Perfluorodicarbonyl (PFDC) compounds may be emitted directly into the atmosphere or formed in the atmospheric degradation of trace fluorinated gases, such as unsaturated perfluoro cyclic compounds. A ...potential atmospheric removal process for PFDCs is UV photolysis, which is presently not well-characterized. In this work, UV and infrared absorption spectra of FC(O)C(O)F, FC(O)CF2C(O)F, and FC(O)CF2CF2C(O)F (three of the simplest PFDCs) and their 248 nm photolysis products are reported. UV spectra were measured at 296 K between 190 and 320 nm using single wavelength and broadband diode array spectroscopic measurement techniques. Infrared absorption spectra were measured at 296 K using Fourier transform infrared spectroscopy between 500 and 4000 cm–1. The PFDCs are shown to be potent greenhouse gases with radiative efficiencies (well-mixed) of 0.142, 0.218, and 0.293 W m–2 ppb–1 for FC(O)C(O)F, FC(O)CF2C(O)F, and FC(O)CF2CF2C(O)F, respectively. Photolysis product yields (248 nm) were measured using pulsed laser photolysis combined with infrared absorption detection of radical products scavenged to stable bromides by reaction with Br2. BrC(O)F was identified as a major stable end product in all systems with a yield greater than ∼90%. The infrared spectrum of BrC(O)F is reported as part of this study. FC(O)CBrF2 and FC(O)CF2CBrF2 were also identified as products in the photolysis of FC(O)CF2C(O)F and FC(O)CF2CF2C(O)F, respectively, by comparison with theoretically calculated infrared absorption spectra. A carbonyl difluoride (CF2O) primary photolysis yield of ∼10% was measured in the photolysis of FC(O)C(O)F.
The reaction CH3C(O)O2-+-HO2 arrow right CH3C(O)OOH-+-O2 (Reaction R5a), CH3C(O)OH-+-O3 (Reaction R5b), CH3-+-CO2-+-OH-+-O2 (Reaction R5c) was studied in a series of experiments conducted at ...1000-mbar and (293-±-2)-K in the HIRAC simulation chamber. For the first time, products, (CH3C(O)OOH, CH3C(O)OH, O3 and OH) from all three branching pathways of the reaction have been detected directly and simultaneously. Measurements of radical precursors (CH3OH, CH3CHO), HO2 and some secondary products HCHO and HCOOH further constrained the system. Fitting a comprehensive model to the experimental data, obtained over a range of conditions, determined the branching ratios α(R5a)- = -0.37-±-0.10, α(R5b) = -0.12-±-0.04 and α(R5c) = -0.51-±-0.12 (errors at 2σ level). Improved measurement/model agreement was achieved using k(R5) = (2.4-±-0.4)- × -10-11-cm3-molecule-1-s-1, which is within the large uncertainty of the current IUPAC and JPL recommended rate coefficients for the title reaction. The rate coefficient and branching ratios are in good agreement with a recent study performed by Groß et al. (2014b); taken together, these two studies show that the rate of OH regeneration through Reaction (R5) is more rapid than previously thought. GEOS-Chem has been used to assess the implications of the revised rate coefficients and branching ratios; the modelling shows an enhancement of up to 5 % in OH concentrations in tropical rainforest areas and increases of up to 10 % at altitudes of 6-8 km above the equator, compared to calculations based on the IUPAC recommended rate coefficient and yield. The enhanced rate of acetylperoxy consumption significantly reduces PAN in remote regions (up to 30 %) with commensurate reductions in background NOx.
A wide range of chemical and physical parameters has been observed over the course of a severe Europe-wide air pollution episode in August 2003. Detailed surface observations made at the rural ...perimeter edge of London, UK, indicated significantly elevated levels of primary volatile organic compounds (VOCs), ozone (>110
ppbV), other photochemical by-products such as PAN, HCHO, and oxygenates but not NO
x
. Reactive tracers in combination with Doppler wind radar and back trajectories have been used to establish that initial morning rises in O
3 during the episode were caused by entrainment of air from aloft, polluted on regional scales from mainland Europe. Total VOC reactivity to OH approximately doubled during this episode, with similar distribution between functional groups, but showing a temperature dependent contribution from biogenic isoprene (maximum 1.2
ppbv). Under episodic conditions total peroxy radicals in excess of 60
pptv were observed in late afternoon with strong correlation to a later and higher peak in ozone. Episodic alkyl peroxy radicals were calculated to be substantially influenced by peroxy acetyl, via the thermal dissociation of PAN at high temperatures.
