Diagnosis of NOx and HNO3 air snow transfer chemistry at Summit, Greenland
GISP2 Drilling Site. Larger
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Eric J. Steig
(Earth and Space Sciences)
Meredith Hastings
(Atmospheric Sciences)
Julia Jarvis and Shelley Kunasek
(Earth and Space Sciences)
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 polar ice cores offer a means to 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. Significantly more can be gained by utilizing the additional information available from nitrate isotope ratios.
Recent work by our group 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 (Hastings, Steig & Sigman, 2004; Jarvis, 2005). Our results also show that there is potential to diagnose changes in NOx source. Our 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 μ) nitrate concentrations, necessary for the low atmospheric and wet-deposition concentrations typical of remote regions. Before we can fully utilize the potential of nitrate-isotope measurements in ice core research, we need more complete knowledge of the various factors controlling NOx and HNO3 isotope ratios.
Current research addresses the field-based aspect of the problem at Summit Greenland, where the deep GISP2 and GRIP ice cores were drilled. Our goal is to better characterize the isotopic variability in NOy (= NOx + HNO3, HONO, etc.) in air and snow. We are collecting and analyzing 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. With help from the staff of the Greenland Summit Observatory, we are also collecting fresh snowfall and surface snow on a daily basis throughout the year. These snow and firn measurements will are 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 peroxyacetylnitrate (PAN), and aerosol filters for particulate NO3-).
Our atmospheric sampling work is complemented by parallel meteorological and atmospheric sampling programs that are ongoing at the Greenland Summit Observatory and funded by NSF, NOAA and ESF,allowing a comprehensive record of both isotope variability and relevant mixing ratios of important chemical variables. This project also includes a new 10 cm diameter ice core to a depth of ~100 m, to obtain a record of the last ~200 - 300 years at subannual resolution, allowing us to investigate longer term variations in mean nitrate isotope ratios. 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-), an important contributor to acid rain and a source of nitrogen fertilization, our work will also contribute to ecological studies, particularly in Arctic regions.
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