Theoretical and experimental study of the OH radical reaction with HCN

Atmospheric chemistry of CF3CN: kinetics and products of reaction with OH radicals, Cl atoms and O3

Long path length FTIR-smog chamber techniques were used to study the title reactions in 700 Torr of N₂, oxygen or air diluent at 296 ± 2 K. Values of k(Cl + CF₃CN) = (2.43 ± 0.33) × 10^-15 and k(OH + CF₃CN) = (4.61 ± 0.34) × 10^-15 cm³ molecule^-1 s^-1 were measured. There was no discernible reaction of CF₃CN with O₃ and an upper limit of k(O₃ + CF₃CN) ≤ 7.9 × 10^-24 cm³ molecule^-1 s^-1 was established. The IR spectra of CF₃CN and CF₃CF₂CN are reported. The atmospheric lifetime of CF₃CN is determined by the reaction with OH and is approximately 6.9 years. Reaction of CF₃CN with Cl atoms in a chamber study gives (Z-) and/or (E-) CF₃CClNCl and CF₃C(O)Cl as major primary products. Under environmental conditions, the OH radical initiated oxidation gives COF₂ in a yield of (96 ± 8)%. The global warming potential for CF₃CN is estimated as 1030 for a 100 year time horizon.

Extension of a gaseous dry deposition algorithm to oxidized volatile organic compounds and hydrogen cyanide for application in chemistry transport models

The dry deposition process refers to flux loss of an atmospheric pollutant due to uptake of the pollutant by the Earth's surfaces, including vegetation, underlying soil, and any other surface types. In chemistry transport models (CTMs), the dry deposition flux of a chemical species is typically calculated as the product of its surface layer concentration and its dry deposition velocity (V _d); the latter is a variable that needs to be highly empirically parameterized due to too many meteorological, biological, and chemical factors affecting this process. The gaseous dry deposition scheme of Zhang et al. (2003) parameterizes V _d for 31 inorganic and organic gaseous species. The present study extends the scheme of Zhang et al. (2003) to include an additional 12 oxidized volatile organic compounds (oVOCs) and hydrogen cyanide (HCN), while keeping the original model structure and formulas, to meet the demand of CTMs with increasing complexity. Model parameters for these additional chemical species are empirically chosen based on their physicochemical properties, namely the effective Henry's law constants and oxidizing capacities. Modeled V _d values are compared against field flux measurements over a mixed forest in the southeastern US during June 2013. The model captures the basic features of the diel cycles of the observed V _d. Modeled V _d values are comparable to the measurements for most of the oVOCs at night. However, modeled V _d values are mostly around 1 cm s^-1 during daytime, which is much smaller than the observed daytime maxima of 2-5 cm s^-1. Analysis of the individual resistance terms and uptake pathways suggests that flux divergence due to fast atmospheric chemical reactions near the canopy was likely the main cause of the large model-measurement discrepancies during daytime. The extended dry deposition scheme likely provides conservative V _d values for many oVOCs. While higher V _d values and bidirectional fluxes can be simulated by coupling key atmospheric chemical processes into the dry deposition scheme, we suggest that more experimental evidence of high oVOC V _d values at additional sites is required to confirm the broader applicability of the high values studied here. The underlying processes leading to high measured oVOC V _d values require further investigation.

Atmospheric Chemistry of (CF3)2CF-C≡N: A Replacement Compound for the Most Potent Industrial Greenhouse Gas, SF6

FTIR/smog chamber experiments and ab initio quantum calculations were performed to investigate the atmospheric chemistry of (CF₃)₂CFCN, a proposed replacement compound for the industrially important sulfur hexafluoride, SF₆. The present study determined k(Cl + (CF₃)₂CFCN) = (2.33 ± 0.87) × 10^-17, k(OH + (CF₃)₂CFCN) = (1.45 ± 0.25) × 10^-15, and k(O₃ + (CF₃)₂CFCN) ≤ 6 × 10^-24 cm³ molecule^-1 s^-1, respectively, in 700 Torr of N₂ or air diluent at 296 ± 2 K. The main atmospheric sink for (CF₃)₂CFCN was determined to be reaction with OH radicals. Quantum chemistry calculations, supported by experimental evidence, shows that the (CF₃)₂CFCN + OH reaction proceeds via OH addition to -C(≡N), followed by O₂ addition to -C(OH)═N·, internal H-shift, and OH regeneration. The sole atmospheric degradation products of (CF₃)₂CFCN appear to be NO, COF₂, and CF₃C(O)F. The atmospheric lifetime of (CF₃)₂CFCN is approximately 22 years. The integrated cross section (650-1500 cm^-1) for (CF₃)₂CFCN is (2.22 ± 0.11) × 10^-16 cm² molecule^-1 cm^-1 which results in a radiative efficiency of 0.217 W m^-2 ppb^-1. The 100-year Global Warming Potential (GWP) for (CF₃)₂CFCN was calculated as 1490, a factor of 15 less than that of SF₆.