important chemical (NOx, O3) and environmental variables. A new 4" diameter ice core will be obtained to a depth of ~100 m, to obtain a record of the last ~200-300 years at subannual resolution, to investigate longer term variations in mean nitrate isotope ratios. Broader impacts resulting from the proposed activity. This project will result in a comprehensive baseline data set for use in validation and refinement of theoretical understanding of natural and anthropogenic NOx variability. Because NOx has a significant impact on terrestrial and aqueous chemistry through deposition as nitrate (NO3) important contributor to acid rain and a source of nitrogen fertilization, this work will also contribute to ecological studies, particularly in Arctic regions. This project will support the education and career development of a graduate student and a post doc at the University of Washington; both have contributed to this proposal and have considerable experience with the analytical methods.