The atmospheric processing of (E)- and (Z)-1,2-dichlorohexafluoro-cyclobutane (1,2-c-C4F6Cl2, R-316c) was examined in this work as the ozone depleting (ODP) and global warming (GWP) potentials of ...this proposed replacement compound are presently unknown. The predominant atmospheric loss processes and infrared absorption spectra of the R-316c isomers were measured to provide a basis to evaluate their atmospheric lifetimes and, thus, ODPs and GWPs. UV absorption spectra were measured between 184.95 to 230 nm at temperatures between 214 and 296 K and a parametrization for use in atmospheric modeling is presented. The Cl atom quantum yield in the 193 nm photolysis of R-316c was measured to be 1.90 ± 0.27. Hexafluorocyclobutene (c-C4F6) was determined to be a photolysis co-product with molar yields of 0.7 and 1.0 (±10%) for (E)- and (Z)-R-316c, respectively. The 296 K total rate coefficient for the O(1D) + R-316c reaction, i.e., O(1D) loss, was measured to be (1.56 ± 0.11) × 10–10 cm3 molecule–1 s–1 and the reactive rate coefficient, i.e., R-316c loss, was measured to be (1.36 ± 0.20) × 10–10 cm3 molecule–1 s–1 corresponding to a ∼88% reactive yield. Rate coefficient upper-limits for the OH and O3 reaction with R-316c were determined to be <2.3 × 10–17 and <2.0 × 10–22 cm3 molecule–1 s–1, respectively, at 296 K. The quoted uncertainty limits are 2σ and include estimated systematic errors. Local and global annually averaged lifetimes for the (E)- and (Z)-R-316c isomers were calculated using a 2-D atmospheric model to be 74.6 ± 3 and 114.1 ± 10 years, respectively, where the estimated uncertainties are due solely to the uncertainty in the UV absorption spectra. Stratospheric photolysis is the predominant atmospheric loss process for both isomers with the O(1D) reaction making a minor, ∼2% for the (E) isomer and 7% for the (Z) isomer, contribution to the total atmospheric loss. Ozone depletion potentials for (E)- and (Z)-R-316c were calculated using the 2-D model to be 0.46 and 0.54, respectively. Infrared absorption spectra for (E)- and (Z)-R-316c were measured at 296 K and used to estimate their radiative efficiencies (REs) and GWPs; 100-year time-horizon GWPs of 4160 and 5400 were obtained for (E)- and (Z)-R-316c, respectively. Both isomers of R-316c are shown in this work to be long-lived ozone depleting substances and potent greenhouse gases.
Measurements of OH reactivity were made at the Weybourne Atmospheric Observatory on the North Norfolk coast, UK in May 2004. A wide range of supporting species was also measured concurrently as part ...of the TORCH-2 field campaign, allowing a detailed study of the OH oxidation chemistry to be carried out. Measurements were made in a variety of air masses, with the 3 most prevalent being air from the Atlantic that arrived at the site from over mainland UK in a South Westerly direction, and much cleaner Northerly air that originated over the far North Sea or Arctic, passed over the North Sea and arrived at the site from a North/North Easterly direction. Direct OH reactivity measurements were made on 6 days during the campaign and with influence of 2 of the 3 air masses prevalent during the study period. The average, minimum and maximum measured OH reactivity are: 4.9, 1.3 and 9.7 respectively. The measured OH reactivity was compared to key OH sinks such as NO₂ and CO and a general positive correlation was observed. OH reactivity (k′) was then calculated using the full range of OH sinks species that were measured (including >30 NMHCs) and their pseudo first order rate constants for reaction with OH. For much of the measurement period there is a significant difference between the measured and calculated k′, with an average value of k′meas- k′calc = 1.9 s⁻¹, indicative of unmeasured OH sinks. A zero-dimensional box model containing a subset of the Master Chemical Mechanism was used to calculate the OH reactivity more accurately. The simultaneously measured trace species were used as inputs to the model and their oxidative degradation was described by a chemical mechanism containing ~5,000 species. The extra OH sinks species produced by the model, resulted in an improvement in the agreement between k′meas and k′calc, however the averaged missing OH reactivity across the entire measurement period remained at 1.4 s⁻¹. Speculation is made as to the source of this missing reactivity, including reference to studies showing that a potentially large number of high molecular weight aromatic species could be unmeasured by standard instrumentation.
Absolute quantum yields for the radical (H + HCO) channel of HCHO photolysis, ΦHCO, have been measured for the tropospherically relevant range of wavelengths (λ) between 300 and 330 nm. The HCO ...photoproduct was directly detected by using a custom-built, combined ultra-violet (UV) absorption and cavity ring down (CRD) detection spectrometer. This instrument was previously employed for high-resolution (spectral resolution ∼0.0035 nm) measurements of absorption cross-sections of HCHO, σHCHO(λ), and relative HCO quantum yields. Absolute ΦHCO values were measured at seven wavelengths, λ = 303.70, 305.13, 308.87, 314.31, 320.67, 325.59, and 329.51 nm, using an independent calibration technique based on the simultaneous UV photolysis of HCHO and Cl2. These ΦHCO measurements display greater variability as a function of wavelength than the current NASA-JPL recommendations for ΦHCO. The absolute ΦHCO(λ) determinations and previously measured σHCHO(λ) were used to scale an extensive set of relative HCO yield measurements. The outcome of this procedure is a full suite of data for the product of the absolute radical quantum yield and HCHO absorption cross-section, ΦHCO(λ)σHCHO(λ), at wavelengths from 302.6 to 331.0 nm with a wavelength resolution of 0.005 nm. This product of photochemical parameters is combined with high-resolution solar photon flux data to calculate the integrated photolysis rate of HCHO to the radical (H + HCO) channel, J(HCO). Comparison with the latest NASA-JPL recommendations, reported at 1 nm wavelength resolution, suggests an increased J(HCO) of 25% at 0° solar zenith angle (SZA) increasing to 33% at high SZA (80°). The differences in the calculated photolysis rate compared with the current HCHO data arise, in part, from the higher wavelength resolution of the current data set and highlight the importance of using high-resolution spectroscopic techniques to achieve a complete and accurate picture of HCHO photodissociation processes. All experimental ΦHCO(λ)σHCHO(λ) data are available for the wavelength range 302.6−331.0 nm (at 294 and 245 K and under 200 Torr of N2 bath gas) as Supporting Information with wavelength resolutions of 0.005, 0.1, and 1.0 nm. Equivalent data sets of ΦH2+CO(λ)σHCHO(λ) for the molecular (H2 + CO) photofragmentation channel, produced using the measured ΦHCO(λ) σHCHO(τ) values, are also provided at 0.1 and 1.0 nm resolution.
