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Oxidation of Volatile Organic Compounds in Aqueous Solution and at the Air-water Interface of Aqueous Microdroplets

Citation

Kameel, Fathima Rifkha (2015) Oxidation of Volatile Organic Compounds in Aqueous Solution and at the Air-water Interface of Aqueous Microdroplets. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9MP5174. http://resolver.caltech.edu/CaltechTHESIS:10262014-094813435

Abstract

Isoprene (ISO),the most abundant non-methane VOC, is the major contributor to secondary organic aerosols (SOA) formation. The mechanisms involved in such transformation, however, are not fully understood. Current mechanisms, which are based on the oxidation of ISO in the gas-phase, underestimate SOA yields. The heightened awareness that ISO is only partially processed in the gas-phase has turned attention to heterogeneous processes as alternative pathways toward SOA.

During my research project, I investigated the photochemical oxidation of isoprene in bulk water. Below, I will report on the λ > 305 nm photolysis of H2O2 in dilute ISO solutions. This process yields C10H15OH species as primary products, whose formation both requires and is inhibited by O2. Several isomers of C10H15OH were resolved by reverse-phase high-performance liquid chromatography and detected as MH+ (m/z = 153) and MH+-18 (m/z = 135) signals by electrospray ionization mass spectrometry. This finding is consistent with the addition of ·OH to ISO, followed by HO-ISO· reactions with ISO (in competition with O2) leading to second generation HO(ISO)2· radicals that terminate as C10H15OH via β-H abstraction by O2.

It is not generally realized that chemistry on the surface of water cannot be deduced, extrapolated or translated to those in bulk gas and liquid phases. The water density drops a thousand-fold within a few Angstroms through the gas-liquid interfacial region and therefore hydrophobic VOCs such as ISO will likely remain in these relatively 'dry' interfacial water layers rather than proceed into bulk water. In previous experiments from our laboratory, it was found that gas-phase olefins can be protonated on the surface of pH < 4 water. This phenomenon increases the residence time of gases at the interface, an event that makes them increasingly susceptible to interaction with gaseous atmospheric oxidants such as ozone and hydroxyl radicals.

In order to test this hypothesis, I carried out experiments in which ISO(g) collides with the surface of aqueous microdroplets of various compositions. Herein I report that ISO(g) is oxidized into soluble species via Fenton chemistry on the surface of aqueous Fe(II)Cl2 solutions simultaneously exposed to H2O2(g). Monomer and oligomeric species (ISO)1-8H+ were detected via online electrospray ionization mass spectrometry (ESI-MS) on the surface of pH ~ 2 water, and were then oxidized into a suite of products whose combined yields exceed ~ 5% of (ISO)1-8H+. MS/MS analysis revealed that products mainly consisted of alcohols, ketones, epoxides and acids. Our experiments demonstrated that olefins in ambient air may be oxidized upon impact on the surface of Fe-containing aqueous acidic media, such as those of typical to tropospheric aerosols.

Related experiments involving the reaction of ISO(g) with ·OH radicals from the photolysis of dissolved H2O2 were also carried out to test the surface oxidation of ISO(g) by photolyzing H2O2(aq) at 266 nm at various pH. The products were analyzed via online electrospray ionization mass spectrometry. Similar to our Fenton experiments, we detected (ISO)1-7H+ at pH < 4, and new m/z+ = 271 and m/z- = 76 products at pH > 5.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Volatile Organic Components(VOC), Isoprene (ISO), Secondary Organic Aerosol(SOA), Electro Spray Ionization (ESI), Fenton Chemistry
Degree Grantor:California Institute of Technology
Division:Geological and Planetary Sciences
Major Option:Environmental Science and Engineering
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Hoffmann, Michael R.
Thesis Committee:
  • Wennberg, Paul O. (chair)
  • Okumura, Mitchio
  • Goddard, William A., III
  • Hoffmann, Michael R.
Defense Date:8 October 2014
Record Number:CaltechTHESIS:10262014-094813435
Persistent URL:http://resolver.caltech.edu/CaltechTHESIS:10262014-094813435
DOI:10.7907/Z9MP5174
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1016/j.cplett.2014.04.033DOIArticle adapted for ch. 2
http://dx.doi.org/10.1021/jp4026267DOIArticle adapted for ch. 3
http://dx.doi.org/10.1021/jp505010eDOIArticle adapted for ch. 4
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:8705
Collection:CaltechTHESIS
Deposited By: Fathima Kameel
Deposited On:11 Nov 2014 01:24
Last Modified:09 Mar 2016 17:47

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