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1. A technique for rapid source apportionment applied to ambient organic aerosol measurements from a thermal desorption aerosol gas chromatograph (TAG)

   https://doi.org/10.5194/amt-9-5637-2016  

   Zhang, Yaping ; Williams, Brent J. ; Goldstein, Allen H. ; Docherty, Kenneth S. ; Jimenez, Jose L. ( January 2016 , Atmospheric Measurement Techniques)

   null (Ed.)

   Abstract. We present a rapid method for apportioning the sources of atmospheric organic aerosol composition measured by gas chromatography–mass spectrometry methods. Here, we specifically apply this new analysis method to data acquired on a thermal desorption aerosol gas chromatograph (TAG) system. Gas chromatograms are divided by retention time into evenly spaced bins, within which the mass spectra are summed. A previous chromatogram binning method was introduced for the purpose of chromatogram structure deconvolution (e.g., major compound classes) (Zhang et al., 2014). Here we extend the method development for the specific purpose of determining aerosol samples' sources. Chromatogram bins are arranged into an input data matrix for positive matrix factorization (PMF), where the sample number is the row dimension and the mass-spectra-resolved eluting time intervals (bins) are the column dimension. Then two-dimensional PMF can effectively do three-dimensional factorization on the three-dimensional TAG mass spectra data. The retention time shift of the chromatogram is corrected by applying the median values of the different peaks' shifts. Bin width affects chemical resolution but does not affect PMF retrieval of the sources' time variations for low-factor solutions. A bin width smaller than the maximum retention shift among all samples requires retention time shift correction. A six-factor PMF comparison among aerosol mass spectrometry (AMS), TAG binning, and conventional TAG compound integration methods shows that the TAG binning method performs similarly to the integration method. However, the new binning method incorporates the entirety of the data set and requires significantly less pre-processing of the data than conventional single compound identification and integration. In addition, while a fraction of the most oxygenated aerosol does not elute through an underivatized TAG analysis, the TAG binning method does have the ability to achieve molecular level resolution on other bulk aerosol components commonly observed by the AMS. 

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2. New insights into atmospherically relevant reaction systems using direct analysis in real-time mass spectrometry (DART-MS)

   https://doi.org/10.5194/amt-10-1373-2017  

   Zhao, Yue ; Fairhurst, Michelle C. ; Wingen, Lisa M. ; Perraud, Véronique ; Ezell, Michael J. ; Finlayson-Pitts, Barbara J. ( January 2017 , Atmospheric Measurement Techniques)

   The application of direct analysis in real-time mass spectrometry (DART-MS), which is finding increasing use in atmospheric chemistry, to two different laboratory model systems for airborne particles is investigated: (1) submicron C₃–C₇ dicarboxylic acid (diacid) particles reacted with gas-phase trimethylamine (TMA) or butylamine (BA) and (2) secondary organic aerosol (SOA) particles from the ozonolysis of α-cedrene. The diacid particles exhibit a clear odd–even pattern in their chemical reactivity toward TMA and BA, with the odd-carbon diacid particles being substantially more reactive than even ones. The ratio of base to diacid in reacted particles, determined using known diacid–base mixtures, was compared to that measured by high-resolution time-of-flight aerosol mass spectrometry (HR-ToF-AMS), which vaporizes the whole particle. Results show that DART-MS probes  ∼  30 nm of the surface layer, consistent with other studies on different systems. For α-cedrene SOA particles, it is shown that varying the temperature of the particle stream as it enters the DART-MS ionization region can distinguish between specific components with the same molecular mass but different vapor pressures. These results demonstrate the utility of DART-MS for (1) examining reactivity of heterogeneous model systems for atmospheric particles and (2) probing components of SOA particles based on volatility. 

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3. Effect of Aerosol Acidity on the Kinetics and Products of Heterogeneous Hydroxyl Radical Oxidation of Isoprene Epoxydiol-Derived Secondary Organic Aerosol

   YAN, JIN ; Armstrong, N. Cazimir ; Fankhauser, Alison ; Cooke, Madeline ; Buchenau, Nicolas A. ; Xiao, Yao ; Zhang, Zhenfa ; Lambe, Andrew ; Gold, Avram ; Ault, Andrew ; et al ( October 2022 , AAAR 40th Annual Conference)

   Abstract We recently demonstrated that the heterogeneous hydroxyl radical (·OH) oxidation is an important aging process for isoprene epoxydiol-derived secondary organic aerosol (IEPOX-SOA) that alters its chemical composition, and thus, aerosol physicochemical properties. Notably, dimeric species in IEPOX-SOA were found to heterogeneously react with ·OH at a much faster rate than monomers, suggesting that the initial oligomeric content of freshly-generated IEPOX-SOA particles may affect its subsequent atmospheric oxidation. Aerosol acidity could in principle influence this aging process by enhancing the formation of sulfated and non-sulfated oligomers in freshly-generated IEPOX-SOA. Many multifunctional organosulfate (OS) products derived from heterogeneous ·OH oxidation of sulfur-containing IEPOX-SOA have been observed in cloud water residues and ice nucleating particles and could affect the ability of aged IEPOX-SOA particles to act as cloud condensation nuclei. Hence, this study systematically investigated the effect of aerosol acidity on the kinetics and products resulting from heterogeneous ·OH oxidation of IEPOX-SOA particles. Gas-phase IEPOX was reacted with inorganic sulfate particles of varying pH (0.5 to 2.0) in an indoor smog chamber operated under dark, steady-state conditions to form freshly-generated IEPOX-SOA particles. These particles were then aged at a relative humidity of 60% in an oxidation flow reactor (OFR) for 0-15 days of equivalent atmospheric ·OH exposure. Aged IEPOX-SOA particles were sampled by an online aerosol chemical speciation monitor (ACSM) to measure real-time aerosol mass and chemical changes of the SOA particles, and were also collected onto Teflon filters and into PILS vials for molecular-level chemical analyses by hydrophilic liquid interaction chromatography method interfaced to electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (HILIC/ESI-HR-QTOFMS), ion chromatography, and total OS mass amounts. 