Alkyl nitrates (C
1–C
5) were measured at two sites (near urban and rural) in southeast England during the Tropospheric Organic Chemistry Experiment (TORCH). Methyl nitrate was the dominant species ...during both campaigns accounting for on average about one third of the total measured alkyl nitrates. High mixing ratios (>50 pptv) and variability of methyl nitrate were observed at the near urban site (TORCH1) that were not seen at the rural site (TORCH2) and which could not be explained by local photochemical production or direct emissions. The diurnal variation of methyl nitrate during TORCH1 showed a morning maximum that would be consistent with nighttime chemistry followed by transport to the surface by boundary layer dynamics. Similarly, elevated morning mixing ratios were also observed during TORCH2 although the magnitudes were much smaller. As a result, methyl nitrate could represent a tracer for nighttime chemistry seen at the ground the following day. At both campaigns, the dominant source of short chain alkyl nitrates and carbonyl precursor radicals (≤C
4) were from decomposition of larger compounds. The magnitude of the source increased with decreasing carbon number consistent with increasing total precursor abundance. Non-photochemical emissions of acetaldehyde and acetone could not be accounted for by automobile exhaust emissions alone and indicated that other direct sources are likely important in this environment.
The lysyl oxidase family represents a promising target in stromal targeting of solid tumors due to the importance of this family in crosslinking and stabilizing fibrillar collagens and its known role ...in tumor desmoplasia. Using small-molecule drug-design approaches, we generated and validated PXS-5505, a first-in-class highly selective and potent pan-lysyl oxidase inhibitor. We demonstrate in vitro and in vivo that pan-lysyl oxidase inhibition decreases chemotherapy-induced pancreatic tumor desmoplasia and stiffness, reduces cancer cell invasion and metastasis, improves tumor perfusion and enhances the efficacy of chemotherapy in the autochthonous genetically engineered KPC model, while also demonstrating antifibrotic effects in human patient-derived xenograft models of pancreatic cancer. PXS-5505 is orally bioavailable, safe and effective at inhibiting lysyl oxidase activity in tissues. Our findings present the rationale for progression of a pan-lysyl oxidase inhibitor aimed at eliciting a reduction in stromal matrix to potentiate chemotherapy in pancreatic ductal adenocarcinoma.
The reaction CH3C(O)O2 + HO2 → CH3C(O)OOH + O2 (Reaction R5a), CH3C(O)OH + O3 (Reaction R5b), CH3 + CO2 + OH + O2 (Reaction R5c) was studied in a series of experiments conducted at 1000 mbar and ...(293 ± 2) K in the HIRAC simulation chamber. For the first time, products, (CH3C(O)OOH, CH3C(O)OH, O3 and OH) from all three branching pathways of the reaction have been detected directly and simultaneously. Measurements of radical precursors (CH3OH, CH3CHO), HO2 and some secondary products HCHO and HCOOH further constrained the system. Fitting a comprehensive model to the experimental data, obtained over a range of conditions, determined the branching ratios α(R5a) = 0.37 ± 0.10, α(R5b) = 0.12 ± 0.04 and α(R5c) = 0.51 ± 0.12 (errors at 2σ level). Improved measurement/model agreement was achieved using k(R5) = (2.4 ± 0.4) × 10−11 cm3 molecule−1 s−1, which is within the large uncertainty of the current IUPAC and JPL recommended rate coefficients for the title reaction. The rate coefficient and branching ratios are in good agreement with a recent study performed by Groß et al. (2014b); taken together, these two studies show that the rate of OH regeneration through Reaction (R5) is more rapid than previously thought. GEOS-Chem has been used to assess the implications of the revised rate coefficients and branching ratios; the modelling shows an enhancement of up to 5 % in OH concentrations in tropical rainforest areas and increases of up to 10 % at altitudes of 6–8 km above the equator, compared to calculations based on the IUPAC recommended rate coefficient and yield. The enhanced rate of acetylperoxy consumption significantly reduces PAN in remote regions (up to 30 %) with commensurate reductions in background NOx.
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