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1. Simultaneous detection of ozone and nitrogen dioxide by oxygen anion chemical ionization mass spectrometry: a fast-time-response sensor suitable for eddy covariance measurements

   https://doi.org/10.5194/amt-13-1887-2020  

   Novak, Gordon A. ; Vermeuel, Michael P. ; Bertram, Timothy H. ( January 2020 , Atmospheric Measurement Techniques)

   Abstract. We report on the development, characterization, and fielddeployment of a fast-time-response sensor for measuring ozone (O3) andnitrogen dioxide (NO2) concentrations utilizing chemical ionizationtime-of-flight mass spectrometry (CI-ToFMS) with oxygen anion(O2-) reagent ion chemistry. Wedemonstrate that the oxygen anion chemical ionization mass spectrometer(Ox-CIMS) is highly sensitive to both O3 (180 counts s−1 pptv−1) and NO2 (97 counts s−1 pptv−1), corresponding todetection limits (3σ, 1 s averages) of 13 and 9.9 pptv,respectively. In both cases, the detection threshold is limited by themagnitude and variability in the background determination. The short-termprecision (1 s averages) is better than 0.3 % at 10 ppbv O3 and 4 %at 10 pptv NO2. We demonstrate that the sensitivity of the O3measurement to fluctuations in ambient water vapor and carbon dioxide isnegligible for typical conditions encountered in the troposphere. Theapplication of the Ox-CIMS to the measurement of O3 vertical fluxesover the coastal ocean, via eddy covariance (EC), was tested during the summer of2018 at Scripps Pier, La Jolla, CA. The observed mean ozone depositionvelocity (vd(O3)) was 0.013 cm s−1 with a campaign ensemblelimit of detection (LOD) of 0.0027 cm s−1 at the 95 % confidencelevel, from each 27 min sampling period LOD. The campaign mean and 1standard deviation range of O3 mixing ratios was 41.2±10.1 ppbv. Several fast ozone titration events from local NO emissions weresampled where unit conversion of O3 to NO2 was observed,highlighting instrument utility as a total odd-oxygen (Ox=O3+NO2) sensor. The demonstrated precision, sensitivity, and timeresolution of this instrument highlight its potential for directmeasurements of O3 ocean–atmosphere and biosphere–atmosphere exchangefrom both stationary and mobile sampling platforms. 

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2. Using collision-induced dissociation to constrain sensitivity of ammonia chemical ionization mass spectrometry (NH₄⁺ CIMS) to oxygenated volatile organic compounds

   https://doi.org/10.5194/amt-12-1861-2019  

   Zaytsev, Alexander ; Breitenlechner, Martin ; Koss, Abigail R. ; Lim, Christopher Y. ; Rowe, James C. ; Kroll, Jesse H. ; Keutsch, Frank N. ( January 2019 , Atmospheric Measurement Techniques)

   Abstract. Chemical ionization massspectrometry (CIMS) instruments routinely detect hundreds of oxidized organic compoundsin the atmosphere. A major limitation of these instruments is the uncertaintyin their sensitivity to many of the detected ions. We describe thedevelopment of a new high-resolution time-of-flight chemical ionization massspectrometer that operates in one of two ionization modes: using eitherammonium ion ligand-switching reactions such as for ${\mathrm{NH}}_{\mathrm{4}}^{+}$ CIMS orproton transfer reactions such as for proton-transfer-reaction massspectrometer (PTR-MS). Switching between the modes can be done within 2 min.The ${\mathrm{NH}}_{\mathrm{4}}^{+}$ CIMS mode of the new instrument has sensitivities of upto 67 000 dcps ppbv⁻¹ (duty-cycle-corrected ion counts per second perpart per billion by volume) and detection limits between 1 and 60 pptv at2σ for a 1 s integration time for numerous oxygenated volatileorganic compounds. We present a mass spectrometric voltage scanning procedurebased on collision-induced dissociation that allows us to determine thestability of ammonium-organic ions detected by the ${\mathrm{NH}}_{\mathrm{4}}^{+}$ CIMS instrument.Using this procedure, we can effectively constrain the sensitivity of theammonia chemical ionization mass spectrometer to a wide range of detectedoxidized volatile organic compounds for which no calibration standards exist.We demonstrate the application of this procedure by quantifying thecomposition of secondary organic aerosols in a series of laboratoryexperiments.

