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1. Measurement of iodine species and sulfuric acid using bromide chemical ionization mass spectrometers

   https://doi.org/10.5194/amt-14-4187-2021  

   Wang, Mingyi; He, Xu-Cheng; Finkenzeller, Henning; Iyer, Siddharth; Chen, Dexian; Shen, Jiali; Simon, Mario; Hofbauer, Victoria; Kirkby, Jasper; Curtius, Joachim; et al (January 2021, Atmospheric Measurement Techniques)

   null (Ed.)

   Abstract. Iodine species are important in the marine atmosphere foroxidation and new-particle formation. Understanding iodine chemistry andiodine new-particle formation requires high time resolution, highsensitivity, and simultaneous measurements of many iodine species. Here, wedescribe the application of a bromide chemical ionization mass spectrometer(Br-CIMS) to this task. During the iodine oxidation experiments in theCosmics Leaving OUtdoor Droplets (CLOUD) chamber, we have measured gas-phaseiodine species and sulfuric acid using two Br-CIMS, one coupled to aMulti-scheme chemical IONization inlet (Br-MION-CIMS) and the other to aFilter Inlet for Gasses and AEROsols inlet (Br-FIGAERO-CIMS). From offlinecalibrations and intercomparisons with other instruments, we havequantified the sensitivities of the Br-MION-CIMS to HOI, I2, andH2SO4 and obtained detection limits of 5.8 × 106,3.8 × 105, and 2.0 × 105 molec. cm−3,respectively, for a 2 min integration time. From binding energycalculations, we estimate the detection limit for HIO3 to be1.2 × 105 molec. cm−3, based on an assumption of maximumsensitivity. Detection limits in the Br-FIGAERO-CIMS are around 1 order ofmagnitude higher than those in the Br-MION-CIMS; for example, the detectionlimits for HOI and HIO3 are 3.3 × 107 and 5.1 × 106 molec. cm−3, respectively. Our comparisons of the performanceof the MION inlet and the FIGAERO inlet show that bromide chemicalionization mass spectrometers using either atmospheric pressure or reducedpressure interfaces are well-matched to measuring iodine species andsulfuric acid in marine environments. 

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2. OH and HO₂ radical chemistry in a midlatitude forest: measurements and model comparisons

   https://doi.org/10.5194/acp-20-9209-2020  

   Lew, Michelle M.; Rickly, Pamela S.; Bottorff, Brandon P.; Reidy, Emily; Sklaveniti, Sofia; Léonardis, Thierry; Locoge, Nadine; Dusanter, Sebastien; Kundu, Shuvashish; Wood, Ezra; et al (January 2020, Atmospheric Chemistry and Physics)

   null (Ed.)

