AVS 56th International Symposium & Exhibition
    Surface Science Wednesday Sessions
       Session SS1-WeM

Paper SS1-WeM9
The Composition of the Liquid/Vapor Interface of Aqueous Nitrate and Nitric Acid Solutions

Wednesday, November 11, 2009, 10:40 am, Room M

Session: Water/Surface Interactions & Environmental Chemistry I
Presenter: T.L. Lewis, Univ. of California, Irvine
Authors: T.L. Lewis, Univ. of California, Irvine
M.A. Brown, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany
K. Callahan, Univ. of California, Irvine
M. Faubel, Max-Planck-Institut für Dynamik und Selbstorganisation, Germany
D. Tobias, Univ. of California, Irvine
B. Winter, Hemholtz-Zentrum Berlin für Materialien und Energie, Germany
J.C. Hemminger, Univ. of California, Irvine
Correspondent: Click to Email
Nitrate anions become incorporated into sea salt aerosols through the uptake of gas-phase nitric acid in atmospheric aging processes. It has been suggested that photochemical decomposition of nitrate ions in the near surface region of such aerosols leads to OH radical production and subsequent halogen oxidation. However, the concentration of nitrate ions at the surface of aqueous solutions has been a controversial topic in the literature. We obtain a depth profile of the interfacial composition of nitrate anions in aqueous solution in the presence of alkali halide salts. All experiments were performed with XPS on a liquid micro jet and were acquired using tunable synchrotron radiation from the U41-PGM undulator beamline of the BESSY facility in Berlin, where we can vary the photoelectron kinetic energy to obtain depth dependent composition information. Our previous work shows that while nitrate and nitrite anions have a finite concentration at the interface, both prefer bulk solvation. Current results indicate that co-dissolved alkali halide salts in a sodium nitrate solution push the nitrate anion further into the bulk and away from the liquid-vapor interface. A greater effect is seen with increasing concentration of alkali halide as well as increasing size and polarizability of the halide anion. Comparison will be made with MD simulations of the corresponding solutions. In studies of nitric acid, another key component in the atmospheric chemistry of sea salt aerosols, tunable XPS elucidates the depth dependence of undissociated nitric acid. Results show that a higher concentration of undissociated nitric acid is present in the surface region relative to the bulk. Additionally, the relative concentration of undissociated nitric acid increases with increasing nitric acid concentration. These results have significant implications for the chemistry and photochemistry of sea salt aerosols.