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1. Emission factors of long-lived volatile organic compounds from the 2019–2020 Australian wildfires during the COALA campaign

   https://doi.org/10.5194/acp-2021-742  

   Mouat, A. P.; Paton-Walsh, C.; Simmons, J. B.; Ramirez-Gamboa, J.; Griffith, D. W.; Kaiser, J. (January 2021, Atmospheric chemistry and physics discussion)

   In 2019/2020, Australia experienced its largest wildfire season on record. Smoke covered hundreds of square kilometers across the southeastern coast and reached the site of the 2020 COALA (Characterizing Organics and Aerosol Loading over Australia) field campaign in New South Wales. Using a subset of nighttime observations made by a proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS), we calculate emission ratios (ERs) and factors (EFs) for 21 volatile organic compounds (VOCs). We restrict our analysis to VOCs with sufficiently high lifetimes to be minimally impacted by oxidation over the ~8 h between when the smoke was emitted and when it arrived at the field site. We use oxidized VOC to VOC ratios to assess the total amount of radical oxidation: maleic anhydride/furan to assess OH oxidation, and (cis-2-butenediol + furanone)/furan to assess NO3 oxidation. We compare ERs calculated from the freshest portion of the plume to ERs calculated using the entire nighttime period. Finding good agreement between the two, we are able to extend our analysis to VOCs measured in more chemically aged portions of the plume. Our analysis provides ERs and EFs for 9 compounds not previously reported for temperate forests in Australia: acrolein, pentanones/methylbutanal, methyl propanoate, methyl methacrylate, propene, maleic anhydride, benzaldehyde, methyl guaiacol, and methylbenzoic acid. We compare our results with two studies in similar Australian biomes, and two studies focused on US temperate forests. We find mixed agreement for EFs presented from previous studies of Australian wildfires, and generally good agreement with studies focused on fires in the Western US. This suggests that comprehensive field measurements of biomass burning VOC emissions in other regions may be applicable to Australian temperate forests. 

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2. Mechanistic study of the formation of ring-retaining and ring-opening products from the oxidation of aromatic compounds under urban atmospheric conditions

   https://doi.org/10.5194/acp-19-15117-2019  

   Zaytsev, Alexander; Koss, Abigail R.; Breitenlechner, Martin; Krechmer, Jordan E.; Nihill, Kevin J.; Lim, Christopher Y.; Rowe, James C.; Cox, Joshua L.; Moss, Joshua; Roscioli, Joseph R.; et al (January 2019, Atmospheric Chemistry and Physics)

   Abstract. Aromatic hydrocarbons make up a large fraction of anthropogenic volatile organic compounds and contribute significantly to the production of tropospheric ozone and secondary organic aerosol (SOA). Four toluene and four 1,2,4-trimethylbenzene (1,2,4-TMB) photooxidation experiments were performed in an environmental chamber under relevantpolluted conditions (NOx∼10 ppb). An extensive suite of instrumentation including two proton-transfer-reaction mass spectrometers (PTR-MS) and two chemical ionisation mass spectrometers (NH4+ CIMS and I− CIMS) allowed for quantification of reactive carbon in multiple generations of hydroxyl radical (OH)-initiated oxidation. Oxidation of both species produces ring-retaining products such as cresols, benzaldehydes, and bicyclic intermediate compounds, as well as ring-scission products such as epoxides and dicarbonyls. We show that the oxidation of bicyclic intermediate products leads to the formation of compounds with high oxygen content (an O:C ratio of up to 1.1). These compounds, previously identified as highly oxygenated molecules (HOMs), are produced by more than one pathway with differing numbers of reaction steps with OH, including both auto-oxidation and phenolic pathways. We report the elemental composition of these compounds formed under relevant urban high-NO conditions. We show that ring-retaining products for these two precursors are more diverse and abundant than predicted by current mechanisms. We present the speciated elemental composition of SOA for both precursors and confirm that highly oxygenated products make up a significant fraction of SOA. Ring-scission products are also detected in both the gas and particle phases, and their yields and speciation generally agree with the kinetic model prediction. 

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3. Emissions of Trace Organic Gases From Western U.S. Wildfires Based on WE‐CAN Aircraft Measurements

   https://doi.org/10.1029/2020JD033838  

   Permar, Wade; Wang, Qian; Selimovic, Vanessa; Wielgasz, Catherine; Yokelson, Robert J.; Hornbrook, Rebecca S.; Hills, Alan J.; Apel, Eric C.; Ku, I‐Ting; Zhou, Yong; et al (June 2021, Journal of Geophysical Research: Atmospheres)

