HR: 12:05h
AN: A22C-08    [Abstracts]
TI: Controls on the isotopic composition of reactive nitrogen species
AU: * Jarvis, J C
EM: jjarvis@u.washington.edu
AF: Earth and Space Sciences, University of Washington, Seattle, WA 98195 United States
AU: Steig, E J
EM: steig@u.washington.edu
AF: Earth and Space Sciences, University of Washington, Seattle, WA 98195 United States
AU: Hastings, M G
EM: mhastings@princeton.edu
AF: Geosciences, Princeton University, Princeton, NJ 08544 United States
AB: The importance of sunlit snow surfaces in influencing reactive nitrogen chemistry is now well known. However, quantitatively separating the competing roles of transport, air-to snow transfer, and snow-to-air transfer remains challenging. Isotopic measurements represent a powerful new tool for addressing this problem. Recent innovations in analysis techniques - notably, the use of bacterial denitrification to convert nitrate to $N_{2}O$ for mass spectrometry - allow for the measurement of isotope ratios in snow and in gas-phase and aerosol species. To illustrate the potential utility of such measurements, we present a simple model of the controls on the nitrogen and oxygen isotopic composition of nitrate in remote areas such as central Greenland and west Antarctica. Given a source of $NO_{x}$ (e.g. PAN thermal decomposition) with a particular isotopic composition, isotopic variability in gas and aerosol phase $HNO_{3}$ is determined by the balance of "day" vs. "night" nitrogen oxide chemistry. With photochemical steady state, high local OH concentrations and low local $NO_{2}/NO_{x}$ ratios in summer, high $^{15}N/^{14}N$ ratios are found in $HNO_{3}$ due to the preference for the heavier isotope in $NO_{2}$. Under the same conditions, $^{18}O/^{16}O$ and $^{17}O/^{16}O$ ratios are low, due to low ratios in OH. In winter, $^{15}N/^{15}N$ ratios are low, but $^{18}O/^{16}O$ and $^{17}O/^{16}O$ high, due to the very high ratios characteristic of $O_{3}$, which is the source of virtually all $HNO_{3}$ oxygen during this season. Using output from the GEOS-CHEM global chemical transport model to estimate $NO_{x}$, $NO_{y}$, OH and $O_{3}$ concentrations, and published estimates of the isotopic composition of these compounds, we compare our results with measured seasonal profiles of nitrate in snow pits from Summit, Greenland, and Byrd Station, West Antarctica. Our model accommodates all of the observed variability in oxygen isotope ratios at both sites, but roughly 50% of the observed nitrogen isotope variability is unexplained and must reflect changes in the source values. These results suggest that measurements of both nitrogen and oxygen isotope ratios can potentially be used to separate variability in the sources of $HNO_{3}$ from local photochemistry.
DE: 4870 Stable isotopes
DE: 1806 Chemistry of fresh water
DE: 0317 Chemical kinetic and photochemical properties
DE: 0365 Troposphere--composition and chemistry
DE: 0394 Instruments and techniques
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting