Generation of energetic He atom beams by a pulsed positive corona discharge

Time-of-flight measurements were made of neutral helium atom beams extracted from a repetitive, pulsed, positive-point corona discharge. Two strong neutral peaks, one fast and one slow, were observed, accompanied by a prompt photon peak and a fast ion peak. All peaks were correlated with the pulsing of the discharge. The two types of atoms appear to be formed by different mechanisms at different stages of the corona discharge. The fast atoms had energies of 190 eV and were formed at the onset of the pulsing, approximately 0.7 µs before the maximum of the photon peak. The slow peak, composed of electronically metastable He atoms, originated 30–50 µs after the photon pulse, and possessed a nearly thermal velocity distribution. The velocity distribution was typical of an undisturbed supersonic expansion with a stagnation temperature of 131 K and a speed ratio of 3.6. Peak intensities and velocities were measured as a function of source voltage, stagnation pressure, and skimmer voltage

Computational Studies of Intramolecular Hydrogen Atom Transfers in the ß-Hydroxyethylperoxy and ß -Hydroxyethoxy Radicals

The ß-hydroxyethylperoxy (I) and ß-hydroxyethoxy (III) radicals are prototypes of species that can undergo hydrogen atom transfer across their intramolecular hydrogen bonds. These reactions may play an important role in both the atmosphere and in combustion systems. We have used density functional theory and composite electronic structure methods to predict the energetics of these reactions, RRKM/master equation simulations to model the kinetics of chemically activated I, and variational transition state theory (TST) to predict thermal rate constants for the 1,5-hydrogen shift in I (Reaction 1) and the 1,4-hydrogen shift in III (Reaction 2). Our multi-coefficient Gaussian-3 calculations predict that Reaction 1 has a barrier of 23.59 kcal/mol, and that Reaction 2 has a barrier of 22.71 kcal/mol. These predictions agree rather well with the MPW1K and BB1K density functional theory predictions but disagree with predictions based on B3LYP energies or geometries. Our RRKM/master equation simulations suggest that almost 50% of I undergoes a prompt hydrogen shift reaction at pressures up to 10 Torr, but the extent to which I is chemically activated is uncertain. For Reaction 1 at 298 K, the variational TST rate constant is ~30% lower than the conventional TST result, and the microcanonical optimized multidimensional tunneling (OMT) method predicts that tunneling accelerates the reaction by a factor of 3. TST calculations on Reaction 2 reveal no variational effect and a 298 K OMT transmission coefficient of 105. The Eckart method overestimates transmission coefficients for both reactions. [ACS abstract]http://pubs.acs.org/cgi-bin/abstract.cgi/jpcafh/asap/abs/jp0704113.htm

A new mechanism for atmospheric mercury redox chemistry:implications for the global mercury budget

Mercury (Hg) is emitted to the atmosphere mainly as volatile elemental Hg0. Oxidation to water-soluble HgII plays a major role in Hg deposition to ecosystems. Here, we implement a new mechanism for atmospheric Hg0 ∕ HgII redox chemistry in the GEOS-Chem global model and examine the implications for the global atmospheric Hg budget and deposition patterns. Our simulation includes a new coupling of GEOS-Chem to an ocean general circulation model (MITgcm), enabling a global 3-D representation of atmosphere–ocean Hg0 ∕ HgII cycling. We find that atomic bromine (Br) of marine organobromine origin is the main atmospheric Hg0 oxidant and that second-stage HgBr oxidation is mainly by the NO2 and HO2 radicals. The resulting chemical lifetime of tropospheric Hg0 against oxidation is 2.7 months, shorter than in previous models. Fast HgII atmospheric reduction must occur in order to match the  ∼  6-month lifetime of Hg against deposition implied by the observed atmospheric variability of total gaseous mercury (TGM  ≡  Hg0 + HgII(g)). We implement this reduction in GEOS-Chem as photolysis of aqueous-phase HgII–organic complexes in aerosols and clouds, resulting in a TGM lifetime of 5.2 months against deposition and matching both mean observed TGM and its variability. Model sensitivity analysis shows that the interhemispheric gradient of TGM, previously used to infer a longer Hg lifetime against deposition, is misleading because Southern Hemisphere Hg mainly originates from oceanic emissions rather than transport from the Northern Hemisphere. The model reproduces the observed seasonal TGM variation at northern midlatitudes (maximum in February, minimum in September) driven by chemistry and oceanic evasion, but it does not reproduce the lack of seasonality observed at southern hemispheric marine sites. Aircraft observations in the lowermost stratosphere show a strong TGM–ozone relationship indicative of fast Hg0 oxidation, but we show that this relationship provides only a weak test of Hg chemistry because it is also influenced by mixing. The model reproduces observed Hg wet deposition fluxes over North America, Europe, and China with little bias (0–30 %). It reproduces qualitatively the observed maximum in US deposition around the Gulf of Mexico, reflecting a combination of deep convection and availability of NO2 and HO2 radicals for second-stage HgBr oxidation. However, the magnitude of this maximum is underestimated. The relatively low observed Hg wet deposition over rural China is attributed to fast HgII reduction in the presence of high organic aerosol concentrations. We find that 80 % of HgII deposition is to the global oceans, reflecting the marine origin of Br and low concentrations of organic aerosols for HgII reduction. Most of that deposition takes place to the tropical oceans due to the availability of HO2 and NO2 for second-stage HgBr oxidation