   Abstract. Reactions of the hydroxyl (OH) and peroxy (HO2 and RO2) radicals playa central role in the chemistry of the atmosphere. In addition to controlling the lifetimes ofmany trace gases important to issues of global climate change, OH radical reactionsinitiate the oxidation of volatile organic compounds (VOCs) which can lead to the production ofozone and secondary organic aerosols in the atmosphere. Previous measurements of these radicalsin forest environments characterized by high mixing ratios of isoprene and low mixing ratios ofnitrogen oxides (NOx) (typically less than 1–2 ppb) have shown seriousdiscrepancies with modeled concentrations. These results bring into question our understanding ofthe atmospheric chemistry of isoprene and other biogenic VOCs under low NOxconditions. During the summer of 2015, OH and HO2 radical concentrations, as well as totalOH reactivity, were measured using laser-induced fluorescence–fluorescence assay by gasexpansion (LIF-FAGE) techniques as part of the Indiana Radical Reactivity and Ozone productioN InterComparison (IRRONIC). This campaign took place in a forested area near Indiana University's Bloomington campus which is characterized by high mixing ratios of isoprene (average daily maximum ofapproximately 4 ppb at 28 ∘C) and low mixing ratios of NO (diurnal averageof approximately 170 ppt). Supporting measurements of photolysis rates, VOCs,NOx, and other species were used to constrain a zero-dimensional box model basedon the Regional Atmospheric Chemistry Mechanism (RACM2) and the Master Chemical Mechanism (MCM 3.2),including versions of the Leuven isoprene mechanism (LIM1) for HOx regeneration(RACM2-LIM1 and MCM 3.3.1). Using an OH chemical scavenger technique, the study revealed thepresence of an interference with the LIF-FAGE measurements of OH that increased with bothambient concentrations of ozone and temperature with an average daytime maximum equivalentOH concentration of approximately 5×106 cm−3. Subtraction of theinterference resulted in measured OH concentrations of approximately4×106 cm−3 (average daytime maximum) that were in better agreement with modelpredictions although the models underestimated the measurements in the evening. The addition ofversions of the LIM1 mechanism increased the base RACM2 and MCM 3.2 modeled OH concentrationsby approximately 20 % and 13 %, respectively, with the RACM2-LIM1 mechanism providing thebest agreement with the measured concentrations, predicting maximum daily OH concentrationsto within 30 % of the measured concentrations. Measurements of HO2 concentrationsduring the campaign (approximately a 1×109 cm−3 average daytime maximum)included a fraction of isoprene-based peroxy radicals(HO2*=HO2+αRO2) and were found to agree with modelpredictions to within 10 %–30 %. On average, the measured reactivity was consistent with thatcalculated from measured OH sinks to within 20 %, with modeled oxidation productsaccounting for the missing reactivity, however significant missing reactivity (approximately40 % of the total measured reactivity) was observed on some days. 

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3. OH, HO ₂ , and RO ₂ radical chemistry in a rural forest environment: measurements, model comparisons, and evidence of a missing radical sink

   https://doi.org/10.5194/acp-23-10287-2023  

   Bottorff, Brandon; Lew, Michelle M.; Woo, Youngjun; Rickly, Pamela; Rollings, Matthew D.; Deming, Benjamin; Anderson, Daniel C.; Wood, Ezra; Alwe, Hariprasad D.; Millet, Dylan B.; et al (September 2024, Atmospheric Chemistry and Physics)

   Abstract. The hydroxyl (OH), hydroperoxy (HO2), and organic peroxy (RO2)radicals play important roles in atmospheric chemistry. In the presence ofnitrogen oxides (NOx), reactions between OH and volatile organiccompounds (VOCs) can initiate a radical propagation cycle that leads to theproduction of ozone and secondary organic aerosols. Previous measurements ofthese radicals under low-NOx conditions in forested environmentscharacterized by emissions of biogenic VOCs, including isoprene andmonoterpenes, have shown discrepancies with modeled concentrations. During the summer of 2016, OH, HO2, and RO2 radical concentrationswere measured as part of the Program for Research on Oxidants:Photochemistry, Emissions, and Transport – Atmospheric Measurements ofOxidants in Summer (PROPHET-AMOS) campaign in a midlatitude deciduousbroadleaf forest. Measurements of OH and HO2 were made by laser-inducedfluorescence–fluorescence assay by gas expansion (LIF-FAGE) techniques,and total peroxy radical (XO2) mixing ratios were measured by the Ethane CHemical AMPlifier (ECHAMP) instrument. Supporting measurements ofphotolysis frequencies, VOCs, NOx, O3, and meteorological datawere used to constrain a zero-dimensional box model utilizing either theRegional Atmospheric Chemical Mechanism (RACM2) or the Master ChemicalMechanism (MCM). Model simulations tested the influence of HOxregeneration reactions within the isoprene oxidation scheme from the LeuvenIsoprene Mechanism (LIM1). On average, the LIM1 models overestimated daytimemaximum measurements by approximately 40 % for OH, 65 % for HO2,and more than a factor of 2 for XO2. Modeled XO2 mixing ratioswere also significantly higher than measured at night. Addition of RO2 + RO2 accretion reactions for terpene-derived RO2 radicals tothe model can partially explain the discrepancy between measurements andmodeled peroxy radical concentrations at night but cannot explain thedaytime discrepancies when OH reactivity is dominated by isoprene. Themodels also overestimated measured concentrations of isoprene-derivedhydroxyhydroperoxides (ISOPOOH) by a factor of 10 during the daytime,consistent with the model overestimation of peroxy radical concentrations.Constraining the model to the measured concentration of peroxy radicalsimproves the agreement with the measured ISOPOOH concentrations, suggestingthat the measured radical concentrations are more consistent with themeasured ISOPOOH concentrations. These results suggest that the models maybe missing an important daytime radical sink and could be overestimating therate of ozone and secondary product formation in this forest. 

