10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Probing the Role of Water in Criegee Chemistry Leading to High Molecular Weight Compounds on Aqueous Organic Surfaces
LIJIE LI, Michael Hoffmann, Agustin Colussi, California Institute of Technology
Abstract Number: 1427 Working Group: Aerosol Chemistry
Abstract Ozonolysis of unsaturated organic gas emissions is a fundamental process in the formation and growth of tropospheric aerosols. O3 adds to olefinic double bonds to produce chemically activated ozonides, which decompose into reactive carbonyl oxide Criegee (CIs) intermediates. CIs can react with H2O, SO2, carboxylic acids, aldehydes and alcohols, undergo unimolecular isomerization and dissociation, or bimolecular self-reaction. The competition among different reaction pathways determines the fate of organics and their potential for aerosol formation and growth. Understanding the role of water in these processes is essential, given its pervasiveness in the atmosphere both in the gas and aerosol phases. Recent reports have pointed out that the fate of gas-phase CIs in the troposphere is in fact dominated by their reactions with (H2O)n_clusters. The ozonolysis of low-volatility, hydrophobic vapors, however, largely takes place on air-aerosol interfaces, given their fast reactions with O3(g) and the low solubilities of both reactants in water. It is important to realize that in the steep water density gradient at air-aerosol interfaces, chemistry may proceed along different pathways and at faster rates than similar reactions in bulk gas and aerosol phases. In this work, we investigate the roles of water in the ozonolysis of oleic acid (OL) at the aerial interface of aqueous aerosols.
We focused on the formation of high molecular weight (HMW) compounds during the interfacial ozonolysis of OL. OL dissolved in acetonitrile/water mixtures (AN/W) was exposed to O3(g) for < 50 μs under atmospherically relevant 1 atm of N2, 298 K conditions. Online electrospray mass spectrometry (o-ESI- MS), which probes in situ the composition of the interfacial layers where ozonolysis takes place within 1 ms, was used to search for HMW (> C18), α-acyloxyalkyl hydroperoxides from the addition of H2O to CI, and other C9 products. We explored the participation of water in these processes by performing experiments using D2O and H218O instead of H2O in the AN/W solvent. We found that HMW formation increases linearly both with O3(g) exposure and OL concentration in the bulk solution, but decreases upon increasing the molar fraction of water. The main findings are that water functions as a solvent in the ionization of OL, and as a reactant that affects HMW formation.