A photochemical reactor for studies of atmospheric chemistry

A photochemical reactor for studies of atmospheric kinetics and spectroscopy has been built at the Copenhagen Center for Atmospheric Research. The reactor consists of a vacuum FTIR spectrometer coupled to a 100 L quartz cylinder by multipass optics mounted on electropolished stainless steel end flanges, surrounded by UV-A, UV-C and broadband sun lamps in a temperature-controlled housing. The combination of a quartz vessel and UV-C lamps allows higher concentrations of O(1D) and OH than can be generated by similar chambers. The reactor is able to produce radical concentrations of ca. 8 × 1011 cm-3 for OH, 3 × 106 cm-3 for O(1D), 3.3 × 1010 cm-3 for O(3P) and 1.6 × 1012 cm-3 for Cl. The reactor can be operated at pressures from 10-3 to 103 mbar and temperatures from 240 to 330 K. As a test of the system we have studied the reaction CHCl3 + Cl using the relative rate technique and find kCHCl3+Cl/kCH4+Cl = 1.03 ± 0.11, in good agreement with the accepted value

Isotope Effects in the Reactions of Chloroform Isotopologues with Cl, OH and OD

The kinetic isotope effects in the reactions of CHCl3, CDCl 3, and 13CHCl3 with Cl, OH, and OD radicals have been determined in relative rate experiments at 298 ± 1 K and atmospheric pressure monitored by long path FTIR spectroscopy. The spectra were analyzed using a nonlinear least-squares spectral fitting procedure including line data from the HITRAN database and measured infrared spectra as references. The following relative reaction rates were determined: kCHCl 3+ Cl/kCDCl 3+Cl = 3.28 ± 0.01, k CHCl 3+Cl/k13CHCl 3+Cl = 1.000 ± 0.003, kCHCl 3+OH/kCDCl 3+ OH = 3.73 ± 0.02, kCHCl 3+OH/k 13CHCl 3+OH = 1.023 ± 0.002, k CHCl 3+OD/kCDCl 3+OD = 3.95 ± 0.03, and kCHCl 3+OD/k13 CHCl 3+OD = 1.032 ± 0.004. Larger isotope effects in the OH reactions than in the CI reactions are opposite to the trends for CH4 and CH3C1 reported in the literature. The origin of these differences was investigated using electronic structure calculations performed at the MP2/aug-cc-PVXZ (X = D, T, Q) level of theory and are compared with previously calculated values for the other methane derivatives. The Born-Oppenheimer barrier heights to H abstraction are 12.2 and 17.0 kJ mol -1 at the CCSD(T)/aug-cc-pVTZ level of theory for OH and CI, respectively. The reaction rate coefficients of the two elementary vapor phase reactions including the 2H and 13C kinetic isotope effects were calculated using improved canonical variational theory with small curvature tunneling (ICVT/SCT) and the results compared with experimental data

Pressure dependence of the deuterium isotope effect in the photolysis of formaldehyde by ultraviolet light

The pressure dependence of the relative photolysis rate of HCHO vs. HCDO has been investigated for the first time, using a photochemical reactor at the University of Copenhagen. The dissociation of HCHO vs. HCDO using a UVA lamp was measured at total bath gas pressures of 50, 200, 400, 600 and 1030 mbar. The products of formaldehyde photodissociation are either H<sub>2</sub> + CO (molecular channel) or HCO + H (radical channel), and a photolysis lamp was chosen to emit light at wavelengths that greatly favor the molecular channel. The isotope effect in the dissociation, <i>k</i><sub>HCHO</sub>/<i>k</i><sub>HCDO</sub>, was found to depend strongly on pressure, varying from 1.1 + 0.15/−0.1 at 50 mbar to 1.75±0.10 at 1030 mbar. The results can be corrected for radical channel contribution to yield the kinetic isotope effect for the molecular channel; i.e. the KIE in the production of molecular hydrogen. This is done and the results at 1030 mbar are discussed in relation to previous studies at ambient pressure. In the atmosphere the relative importance of the two product channels changes with altitude as a result of changes in pressure and actinic flux. The study demonstrates that the δD of photochemical hydrogen produced from formaldehyde will increase substantially as pressure decreases

Gas-phase advanced oxidation for effective, efficient in situ control of pollution

In this article, gas-phase advanced oxidation, a new method for pollution control building on the photo-oxidation and particle formation chemistry occurring in the atmosphere, is introduced and characterized. The process uses ozone and UV-C light to produce in situ radicals to oxidize pollution, generating particles that are removed by a filter; ozone is removed using a MnO2 honeycomb catalyst. This combination of in situ processes removes a wide range of pollutants with a comparatively low specific energy input. Two proof-of-concept devices were built to test and optimize the process. The laboratory prototype was built of standard ventilation duct and could treat up to 850 m3/h. A portable continuous-flow prototype built in an aluminum flight case was able to treat 46 m3/h. Removal efficiencies of >95% were observed for propane, cyclohexane, benzene, isoprene, aerosol particle mass, and ozone for concentrations in the range of 0.4-6 ppm and exposure times up to 0.5 min. The laboratory prototype generated a OH * concentration derived from propane reaction of (2.5 ± 0.3) × 1010 cm-3 at a specific energy input of 3 kJ/m3, and the portable device generated (4.6 ± 0.4) × 109 cm-3 at 10 kJ/m3. Based on these results, in situ gas-phase advanced oxidation is a viable control strategy for most volatile organic compounds, specifically those with a OH* reaction rate higher than ca. 5 × 10-13 cm3/s. Gas-phase advanced oxidation is able to remove compounds that react with OH and to control ozone and total particulate mass. Secondary pollution including formaldehyde and ultrafine particles might be generated, depending on the composition of the primary pollution

Atmospheric Chemistry of cis-CF3CH=CHF: Kinetics of reactions with OH radicals and O3 and products of OH radical initiated oxidation

Long path length FTIR-smog chamber techniques were used to measure k(OH + cis-CF3CHCHF) = (1.20 ± 0.14) × 10−12 and k(O3 + cis-CF3CHCHF) = (1.65 ± 0.16) × 10−21 cm3 molecule −1 s−1 in 700 Torr of N2/O2 diluent at 296 K. The OH initiated oxidation of cis-CF3CHCHF gives CF3CHO and HCOF in molar yields which are indistinguishable from 100%. The atmospheric lifetime of cis-CF3CHCHF is determined by its reaction with OH and is approximately 10 days. cis-CF3CHCHF has an integrated IR absorption cross section (600–2000 cm−1) of (1.71 ± 0.09) × 10−16 cm molecule−1 and a global warming potential of approximately 3 (100 year time horizon). Quoted uncertainties reflect two standard deviations from least squares regression analyses