HR: 0800h
AN: A11B-0045    [Abstracts]
TI: Multiple Oxygen Isotope Photochemistry of Nitrate in Ice
AU: * McCabe, J R
EM: jmccabe@ucsd.edu
AF: UC San Diego, Department of Chemistry and Biochemistry 9500 Gilman Dr. 0356 , La Jolla, CA 92093 United States
AU: Boxe, C S
EM: boxe@its.caltech.edu
AF: California Insitiute of Technology, W.M. Keck Laboratories, Pasadena, CA 91125 United States
AU: Colussi, A
EM: ajcoluss@caltech.edu
AF: California Insitiute of Technology, W.M. Keck Laboratories, Pasadena, CA 91125 United States
AU: Hoffman, M R
EM: mrh@caltech.edu
AF: California Insitiute of Technology, W.M. Keck Laboratories, Pasadena, CA 91125 United States
AU: Thiemens, M H
EM: mht@chem.ucsd.edu
AF: UC San Diego, Department of Chemistry and Biochemistry 9500 Gilman Dr. 0356 , La Jolla, CA 92093 United States
AB: Nitrate (NO$_{3}$ $^{-}$) is a major anion in polar ice [de Angelis and Legrand, 1995;Dibb et al., 1998;Silvente and Legrand, 1995]. As the primary sink for atmospheric NOx, nitrate's chemical history is a link to understanding the nitrogen budget and oxidation capacity of the polar atmosphere over time. Our understanding of variations in ice core nitrate concentrations has been limited by depositional and post-depositional loss processes, associated with temperature, accumulation rate, diffusion, photochemistry and volatilization [Wolff, 1995]. The first report of multiple oxygen isotope composition of nitrate in ice cores (Alexander et al., 2004) demonstrates the ability to observe changes in the oxidation capacity of past atmospheres. To accurately interpret this and future isotopic data, we need to ascertain the oxygen isotope fractionation ($\delta$$^{17}$O, $\delta $^{18}$O) associated with the photolysis of nitrate in ice. Post-depositional processes, such as nitrate photolysis and volatilization may alter the initial isotopic signal of nitrate in the snowpack. A series of nitrate photolysis experiments were conducted on 10 mM solutions of Fisher KNO$_{3}$ and USGS-35 NaNO$_{3}$ at 313 \pm$ 20 nm over 12 to 48 hours and between -30 and 25 $\deg$C. With initial mass-dependent nitrate, a strict mass-dependent fractionation was observed in the residual irradiated nitrate. However, after 12 and 24 hours of irradiation, mass-independent USGS-35 NaNO$_{3}$ ($\Delta$$^{17}$O = 21.0 \pm$ 0.4 \permil) displayed a decrease of 1.6 \pm$ 0.4 and 2.0 \pm$ 0.4 % at 25 $\deg$C, 1.2 \pm$ 0.4 and 1.3 \pm$ 0.4 \permil at $-5\deg$C, 0.2 \pm$ 0.4 \permil and 1.1 \pm$ 0.4 \permil at $-30\deg$C, respectively. The greater isotope effect at higher temperatures may be due to a thicker quasi-liquid layer (QLL) allowing faster rates for secondary nitrate producing reactions between H$_{2}$O and photoproducts NO, NO$_{2}$, NO$_{2}$$^{-}$. In the aqueous phase this effect is even greater. Hence, we infer that the production of nitrate via these 'secondary processes' has positive temperature dependence, causing a noticeable but minor decrease in $\Delta$$^{17}$O. The potential application of utilizing $\Delta$$^{17}$O -NO$_{3}$$^{-}$ as a new technique to interpret the nitrate ice record to increase our understanding of the polar paleoatmosphere is evaluated in the context of the present measurements.
DE: 1040 Isotopic composition/chemistry
DE: 0305 Aerosols and particles (0345, 4801)
DE: 0317 Chemical kinetic and photochemical properties
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting