High-resolution chemical ionization mass spectrometry (ToF-CIMS): application to study SOA composition and processing

[摘要] This paper demonstrates the capabilities of chemical ionization massspectrometry (CIMS) to study secondary organic aerosol (SOA) compositionwith a high-resolution (HR) time-of-flight mass analyzer (aerosol-ToF-CIMS).In particular, by studying aqueous oxidation of water-soluble organiccompounds (WSOC) extracted from α-pinene ozonolysis SOA, we assessthe capabilities of three common CIMS reagent ions: (a) protonated waterclusters (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 widerange of common organic aerosol constituents. We find that(H₂O)_nH⁺ is more selective to the detection of less oxidizedspecies, so that the range of O / C and OS_C (carbon oxidation state) inthe SOA spectra is considerably lower than those measured usingCH₃C(O)O⁻ and I(H₂O)_n⁻. Specifically,(H₂O)_nH⁺ ionizes organic compounds with OS_C ≤1.3,whereas CH₃C(O)O⁻ and I(H₂O)_n⁻ both ionize highlyoxygenated organics with OS_C up to 4 with I(H₂O)_n⁻being more selective towards multi-functional organic compounds. In the bulkO / C and H / C space (in a Van Krevelen plot), there is a remarkableagreement in both absolute magnitude and oxidation trajectory betweenToF-CIMS data and those from a high-resolution aerosol mass spectrometer(HR-AMS). Despite not using a sensitivity-weighted response for the ToF-CIMSdata, the CIMS approach appears to capture much of the chemical changeoccurring. As demonstrated by the calibration experiments with standards,this is likely because there is not a large variability in sensitivitiesfrom one highly oxygenated species to another, particularly for theCH₃C(O)O⁻ and I(H₂O)_n⁻ reagent ions. Finally, thedata illustrate the capability of aerosol-ToF-CIMS to monitor specificchemical change, including the fragmentation and functionalization reactionsthat occur during organic oxidation, and the oxidative conversion of dimericSOA species into monomers. Overall, aerosol-ToF-CIMS is a valuable,selective complement to some common SOA characterization methods, such asAMS and spectroscopic techniques. Both laboratory and ambient SOA samplescan be analyzed using the techniques illustrated in the paper.

[学科分类] 几何与拓扑