HR: 17:45h
AN: A42F-08    [PDF]
TI: Seasonal Atmospheric Chemistry at Summit, Greenland Based on N and O Isotopes of Nitrate
AU: * Hastings, M G
EM: mhasting@princeton.edu
AF: Princeton University, Department of Geosciences Guyot Hall, Princeton, NJ 08544 United States
AU: Steig, E J
EM: steig@ess.washington.edu
AF: University of Washington, Quaternary Research Center Department of Earth and Space Sciences 19 Johnson Hall, Seattle, WA 98195 United States
AU: Sigman, D M
EM: sigman@princeton.edu
AF: Princeton University, Department of Geosciences Guyot Hall, Princeton, NJ 08544 United States
AU: Jarvis, J
EM: jjarvis@u.washington.edu
AF: University of Washington, Department of Earth and Space Sciences Box 351310, Seattle, WA 98195 United States
AB: Nitric acid (HNO$_{3}$), or nitrate (NO$_{3}$$^{-}$) is the major sink for reactive nitrogen oxides (NO$_{x}$) in the atmosphere. Ice core records of NO$_{3}$$^{-}$ could provide information about past reactive nitrogen chemistry and oxidative capacity of the atmosphere. However, processes that take place in the surface snow (e.g., evaporation, photolysis) can affect the final NO$_{3}$$^{-}$ concentrations that are archived in the ice cores, making it difficult to interpret past changes in atmospheric chemistry or climate. The isotopic composition of NO$_{3}$$^{-}$ in surface snow provides an additional constraint on the effect of post-depositional processing in the upper meter of snowpack on the NO$_{3}$$^{-}$ content measured in glacial ice cores. Furthermore, the isotopes of NO$_{3}$$^{-}$ in ice cores have the potential to allow for reconstruction of sources of NO$_{x}$ and aspects of atmospheric chemistry in the past. Snow pit samples (sampled every 3 cm to 1 m) collected at Summit, Greenland during August 2001 show seasonal variation in N and O isotopes of NO$_{3}$$^{-}$. $\delta$$^{15}$N of NO$_{3}$$^{-}$ in the upper meter of snow ranges from -15 to +16 per mil (vs. atmospheric N$_{2}$), with higher values found in summertime snow. The $\delta$$^{15}$N of snowpack NO$_{3}$$^{-}$ falls in the range reported for NO$_{3}$$^{-}$ in precipitation from other parts of the world. $\delta$$^{18}$O of NO$_{3}$$^{-}$ in the snowpack ranges from 65 to 80 per mil (vs. VSMOW), similar to high values observed for rainwater and aerosol NO$_{3}$$^{-}$. In contrast to the $\delta$$^{15}$N, the $\delta$$^{18}$O of NO$_{3}$$^{-}$ is lower in summertime snow than in wintertime. Preliminary analyses also show diurnal variation in the isotopes of NO$_{3}$$^{-}$ during summer, with depletion of $^{15}$N and $^{18}$O at "night" followed by enrichment during the "day." The similar behavior of the N and O isotopes is suggestive of a simple physical mechanism for the diurnal change, such as photolysis or evaporation. The seasonal $\delta$$^{15}$N and $\delta$$^{18}$O, however, vary in opposite directions suggesting that these processes are not the dominant mechanism determining the seasonal signals. Our hypothesis is that isotope ratios preserved in the snow are determined by seasonal changes in atmospheric chemistry rather than post-depositional processes. To first order, we can explain the variation in $\delta$$^{18}$O of NO$_{3}$$^{-}$ as a switch in the reactions that produce HNO$_{3}$ from NO$_{x}$ from predominantly homogeneous reactions in the summer ("day chemistry" influenced by OH) to predominantly heterogeneous reactions in the winter ("night chemistry" more influenced by ozone). Given our current information, the seasonal change in $\delta$$^{15}$N of NO$_{3}$$^{-}$ may be determined by seasonal changes in chemistry (i.e., ozone concentrations and its effect on the fraction of NO$_{x}$ as NO$_{2}$), the source of NO$_{x}$, or both.
DE: 0322 Constituent sources and sinks
DE: 0365 Troposphere--composition and chemistry
DE: 0368 Troposphere--constituent transport and chemistry
DE: 1863 Snow and ice (1827)
DE: 9315 Arctic region
SC: Atmospheric Sciences [A]
MN: 2003 Fall Meeting