Atmospheric chemistry of isopropyl formate and tert-butyl formate

Article on the atmospheric chemistry of isopropyl formate and tert-butyl formate

Improved mechanistic model of the atmospheric redox chemistry of mercury

12 pags, 4 figs, 3 tabs. -- The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.est.1c03160.We present a new chemical mechanism for Hg0/HgI/HgII atmospheric cycling, including recent laboratory and computational data, and implement it in the GEOS-Chem global atmospheric chemistry model for comparison to observations. Our mechanism includes the oxidation of Hg0 by Br and OH, subsequent oxidation of HgI by ozone and radicals, respeciation of HgII in aerosols and cloud droplets, and speciated HgII photolysis in the gas and aqueous phases. The tropospheric Hg lifetime against deposition in the model is 5.5 months, consistent with observational constraints. The model reproduces the observed global surface Hg0 concentrations and HgII wet deposition fluxes. Br and OH make comparable contributions to global net oxidation of Hg0 to HgII. Ozone is the principal HgI oxidant, enabling the efficient oxidation of Hg0 to HgII by OH. BrHgIIOH and HgII(OH)2, the initial HgII products of Hg0 oxidation, respeciate in aerosols and clouds to organic and inorganic complexes, and volatilize to photostable forms. Reduction of HgII to Hg0 takes place largely through photolysis of aqueous HgII-organic complexes. 71% of model HgII deposition is to the oceans. Major uncertainties for atmospheric Hg chemistry modeling include Br concentrations, stability and reactions of HgI, and speciation and photoreduction of HgII in aerosols and clouds.This work was funded by the USEPA Science to Achieve Results (STAR) Program. This work was also supported by the Slovak Grant Agency VEGA (grant 1/0777/19), the highperformance computing facility of the Centre for Information Technology (https://uniba.sk/en/HPC-Clara) at Comenius University, and the U.S. National Science Foundation under awards 1609848 and 2004100. We thank Helene Angot (CU Boulder) for the Hg measurement data.Peer reviewe

DETECTION OF ATMOSPHERICALLY RELEVANT HYDROCARBONS BY DIODE LASER CAVITY RINGDOWN SPECTROSCOPY

Author Institution: Diagnostic Instrumentation and Analysis Laboratory, Mississippi State University, Starkville, MS 39759; Chemistry Department, State University of New York, College of Environmental Science and Forestry, Syracuse, NY 13210The first overtones of the asymmetric CH stretch of 2-methyl-1,3-butadiene (isoprene), 2,3-dimethyl-1,3-butadiene and 1,3-butadiene were observed in gas phase. The absorption was monitored by means of continuous wave Cavity Ring Down Spectroscopy (cw CRDS) at room temperature. A near infrared diode laser was employed as a light source. The absorption cross-section was determined at 1651.5 nm. This study is a starting point for future studies of the peroxy radicals formed in the OH-initiated degradation of these atmospherically relevant compounds. The preliminary estimates of the absorption cross-section are 2.88 x $10^{-22}$ cm$^{2}$ for 2,3-dimethyl-1,3-butadiene and 7.22 x $10^{-22}$ cm$^{2}$ for isoprene, respectively. The NIR absorption cross-sections for isoprene will be calibrated using the known UV absorption cross-section

Structures, Vibrational Frequencies, and Bond Energies of the BrHgOX and BrHgXO Species Formed in Atmospheric Mercury Depletion Events

Photochemistry during the polar spring leads to atmospheric mercury depletion events (AMDEs): Hg(0), which typically lives for months in the atmosphere, and can experience losses of more than 90% in less than a day. These dramatic losses are known to be initiated largely by Br + Hg + M → BrHg• + M, but the fate of BrHg• is a matter of guesswork. It is believed that BrHg• largely reacts with halogen oxides XO (X = Cl, Br, and I) to form BrHgOX compounds, but these species have never been studied experimentally. Here, we use quantum chemistry to characterize the structures, vibrational frequencies, and thermodynamics of these BrHgOX species and their BrHgXO isomers. The BrHgXO isomers have never previously been studied in experiments or computations. We find the BrHgOX species are 24–28 kcal/mol more stable than their BrHgXO isomers. When formed during polar AMDEs, BrHgBrO and BrHgIO appear sufficiently stable in that they will not dissociate before undergoing deposition, but BrHgClO is probably not that stable