   Abstract We present emission measurements of volatile organic compounds (VOCs) for western U.S. wildland fires made on the NSF/NCAR C‐130 research aircraft during the Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE‐CAN) field campaign in summer 2018. VOCs were measured with complementary instruments onboard the C‐130, including a proton‐transfer‐reaction time‐of‐flight mass spectrometer (PTR‐ToF‐MS) and two gas chromatography (GC)‐based methods. Agreement within combined instrument uncertainties (<60%) was observed for most co‐measured VOCs. GC‐based measurements speciated the isomeric contributions to selected PTR‐ToF‐MS ion masses and generally showed little fire‐to‐fire variation. We report emission ratios (ERs) and emission factors (EFs) for 161 VOCs measured in 31 near‐fire smoke plume transects of 24 specific individual fires sampled in the afternoon when burning conditions are typically most active. Modified combustion efficiency (MCE) ranged from 0.85 to 0.94. The measured campaign‐average total VOC EF was 26.1 ± 6.9 g kg⁻¹, approximately 67% of which is accounted for by oxygenated VOCs. The 10 most abundantly emitted species contributed more than half of the total measured VOC mass. We found that MCE alone explained nearly 70% of the observed variance for total measured VOC emissions (r² = 0.67) and >50% for 57 individual VOC EFs representing more than half the organic carbon mass. Finally, we found little fire‐to‐fire variability for the mass fraction contributions of individual species to the total measured VOC emissions, suggesting that a single speciation profile can describe VOC emissions for the wildfires in coniferous ecosystems sampled during WE‐CAN. 

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4. HPALD2 is a Peroxyhemiacetal and a Source of SOA

   RICE, REBECCA; Surratt, Jason; Zhang, Zhenfa; Gold, Avram (October 2023, 2023 AAAR Meeting)

   Abstract Number: 51 Working Group: Aerosol Chemistry Abstract Isoprene, a volatile organic compound (VOC) is emitted largely by vegetation at a rate of 512 Tg/yr. Based on theoretical calculations and mass spectrometric evidence, Z-δ-hydroperoxyalkenal structures (HPALD1 and HPALD2) have been assigned to C5H8O3 gas-phase compounds accounting for up to 12% of the total first-generation isoprene oxidation products. The putative HPALDs are conjugated carbonyls expected to have a significant absorption cross section at ambient UV wavelengths (> 315 nm). Fast internal energy transfer from the excited alkenal to the O-OH bond is predicted to cause rapid bond dissociation degradation into volatile fragments, with little or no formation of SOA. We undertook synthesis of HPALD2 to verify the structure assigned solely from mass spectrometry. By proton NMR, HPALD2 exists exclusively as the peroxyhemiacetal tautomer, with no carbonyl detected, even in D2O. Tautomerization to the cyclic peroxyhemiacetal is strongly favored by the Z geometry of HPALD2. The peroxyhemiacetal structure of the isoprene photochemical oxidation product was confirmed by matching the IMS drift time of the synthetic standard with a major C5H8O3 product from hydroxyl radical oxidation of isoprene. Lacking the conjugated chromophore, the peroxyhemiacetal does not absorb at > 250 nm will persist at ambient UV wavelengths. In chamber experiments, OH oxidation caused rapid nucleation in the absence of seed, and reactive uptake in the presence of both (NH4)2SO4 and (NH4)HSO4 seed. Products at m/z C5H8O5, C5H10O5, C5H10O6 were detected by on-line monitoring of the gas phase by an iodide-CIMS-high resolution time-of-flight mass spectrometer (HR-ToF-MS). Analysis of filter extracts by hydrophilic interaction liquid chromatography coupled to an electrospray ionization HR-ToF-MS detector operated in the negative mode showed major products with compositions C5H10O5 in all experiments, and major sulfated products with compositions C5H10O8S and C3H6O6S in seeded experiments. 

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5. HPALD2 is a Peroxyhemiacetal and a Source of SOA

   REBECCA RICE, Jason Surratt (October 2023, 41st Meeting American Association of Aerosol Research)

   Isoprene, a volatile organic compound (VOC) is emitted largely by vegetation at a rate of 512 Tg/yr. Based on theoretical calculations and mass spectrometric evidence, Z-δ-hydroperoxyalkenal structures (HPALD1 and HPALD2) have been assigned to C5H8O3 gas-phase compounds accounting for up to 12% of the total first-generation isoprene oxidation products. The putative HPALDs are conjugated carbonyls expected to have a significant absorption cross section at ambient UV wavelengths (> 315 nm). Fast internal energy transfer from the excited alkenal to the O-OH bond is predicted to cause rapid bond dissociation degradation into volatile fragments, with little or no formation of SOA. We undertook synthesis of HPALD2 to verify the structure assigned solely from mass spectrometry. By proton NMR, HPALD2 exists exclusively as the peroxyhemiacetal tautomer, with no carbonyl detected, even in D2O. Tautomerization to the cyclic peroxyhemiacetal is strongly favored by the Z geometry of HPALD2. The peroxyhemiacetal structure of the isoprene photochemical oxidation product was confirmed by matching the IMS drift time of the synthetic standard with a major C5H8O3 product from hydroxyl radical oxidation of isoprene. Lacking the conjugated chromophore, the peroxyhemiacetal does not absorb at > 250 nm will persist at ambient UV wavelengths. In chamber experiments, OH oxidation caused rapid nucleation in the absence of seed, and reactive uptake in the presence of both (NH4)2SO4 and (NH4)HSO4 seed. Products at m/z C5H8O5, C5H10O5, C5H10O6 were detected by on-line monitoring of the gas phase by an iodide-CIMS-high resolution time-of-flight mass spectrometer (HR-ToF-MS). Analysis of filter extracts by hydrophilic interaction liquid chromatography coupled to an electrospray ionization HR-ToF-MS detector operated in the negative mode showed major products with compositions C5H10O5 in all experiments, and major sulfated products with compositions C5H10O8S and C3H6O6S in seeded experiments. 

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