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Nitrate photochemsitry and interrelated chemical phenomena in ice: influence of the quasi-liquid layer (QLL)

Citation

Boxe, Christopher Shawn (2005) Nitrate photochemsitry and interrelated chemical phenomena in ice: influence of the quasi-liquid layer (QLL). Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-05272005-143636

Abstract

The detailed photochemistry of nitrate ions trapped within polycrystalline ice has been investigated. Using a variety of experimental techniques, the photolysis of NO3- within ice is shown to produce NO, HONO, NO2- and NO2 as stable and metastable products. The gaseous products are released to the overlying gas phase. Implications of these results for chemical transformations in polar regions are discussed.

The isotopic composition of nitrate in polar ice cores exhibits mass-independent fractionation (?????~ 25 ‰). However, in this study it is shown that nitrate photolysis is a mass-dependent process and that nitrate can be a conservative tracer for past atmospheric conditions.

Photolysis of NO3- produces nitrite ion within ice and the rate of photolysis increases with increasing temperature. A linear Arrhenius temperature dependence for the measured quantum yield, , was found both above and below the normal melting point of water ice. These results indicate that nitrate photochemistry occurs primarily in the quasi-liquid layer (QLL), which is a disordered layer of ice that has chemical properties closer to water than solid crystalline ice.

The photochemical production of gas-phase NO and NO2 from ice-phase nitrate was determined in real-time using laser-induced fluorescence, two-photon laser-induced fluorescence, and chemiluminescence. These results showed that NO and NO2, which was produced at lower temperatures is released in much larger quantities at ?7 ± 2 oC from subsurface regions of the QLL or at the ice/vapor interface. These results highlight the importance of the QLL and sub-eutectic sub-surface solutions at grain boundaries in polycrystalline ice. In addition, it was shown that the total amount of NO2 that is photodesorbed scales nonlinearly with [NO3?]o or the heating rate. Evidence for extensive chemical and photochemical losses of NO2 before release into the gas-phase is presented. In the case of NO, photo-induced desportion occured at ~ ?20 oC, which suggests a preferential retention of NO2 over NO in the nanoporous water channels of the QLL domains.

The impact of the specific nature of the chemical solutes in water on the nominal pH of the QLL was explored using the temperature dependence of 19F-NMR chemical shifts of 3-fluorobenzoic acid. The 19F-NMR measurements during the freezing of NaCl, NaNO2 and KNO3 solutions indicated that the QLL domains within the polycrystalline ice were shifted to higher pH values, while the freezing of NH4OH or Na2SO4 solutions produced QLL domains that showed a drop in pH (i.e., protons were released from the ice-phase to the QLL). These results help explain the release of HONO which is formed by protonation of NO2- within the QLL domains and are relevant to the observed release of nitrous acid to the atmospheric boundary layer in the Arctic and Antarctic.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:nitrate photochemistry; QLL; quasi-liquid layer
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Environmental Science and Engineering
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Hoffmann, Michael R.
Thesis Committee:
  • Hoffmann, Michael R. (chair)
  • Ingersoll, Andrew P.
  • Okumura, Mitchio
  • Yung, Yuk L.
Defense Date:26 May 2005
Author Email:boxe (AT) caltech.edu
Record Number:CaltechETD:etd-05272005-143636
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-05272005-143636
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:2154
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:06 Jun 2005
Last Modified:26 Dec 2012 02:47

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