G. W. Harris

Carbon kinetic isotope effect in the reaction of CH4 with Cl atoms

The carbon kinetic isotope effect in the reaction between Cl and CH4 (KIECl) has been measured using tunable diode laser absorption spectroscopy to determine 13CH4/12CH4 ratios. Cl atoms were generated by the irradition of Cl2 in static mixtures of Cl2/CH4/N2 or Cl2/CH4/N2/O2. Both methods resulted in a (KIECl) of 1.066±0.002 at 297 K. The KIECl displayed a slight temperature dependence, increasing to 1.075±0.005 at 223 K. This result suggests a significant influence of the title reaction on the stratospheric CH4 isotopic composition and may help to resolve discrepancies between measurements of stratospheric 13CH4/12CH4 profiles and laboratory measurements of KIEOH.

Modelling the chemistry of ozone, halogen compounds, and hydrocarbons in the arctic troposphere during spring

The box model Moccalce has been developed to study the chemistry of the arctic boundary layer. It treats chemical reactions in the gas phase and in the aerosol, as well as exchange between the 2 phases. Photolysis rates vary according to the solar declination during polar sunrise. Apart from the standard tropospheric chemistry of ozone, hydrocarbons, and nitrogen species, the reaction mechanism includes sulfur and the halogens Cl, Br, and I. Modeling an ozone depletion event, we found that iodine species contribute to the chemical destruction of ozone significantly if 10 mixing ratios are about 1 pmol/mol. The reactions of BrO with Bra and 10 are the main pathways of the ozone destruction cycle. Hydrocarbon concentrations decrease during ozone depletion events due to reaction with halogen atoms. The rate of ozone destruction depends on whether the addition of Br to C2H4 and C2 H2 yields inert products or intermediates from which Br can be regenerated. Bromine and HCHO are positively correlated. The model produces HCHO during ozone depletion events, though not as much as reported from field observations. After the destruction of ozone has been competed, the halogen species are converted to halides and subsequently scavenged by aerosol particles.

Carbon kinetic isotope effect in the reaction CH4+Cl: a relative rate study using FTIR spectroscopy

Abstract The 12 C / 13 C kinetic isotope effect in the reaction of methane with chlorine atoms (KIECl12C/13C) was investigated at 298 K using photolysis of static Cl2/CH4/N2 mixtures, with in-situ analysis of isotopic composition by FTIR absorption spectroscopy. Least squares fitting of composite 12 CH 4 / 13 CH 4 spectra to reference spectra obtained at the same temperature, pressure and resolution enabled the relative 12 CH 4 : 13 CH 4 content to be measured at a high precision. The result obtained was KIECl12C/13C(298 K)=1.066±0.002 (2σ), in excellent agreement with the single previous experimental determination at this temperature. This result confirms the unexpectedly high fractionation in the title reaction.

D/H kinetic isotope effect in the reaction CH4+Cl

The D/H kinetic isotope effect (KIEClD) in the reaction of CH4 with Cl has been investigated using a tunable diode laser absorption spectrometer (TDLAS) to measure 12CH3D/12CH4 ratios. A KIEClD of 1.508±0.041 was obtained at room temperature (296 K). Experiments employing Fourier transform infrared measurements of 12CH3D/12CH4 ratios confirmed this result. An investigation of the temperature dependence showed an increase of KIEClD (1.592±0.057 at 223 K) with decreasing temperature. This very large D/H fractionation effect affects the overall isotope fractionation in the middle and upper stratosphere, where Cl contributes substantially to the total removal of CH4.

Fast Response tunable diode laser spectroscopy for trace gas flux measurements

A fast tunable diode laser spectrometer was applied for measurements of N 2 O emissions from a harvested wheat field located in Sealand, Denmark. The dual-channel instrument uses two tone frequency modulation and signal detection at 11 MHz in conjunction with rapid (1 ms) scanning of the laser. A dichroic beam combiner and a mechanical chopper allow time multiplexing. The absorption signals are recorded and analysed on-line. The combination of a software, current and temperature controlled line locking scheme results in high stability of the instrument. The time response (200 ms) of the spectrometer is limited by the measurement-cell gas exchange. For eddy correlation measurements, the trace gas mixing ratio and wind data were sampled synchronously at a rate of 10 Hz using an interrupt-driven algorithm. Mean N 2 O flux detection limits were 6 pptv m s -1 , corresponding to a deposition velocity of 2 x 10 -5 m s -1 for a half hour measurement time. In addition, concentration gradients were determined from measurements at four different inlet heights at 0.1 ppbv precision in a shared integration time of 30 min. These measurements resulted in a flux detection limit of 7 pptv m s -1 for the boundary layer mixing conditions encountered.

