Ozone and Hydroxyl Radical Oxidation of Phospholipid-Coated Salt Mixtures: Model Systems For Investigating Sea-Salt Aerosol Processing In the Atmosphere.

摘要

Sea-salt aerosol (SSA) is a dominant component of marine aerosol, and organic material can account for more than half of the particles' composition by mass. The oxidation of this organic material and the salt portion of particles are important processes in the atmosphere. In this dissertation, model systems consisting of salt mixtures coated with phospholipids have been used to study the mechanisms and kinetics of the reactions of ozone and hydroxyl radicals (OH) with organic-coated sea-salt aerosol particles. Diffuse reflection infrared Fourier transform spectrometry (DRIFTS) and matrix-assisted laser desorption/ionization time-of-flight-mass spectrometry (MALDI-TOF-MS) were used to identify possible structures of surface-bound reaction products.;The ozone oxidation of 1-oleoyl-2-palmitoyl-sn-glycero-3-phosphocholine (OPPC) adsorbed on NaCl (OPPC/NaCl) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) on (1) a mixture of MgCl2*6H2O and NaCl (POPC/MgCl 2*6H2O/NaCl), and (2) Instant Ocean® synthetic sea salt (POPC/IO) were studied. The products identified in these reactions included a secondary ozonide, carboxylic acids, and aldehydes. The branching ratio to give the secondary ozonide versus carboxylic acids and aldehydes was smaller for reaction of ozone with POPC/MgCl2*6H2O/NaCl and POPC/IO than with OPPC/NaCl and decreased upon addition of water vapor to the system because of increased trapping of the Criegee intermediates by adsorbed water. With an ozone concentration of 2.5 × 1012 molecules cm-3 (100 ppb), the lifetime of POPC with respect to ozone is expected to be 3 - 6 hours.;The OH oxidation of phospholipids adsorbed on salt was studied using both OH generated at the surface of salt particles (via photolysis at λ ≥ 290 nm in air of NaNO2 mixed with NaCl) and OH generated in the gas phase (via photolysis of isopropyl nitrite in N2/air). Surface-bound aldehydes, ketones, organic nitrates, and nitrate ions were identified as products of these reactions and structures of potential products were proposed based on mechanistic considerations combined with the MALDI-TOF-MS and DRIFTS spectra. A greater variety of products were observed with OH generated in the gas phase because more secondary chemistry occurs. These studies suggest that under atmospheric conditions with an OH concentration of 5 × 10 6 radicals cm-3, the lifetime of POPC with respect to OH is < 6 days.