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Application of high resolution Chemical Ionization Mass Spectrometry (CI-ToFMS) to study SOA composition: focus on formation of oxygenated species via aqueous phase processing

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Author(s): D. Aljawhary | A. K. Y. Lee | J. P. D. Abbatt

Journal: Atmospheric Measurement Techniques Discussions
ISSN 1867-8610

Volume: 6;
Issue: 4;
Start page: 6147;
Date: 2013;
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ABSTRACT
This paper demonstrates the capabilities of Chemical Ionization Mass Spectrometry (CIMS) to study secondary organic aerosol (SOA) composition with a high resolution (HR) time-of-flight mass analyzer (aerosol-CI-ToFMS). In particular, by studying aqueous oxidation of Water Soluble Organic Compounds (WSOC) extracted from α-pinene ozonolysis SOA, we assess the capabilities of three common CIMS reagent ions: (a) protonated water clusters (H2O)nH+, (b) acetate CH3C(O)O− and (c) iodide water clusters I(H2O)n− to monitor SOA composition. As well, we report the relative sensitivity of these reagent ions to a wide range of common organic aerosol constituents. We find that (H2O)nH+ is more selective to the detection of less oxidized species, so that the range of O/C and OSC (carbon oxidation state) in the SOA spectra is considerably lower than those measured using CH3C(O)O− and I(H2O)n−. Specifically, (H2O)nH+ ionizes organic compounds with OSC ≤ 1.3, whereas CH3C(O)O− and I(H2O)n− both ionize highly oxygenated organics with OSC up to 4 with I(H2O)n− being more selective towards multi-functional organic compounds. In the bulk O/C and H/C space, i.e. in a Van Krevelen plot, there is a remarkable agreement in both absolute magnitude and oxidation trajectory between CI-ToFMS data and those from a high resolution aerosol mass spectrometer (HR-AMS). This indicates that the CI-ToFMS data captures much of the chemical change occurring in the particle and that gas phase species, which are not detected by the HR-AMS, do not dominate the overall ion signal. Finally, the data illustrate the capability of aerosol-CI-ToFMS to monitor specific chemical change, including the fragmentation and functionalization reactions that occur during organic oxidation, and the oxidative conversion of dimeric SOA species into monomers. Overall, aerosol-CI-ToFMS is a valuable, selective complement to some common SOA characterization methods, such as AMS and spectroscopic techniques. Both laboratory and ambient SOA samples can be analyzed using the techniques illustrated in the paper.
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