    

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3. Molecular characterization of ultrafine particles using extractive electrospray time-of-flight mass spectrometry

   https://doi.org/10.1039/d1ea00050k  

   Surdu, Mihnea ; Pospisilova, Veronika ; Xiao, Mao ; Wang, Mingyi ; Mentler, Bernhard ; Simon, Mario ; Stolzenburg, Dominik ; Hoyle, Christopher R. ; Bell, David M. ; Lee, Chuan Ping ; et al ( September 2021 , Environmental Science: Atmospheres)

   Aerosol particles negatively affect human health while also having climatic relevance due to, for example, their ability to act as cloud condensation nuclei. Ultrafine particles (diameter D p < 100 nm) typically comprise the largest fraction of the total number concentration, however, their chemical characterization is difficult because of their low mass. Using an extractive electrospray time-of-flight mass spectrometer (EESI-TOF), we characterize the molecular composition of freshly nucleated particles from naphthalene and β-caryophyllene oxidation products at the CLOUD chamber at CERN. We perform a detailed intercomparison of the organic aerosol chemical composition measured by the EESI-TOF and an iodide adduct chemical ionization mass spectrometer equipped with a filter inlet for gases and aerosols (FIGAERO-I-CIMS). We also use an aerosol growth model based on the condensation of organic vapors to show that the chemical composition measured by the EESI-TOF is consistent with the expected condensed oxidation products. This agreement could be further improved by constraining the EESI-TOF compound-specific sensitivity or considering condensed-phase processes. Our results show that the EESI-TOF can obtain the chemical composition of particles as small as 20 nm in diameter with mass loadings as low as hundreds of ng m −3 in real time. This was until now difficult to achieve, as other online instruments are often limited by size cutoffs, ionization/thermal fragmentation and/or semi-continuous sampling. Using real-time simultaneous gas- and particle-phase data, we discuss the condensation of naphthalene oxidation products on a molecular level. 

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4. Photodissociation dynamics of acetone studied by time-resolved ion imaging and photofragment excitation spectroscopy

   https://doi.org/10.1039/C7CP07320H  

   Toulson, Benjamin W. ; Fishman, Dmitry A. ; Murray, Craig ( January 2018 , Physical Chemistry Chemical Physics)

   The photodissociation dynamics of acetone has been investigated using velocity-map ion imaging and photofragment excitation (PHOFEX) spectroscopy across a range of wavelengths spanning the first absorption band (236–308 nm). The radical products of the Norrish Type I dissociation, methyl and acetyl, as well as the molecular product ketene have been detected by single-photon VUV ionization at 118 nm. Ketene appears to be formed with non-negligible yield at all wavelengths, with a maximum value of Φ ≈ 0.3 at 280 nm. The modest translational energy release is inconsistent with dissociation over high barriers on the S 0 surface, and ketene formation is tentatively assigned to a roaming pathway involving frustrated dissociation to the radical products. Fast-moving radical products are detected at λ ≤ 305 nm with total translational energy distributions that extend to the energetic limit, consistent with dissociation occurring near-exclusively on the T 1 surface following intersystem crossing. At energies below the T 1 barrier a statistical component indicative of S 0 dissociation is observed, although dissociation via the S 1 /S 0 conical intersection is absent at shorter wavelengths, in contrast to acetaldehyde. The methyl radical yield is enhanced over that of acetyl in PHOFEX spectra at λ ≤ 260 nm due to the onset of secondary dissociation of internally excited acetyl radicals. Time-resolved ion imaging experiments using picosecond duration pulses at 266 nm find an appearance time constant of τ = 1490 ± 140 ps for CH 3 radicals formed on T 1 . The associated rate is representative of S 1 → T 1 intersystem crossing. At 284 nm, CH 3 is formed on T 1 with two distinct timescales: a fast <10 ns component is accompanied by a slower component with τ = 42 ± 7 ns. A two-step mechanism involving fast internal conversion, followed by slower intersystem crossing (S 1 → S 0 → T 1 ) is proposed to explain the slow component. 

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5. Highly-excited state properties of cumulenone chlorides in the vacuum-ultraviolet

   https://doi.org/10.1039/D0CP01835J  

   Nguyen, Quynh L. ; Peters, William K. ; Fortenberry, Ryan C. ( June 2020 , Physical Chemistry Chemical Physics)

   null (Ed.)

   Recent observations of chloromethane in interstellar environments suggest that other organohalogens, which are known to be critically important in Earth's atmosphere, may also be of significance beyond our own terrestrial veil. This raises the question of how such molecules behave under extreme conditions such as when exposed to vacuum ultraviolet (VUV) radiation. VUV photons promote molecules to highly excited states that fragment in non-statistical patterns controlled by the initial femtosecond dynamics. A detailed understanding of VUV-driven photochemistry in complex organic molecules that consist of more than one functional group is a particularly challenging task. This quantum chemical analysis reports the electronic states and ionization potentials up to the VUV range (6–11 eV) of the chlorine-substituted cumulenone series molecules. The valence and Rydberg properties of lone-pair terminated, π-conjugated systems are explored for their potential resonance with lone pairs from elsewhere in the system. The carbon chain elongation within the family ClHC n O, where n = 1–4, influences the electronic excitations, associated wavefunctions, and ionization potentials of the molecules. The predicted geometries and ionization potentials are in good agreement with the available experimental photoelectron spectra for formyl chloride and chloroketene, n = 1–2. Furthermore, comparison between the regular cumulenone species and the corresponding chlorinated derivatives exhibit similar behaviors especially for n = 3, where the allene backbone in propadienone chloride is severely bent. Most notably for the excited states is that the Rydberg character becomes more dominant as the energy increases, with some retaining valence characters. 

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