Time-Resolved Electrospray Mass Spectrometric Studies of Atmospheric Gas-liquid Reactions

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Important but poorly constrained multiphase chemical pathways limit current understanding of the atmospheric processing and global budgets of volatile organic compounds. Electrospray mass spectrometry (ES-MS) is a powerful tool with which to investigate interfacial reactions on liquid microjets at high temporal resolution. The approach allows simultaneous detection of reactants, primary products, and intermediates in the absence of secondary chemistry for a wide variety of chemical reactions under realistic conditions. Through this project, the PI will incorporate a technique of gas-beam modulation and pulsed ion-gating detection to his existing coupled reactor and ES-MS system. The new approach will resolve reactive events on surfaces of water droplets in the absence of an applied external field. The new instrument will be used to investigate the heterogeneous chemistry of biogenic gases on the surface of mildly acidic water. Previous evidence indicates that the weakly hydrated H3O+ in small water clusters is able to protonate non-alkane organic gases at the air-water interface below pH 4. It is likely that biogenic olefins would be similarly protonated and engage in cationic oligomerization on typical environmental surfaces. This phenomenon would provide a universal mechanism for the dry deposition of most volatile organic gases. The project will addresses important issues in global atmospheric chemistry using innovative experimental approaches based on a newly proposed technique. Results are expected to fill outstanding gaps in current understanding of the atmospheric chemistry of biogenic gases and secondary aerosol formation and provide fundamental insight into the scavenging and subsequent air-to-water transfer of atmospheric CO2 on water. The project will (1) help resolve multi-phasic interfacial processes that are important to atmospheric chemistry and thereby lay the foundation of more realistic model mechanisms and (2) expose students to new, wider perspectives and skills in global environmental chemistry. Education and outreach activities include mentoring a postdoctoral fellow, a graduate student, and undergraduate students. Educational opportunities will be extended to community college students (through a partnership with Oak Crest Institute of Science) and to members of traditionally underrepresented groups (through Caltech's MURF program and a collaboration with Pasadena City College).