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

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 OS_C (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 OS_C ≤ 1.3, whereas CH₃C(O)O^− and I(H₂O)_n^− both ionize highly oxygenated organics with OS_C 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

Aqueous-phase photooxidation of levoglucosan – A mechanistic study using aerosol time-of-flight chemical ionization mass spectrometry (Aerosol ToF-CIMS)

10.5194/acp-14-9695-2014Atmospheric Chemistry and Physics14189695-970

Aqueous-phase photooxidation of levoglucosan – a mechanistic study using Aerosol Time of Flight Chemical Ionization Mass Spectrometry (Aerosol-ToF-CIMS)

Levoglucosan (LG) is a widely employed tracer for biomass burning (BB). Recent studies have shown that LG can react rapidly with hydroxyl (OH) radicals in the aqueous phase despite many mass balance receptor models assuming it to be inert during atmospheric transport. In the current study, aqueous-phase photooxidation of LG by OH radicals was performed in the laboratory. The reaction kinetics and products were monitored by aerosol time-of-flight chemical ionization mass spectrometry (Aerosol ToF-CIMS). Approximately 50 reaction products were detected by the Aerosol ToF-CIMS during the photooxidation experiments, representing one of the most detailed product studies yet performed. By following the evolution of mass defects of product peaks, unique trends of adding oxygen (+O) and removing hydrogen (−2H) were observed among the products detected, providing useful information for determining potential reaction mechanisms and sequences. Additionally, bond-scission reactions take place, leading to reaction intermediates with lower carbon numbers. We introduce a data analysis framework where the average oxidation state (OSc) is plotted against a novel molecular property: double-bond-equivalence-to-carbon ratio (DBE/#C). The trajectory of LG photooxidation on this plot suggests formation of polycarbonyl intermediates and their subsequent conversion to carboxylic acids as a general reaction trend. We also determined the rate constant of LG with OH radicals at room temperature to be 1.08 ± 0.16 × 109 M−1 s−1. By coupling an aerosol mass spectrometer (AMS) to the system, we observed a rapid decay of the mass fraction of organic signals at mass-to-charge ratio 60 (f60), corresponding closely to the LG decay monitored by the Aerosol ToF-CIMS. The trajectory of LG photooxidation on a f44–f60 correlation plot matched closely to literature field measurement data. This implies that aqueous-phase photooxidation might be partially contributing to aging of BB particles in the ambient atmosphere

Rapid Aqueous-Phase Photooxidation of Dimers in the α‑Pinene Secondary Organic Aerosol

Chemical complexity significantly hinders our understanding of the formation and evolution of secondary organic aerosol (SOA), which is known to have impacts on air quality and global climate. Dimeric substances present in SOA comprise a major fraction of extremely low-volatile organic compounds that are especially poorly characterized. Using online mass spectrometry, we have investigated the aqueous-phase OH oxidation of dimers present in the water-soluble fraction of SOA arising from ozonolysis of α-pinene. This study highlights very rapid OH oxidation of dimeric compounds. In particular, using pinonic acid as a reference compound, we obtained second-order rate constants for the loss of 12 dimers, with an average value of (1.3 ± 0.5) × 10⁹ M^–1 s^–1 at room temperature. For the first time, this study demonstrates that rapid loss of dimeric compounds will occur in cloudwater and potentially also in aqueous aerosols

Formation of hydroxyl radicals from photolysis of secondary organic aerosol material

This paper demonstrates that OH radicals are formed by photolysis of secondary organic aerosol (SOA) material formed by terpene ozonolysis. The SOA is collected on filters, dissolved in water containing a radical trap (benzoic acid), and then exposed to ultraviolet light in a photochemical reactor. The OH formation rates, which are similar for both α-pinene and limonene SOA, are measured from the formation rate of p-hydroxybenzoic acid as measured using offline HPLC analysis. To evaluate whether the OH is formed by photolysis of H₂O₂ or organic hydroperoxides (ROOH), the peroxide content of the SOA was measured using the horseradish peroxidase-dichlorofluorescein (HRP-DCF) assay, which was calibrated using H₂O₂. The OH formation rates from SOA are 5 times faster than from the photolysis of H₂O₂ solutions whose concentrations correspond to the peroxide content of the SOA solutions, assuming that the HRP-DCF signal arises from H₂O₂ alone. The higher rates of OH formation from SOA are likely due to ROOH photolysis, but we cannot rule out a contribution from secondary processes as well. This result is substantiated by photolysis experiments conducted with t-butyl hydroperoxide and cumene hydroperoxide which produce over 3 times more OH than photolysis of equivalent concentrations of H₂O₂. Relative to the peroxide level in the SOA and assuming that the peroxides drive most of the ultraviolet absorption, the quantum yield for OH generation from α-pinene SOA is 0.8 ± 0.4. This is the first demonstration of an efficient photolytic source of OH in SOA, one that may affect both cloud water and aerosol chemistry

Chemical evolution of atmospheric organic carbon over multiple generations of oxidation

The evolution of atmospheric organic carbon as it undergoes oxidation has a controlling influence on concentrations of key atmospheric species, including particulate matter, ozone and oxidants. However, full characterization of organic carbon over hours to days of atmospheric processing has been stymied by its extreme chemical complexity. Here we study the multigenerational oxidation of α-pinene in the laboratory, characterizing products with several state-of-the-art analytical techniques. Although quantification of some early generation products remains elusive, full carbon closure is achieved (within measurement uncertainty) by the end of the experiments. These results provide new insights into the effects of oxidation on organic carbon properties (volatility, oxidation state and reactivity) and the atmospheric lifecycle of organic carbon. Following an initial period characterized by functionalization reactions and particle growth, fragmentation reactions dominate, forming smaller species. After approximately one day of atmospheric aging, most carbon is sequestered in two long-lived reservoirs—volatile oxidized gases and low-volatility particulate matter.National Science Foundation (U.S.). Graduate Research Fellowship Program (AGS-PRF 1433432)National Science Foundation (U.S.). Graduate Research Fellowship Program (Grant AGS-1536939)National Science Foundation (U.S.). Graduate Research Fellowship Program (Grant AGS-1537446)National Science Foundation (U.S.). Graduate Research Fellowship Program (Grant AGS-1536551