Abstract
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-ToF-CIMS). 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. Furthermore, 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 (in a Van Krevelen plot), there is a remarkable agreement in both absolute magnitude and oxidation trajectory between ToF-CIMS data and those from a high-resolution aerosol mass spectrometer (HR-AMS). Despite not using a sensitivity-weighted response for the ToF-CIMS data, the CIMS approach appears to capture much of the chemical change occurring. As demonstrated by the calibration experiments with standards, this is likely because there is not a large variability in sensitivities from one highly oxygenated species to another, particularly for the CH3C(O)O− and I(H2O)n− reagent ions. Finally, the data illustrate the capability of aerosol-ToF-CIMS 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-ToF-CIMS 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.
Highlights
Organic compounds comprise an important subset of atmospheric constituents and can exist in all atmospheric phases
While this study focuses on the aqueous phase chemistry of organics, the technique and the approach discussed here can be pursued in other reaction systems and phases
The sensitivity of 13 organic compounds with known concentrations in the aqueous phase was assessed using the ToFCIMS with the three reagent ions
Summary
Organic compounds comprise an important subset of atmospheric constituents and can exist in all atmospheric phases (i.e. gas, particle and aqueous). While the technique of chemical ionization mass spectrometry (CIMS) has been used in atmospheric chemistry for decades for gas phase measurements, it has only being applied to the study of organic processing that takes place in atmospheric cloud-, fog-, and rain-water, and particles (Thornton et al, 2003; Hearn and Smith, 2004; Thornton and Abbatt, 2005; Sareen et al, 2010; Ervens et al, 2011; Zhao et al, 2012). The water-soluble organic compounds (WSOC) of α-pinene ozonolysis SOA We treat this complex organic mixture and its subsequent aqueous photo-oxidation components as models to highlight the capabilities of high-resolution aerosolToF-CIMS to study organic aerosol composition and processing. The detailed mechanistic interpretation of the oxidative evolution of the hundreds of peaks within the aerosol-ToF-CIMS spectra, along with selected kinetics studies of major constituents, will be presented in a subsequent publication
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