FTIR study of the Cl- and Br-atom initiated oxidation of trichloroethylene

The Cl- and Br- initiated oxidations of CHCl(DOUBLEBOND)CCl2 in 700 torr of air at 296 K have been studied using a Fourier transform infrared spectrometer. Rate constants k(Cl+CHCl(DOUBLEBOND)CCl2)=(7.2±0.8)×10−11 and k(Br+CHCl(DOUBLEBOND)CCl2)=(1.1±0.4)×10−13 cm3 molecule−1 s−1 were determined using a relative rate technique with ethane and ethylene as references, respectively. The major products observed were CHXClC(O)Cl, (X=Cl or Br), CHClO, and CCl2O. Combining results obtained for the Cl-initiated oxidation of CHCl2(SINGLEBOND)CHCl2, we deduced that Cl-addition on trichloroethylene occurs via channel 1a, Cl+CHCl(DOUBLEBOND)CCl2 CHCl2(SINGLEBOND)CCl2, (100±12)%. Self-reaction of the subsequently generated peroxy radicals CHCl2(SINGLEBOND)CCl2O2 leads to CHCl2CCl2O radicals which were found to decompose via channel 8a, CHCl2C(O)Cl+Cl, (91±11)% of the time, and channel 8b, CHCl2+CCl2O, (9±2)%. The reaction Br+CHCl(DOUBLEBOND)CCl2CHBrCl(SINGLEBOND)CCl2 (17a) accounted for ≥(96±11)% of the total reaction. Decomposition of the CHBrCl(SINGLEBOND)CCl2O radicals proceeds (≥93±11)% via CHBrClC(O)Cl+Cl. As part of this work, k(Cl+CHCl2C(O)Cl)=(3.6±0.6)×10−14 and k(Cl+CHCl2(SINGLEBOND)CHCl2)=(1.9±0.2)×10−13 cm3 molecule−1 s−1 were measured. Errors reported above include statistical uncertainties (2σ) and estimated systematic uncertainties.

The TDLAS instrument for the detection of total inorganic chlorine in the stratosphere

A novel method based on tunable diode laser absorption spectroscopy is proposed for the detection of the sum of the stratospheric chlorine components HCl, ClONO2, ClOOCl and ClO. All of the above species are detected as HCl following thermal decomposition (ClONO2 and ClOOCl) to ClO which is converted to Cl via reaction with NO; the Cl atom is then rapidly reacted with a hydrocarbon to form HCl. The method has been investigated in the laboratory for detection of ClONO2, for which the conversion efficiency was found to be 100% under suitable conditions.

Development and application of multilaser TDLAS instruments for ground-based, shipboard, and airborne measurements of trace gas species in the atmosphere

Tunable diode laser absorption spectroscopy (TDLAS) meets the major requirements for atmospheric trace gas monitoring, which are sub-ppbv sensitivity, high detection speed and the potential for simultaneous in-situ measurements of several compounds. In recent years, several multi-laser TDLAS systems have been developed at the Max Planck Institute for Chemistry and used in a number of ground based, shipboard and airborne field campaigns to measure the concentrations of atmospheric trace species, e.g. N2O, CH4, CO, HCHO, H2O2 and NO2, from the boundary layer up to the lower stratosphere at 14 km altitude. During these field measurement on various platforms, detailed comparisons of TDLAS with other techniques have been performed for CO, HCHO, and NO2, yielding an agreement between the various instruments on the order of 10-20 percent. In addition, a TDLAS instrument has been used to measure N2O fluxes from soils by eddy correlation and flux gradient techniques. A particular TDLAS instrument has been developed, which is capable of high-precision direct measurements of 13CH4/12CH4 and 12CH3D/12CH4 ratios. An intercomparison between this instrument and conventional mass spectrometry yielded a mean deviation of (delta) 13C equals 0.5 percent and (delta) D equals 5 percent.