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4. Temperature-Dependent Spin-Driven Dimerization Determines the Ultrafast Dynamics of a Copper(II)-Bound Tripyrrindione Radical

   https://doi.org/10.1021/acs.jpclett.3c02726  

   Kumar, Anshu; Thompson, Benjamin; Gautam, Ritika; Tomat, Elisa; Huxter, Vanessa (December 2023, The Journal of Physical Chemistry Letters)

   Radicals and other open-shell molecules play a central role in chemical transformations and redox chemistry. While radicals are often highly reactive, stable radical systems are desirable for a range of potential applications, ranging from materials chemistry and catalysis to spintronics and quantum information. Here we investigate the ultrafast properties of a stable radical system with temperature-dependent spin-tunable properties. This radical complex, Cu(II) hexaethyl tripyrrin-1,14-dione, accommodates unpaired electrons localized on both the copper metal center and the tripyrrolic ligand. The unusual combination of two unpaired electrons and high stability in this radical molecule enable switchable temperature-dependent spin coupling. Two-dimensional electronic spectroscopy measurements of Cu(II) hexaethyl tripyrrin-1,14-dione were collected at room temperature and at 77 K. At room temperature, the molecules are present as monomers and have short picosecond lifetimes. At 77 K, the molecules are present in a dimer form mediated by ferromagnetic and antiferromagnetic coupling. This reversible spin-driven dimerization changes the optical properties of the system, generating long-lived excitonic states. 

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5. Atmospheric Photo-Oxidation of 2-Ethoxyethanol: Autoxidation Chemistry of Glycol Ethers

   https://doi.org/10.1021/acs.jpca.3c04456  

   Yu, Hongmin; Møller, Kristian H; Buenconsejo, Reina S; Crounse, John D; Kjaergaard, Henrik G; Wennberg, Paul O (November 2023, The Journal of Physical Chemistry A)

   We investigate the gas-phase photo-oxidation of 2-ethoxyethanol (2-EE) initiated by the OH radical with a focus on its autoxidation pathways. Gas-phase autoxidation intramolecular H-shifts followed by O2 additionhas recently been recognized as a major atmospheric chemical pathway that leads to the formation of highly oxygenated organic molecules (HOMs), which are important precursors for secondary organic aerosols (SOAs). Here, we examine the gas-phase oxidation pathways of 2-EE, a model compound for glycol ethers, an important class of volatile organic compounds (VOCs) used in volatile chemical products (VCPs). Both experimental and computational techniques are applied to analyze the photochemistry of the compound. We identify oxidation products from both bimolecular and autoxidation reactions from chamber experiments at varied HO2 levels and provide estimations of rate coefficients and product branching ratios for key reaction pathways. The H-shift processes of 2-EE peroxy radicals (RO2) are found to be sufficiently fast to compete with bimolecular reactions under modest NO/HO2 conditions. More than 30% of the produced RO2 are expected to undergo at least one H-shift for conditions typical of modern summer urban atmosphere, where RO2 bimolecular lifetime is becoming >10 s, which implies the potential for glycol ether oxidation to produce considerable amounts of HOMs at reduced NOx levels and elevated temperature. Understanding the gas-phase autoxidation of glycol ethers can help fill the knowledge gap in the formation of SOA derived from oxygenated VOCs emitted from VCP sources. 

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