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4. Bulk and molecular-level characterization of laboratory-aged biomass burning organic aerosol from oak leaf and heartwood fuels

   https://doi.org/10.5194/acp-18-2199-2018  

   Fortenberry, Claire F. ; Walker, Michael J. ; Zhang, Yaping ; Mitroo, Dhruv ; Brune, William H. ; Williams, Brent J. ( January 2018 , Atmospheric Chemistry and Physics)

   null (Ed.)

   Abstract. The chemical complexity of biomass burning organic aerosol (BBOA) greatlyincreases with photochemical aging in the atmosphere, necessitatingcontrolled laboratory studies to inform field observations. In theseexperiments, BBOA from American white oak (Quercus alba) leaf andheartwood samples was generated in a custom-built emissions and combustionchamber and photochemically aged in a potential aerosol mass (PAM) flowreactor. A thermal desorption aerosol gas chromatograph (TAG) was used inparallel with a high-resolution time-of-flight aerosol mass spectrometer(AMS) to analyze BBOA chemical composition at different levels ofphotochemical aging. Individual compounds were identified and integrated toobtain relative decay rates for key molecules. A recently developedchromatogram binning positive matrix factorization (PMF) technique was usedto obtain mass spectral profiles for factors in TAG BBOA chromatograms,improving analysis efficiency and providing a more complete determination ofunresolved complex mixture (UCM) components. Additionally, the recentlycharacterized TAG decomposition window was used to track molecular fragmentscreated by the decomposition of thermally labile BBOA during sampledesorption. We demonstrate that although most primary (freshly emitted) BBOAcompounds deplete with photochemical aging, certain components eluting withinthe TAG thermal decomposition window are instead enhanced. Specifically, theincreasing trend in the decomposition m∕z 44 signal (CO2+)indicates formation of secondary organic aerosol (SOA) in the PAM reactor.Sources of m∕z 60 (C2H4O2+), typically attributed tofreshly emitted BBOA in AMS field measurements, were also investigated. Fromthe TAG chemical speciation and decomposition window data, we observed adecrease in m∕z 60 with photochemical aging due to the decay ofanhydrosugars (including levoglucosan) and other compounds, as well as anincrease in m∕z 60 due to the formation of thermally labile organic acidswithin the PAM reactor, which decompose during TAG sample desorption. Whenaging both types of BBOA (leaf and heartwood), the AMS data exhibit acombination of these two contributing effects, causing limited change to theoverall m∕z 60 signal. Our observations demonstrate the importance ofchemically speciated data in fully understanding bulk aerosol measurementsprovided by the AMS in both laboratory and field studies. 

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5. Synergistic Multiphase Chemistry of Isoprene Hydroxy Hydroperoxides (ISOPOOH) with Sulfur Dioxide in Acidic Sulfate Aerosols Leading to Secondary Inorganic and Organic Aerosol Formation

   Zhang, Yue ; Yan, Yan ; Chen, Yuzhi ; Armstrong, Cazimir ; Zhang, Zhenfa ; Gold, Avram ; Turpin, Barbara ; Surratt, Jason ( October 2021 , 2021 AAAR 39th Annual Conference)

   In order to examine the reaction products, kinetics, and implications of ISOPOOH with aqueous sulfite, ammonium bisulfate particles were injected into the UNC 10‐m3 indoor environmental chamber under humid (i.e., 72% RH) and dark conditions. After the inorganic sulfate concentration stabilized, selected concentrations of gas‐phase 1,2‐ISOPOOH were injected into the chamber, and aerosols showed a minimal mass increase. Gaseous SO2 was subsequently injected into the chamber and a significant amount of aerosol mass was produced. The gas‐phase ISOPOOH and particle‐phase species were sampled with online instruments, including a chemical ionization mass spectrometer (CIMS), an aerosol chemical speciation monitor (ACSM), a particle‐into‐liquid sampler (PILS) for analysis by ion chromatography analysis (IC), and filter samples were analyzed by an ultra‐performance liquid chromatography coupled to an electrospray ionization highresolution quadrupole time‐of‐flight mass spectrometry (UPLCESI‐ HR‐QTOFMS) to obtain offline molecular‐level information. Results show that a significant amount of inorganic sulfate and organosulfates were formed rapidly after injecting SO2, altering the chemical and physical properties of the particles including phase state, pH, reactivity, and composition. Multifunctional C5‐organic species that were previously measured in atmospheric fine aerosol samples were also reported here as reaction products, including 2‐methyletrols and 2‐methyltetrol sulfates that were previously thought to be only produced from the reactive uptake of isoprene‐derived epoxydiols (IEPOX). Such results indicate that the multiphase reactions of ISOPOOH could have significant impacts on the atmospheric lifecycle of organic aerosols and sulfur, as well as the physicochemical properties of ambient particles. 

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