Use of SEM/EDX Analysis to Study the Oxidation of Aerosol by Hydroxyl Radical
EMSL Project ID
17810
Abstract
Unique chemistry at the air-water interface between chloride ion in sea salt aerosol and the hydroxyl radical (OH) has been suggested to play a major role in the production of increased levels of molecular chlorine in the marine boundary layer, including coastal regions. Molecular chlorine is photolyzed by solar radiation to produce highly reactive chlorine atoms, which can destroy tropospheric ozone (O3) or, alternatively, react with organic molecules in the presence of nitrogen oxides leading to the production of O3. Ozone is a key tropospheric oxidant, a greenhouse gas, and has negative health effects.Previous studies in the Finlayson-Pitts laboratory have shown that the reaction between OH and chloride ion at the interface of deliquesced aerosol particles generates molecular chlorine at levels much greater than can be explained by well-known, bulk aqueous-phase kinetics. Molecular dynamics (MD) simulations of chloride ion solutions and sum frequency generation studies support this result, showing that the chloride ion has a propensity for the air-water interface, making it available for reaction with colliding gases.
It is not known, however, how other inorganic ions in the aerosol will affect the production of chlorine by the OH-chloride reaction. For example, as sea salt aerosol travels inland over polluted coastal regions, some of the chloride ion is replaced by nitrate due to reactions with nitric acid and other nitrogen oxides.
We propose to study how the addition of the nitrate ion to sodium chloride aerosol affects the interface chemistry of Cl-. Two series of experiments are planned: (1) Investigation of the OH-chloride interface reaction when nitrate ion is an added aerosol component; (2) Irradiation of the mixed aerosol alone to probe for OH formation within the aerosol.
Objective 1. Does the replacement of some of the Cl- by NO3 ion during atmospheric processing affect the amount of molecular chlorine formed by the OH-chloride interface reaction? Will we see more, less, or the same amount of Cl2 when NO3 is present?
Approach: Experiments will be carried out in the aerosol chamber in the Finlayson-Pitts laboratory in which Cl2 production will be measured from the reaction of gas phase OH with deliquesced particles of mixed NaCl/NaNO3. Aerosol particles, generated by atomizing a ~1% (w/w) solution of 1:1 NaCl/NaNO3, will be exposed to OH formed by O3 photolysis at 254 nm. Cl2 production will be measured in real time in our lab using atmospheric pressure ionization mass spectrometry. Particles will be collected from the chamber onto copper or gold TEM grids before, during, and after photolysis using a time resolved aerosol collector (TRAC), provided by PNNL. The particles will be analyzed using SEM to ensure they are uniform mixtures of Cl- and NO3 rather than separate particles of each salt. The particles will then be analyzed using SEM-EDX to characterize changes in morphology and elemental composition due to OH exposure. Specifically, we plan to look for Cl- depletion in the particles after OH exposure. The ratio of NO3 to Cl- will be varied to explore a wide range of Cl- replacement by NO3 .
Similar studies will be carried out with mixtures of the SO42- anion with Cl- in comparison to the NO3 /Cl- mixtures since sulfate is not photochemically active in this region and resides mainly in the bulk of the particle. Other ions that compete with Cl- for the air-water interface, but that are photochemically inert, such as HSO3, will be added to the Cl- aerosol to investigate dilution of chloride at the interface.
Objective 2. Does nitrate ion photolysis provide a source of OH radical within the aerosol?
Approach: Aerosol photolysis experiments will be carried out in the absence of O3 to probe for Cl2 production by OH generated within the particle. Nitrate is known to photolyze at wavelengths below ~320 nm to form oxidants, including OH. Photolysis at 254 nm of deliquesced aerosol particles containing mixtures of varied NO3 /Cl- ratios will be carried out. Cl2 formation will again be measured in real time. Particles will be analyzed as above by SEM/EDX before and after photolysis to probe for changes in morphology and elemental composition. We plan to search for Cl and N depletion in the particles after irradiation.
Reacted and unreacted particles will also be analyzed by TOF-SIMS to investigate the depletion of Cl- as a function of depth within the particle to identify where the OH is formed (in the center of the particle or at the surface), and hence where NO3 resides within the particle.
Project Details
Project type
Exploratory Research
Start Date
2006-02-21
End Date
2007-03-19
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members