Transient absorption spectra and decay profiles of HO2 have been measured using cw near-IR two-tone frequency modulation absorption spectroscopy at 297 K and 50 Torr in diluent of N2 in the presence of water. From the depletion of the HO2 absorption peak area following the addition of water, the equilibrium constant of the reaction HO2 + H2O <--> HO2-H2O was determined to be K2 = (5.2 +/- 3.2) x 10(-19) cm3 molecule(-1) at 297 K. Substituting K2 into the water dependence of the HO2 decay rate, the rate coefficient of the reaction HO2 + HO2-H2O was estimated to be (1.5 +/- 0.1) x 10(-11) cm3 molecule(-1) s(-1) at 297 K and 50 Torr with N2 as the diluent. This reaction is much faster than the HO2 self-reaction without water. It is suggested that the apparent rate of the HO2 self-reaction is enhanced by the formation of the HO2-H2O complex and its subsequent reaction. Results are discussed with respect to the kinetics and atmospheric chemistry of the HO2-H2O complex. At 297 K and 50% humidity, the concentration ratio of [HO2-H2O]/[HO2] was estimated from the value of K2 to be 0.19 +/- 0.11.

[1] Complex formation between HO 2 and H 2 O and kinetics of the HO 2 -H 2 O complex were investigated using near-infrared two-tone frequency modulation spectroscopy at 250-350 K and 50 torr with N 2 diluent. From the depletion of the HO 2 signal in the presence of water, the equilibrium constant of the HO 2 -H 2 O complex formation was estimated. The stabilized energy of the HO 2 -H 2 O complex formation was estimated to be 31 ± 4 kJ mol À1 , and it was suggested that the complex has two hydrogen bonds. Using the equilibrium constant obtained from spectral measurements, the rate constant of the reaction HO 2 + HO 2 -H 2 O was estimated at 297-350 K and 50 torr with N 2 diluent. Weak dependence of the rate constant on temperature indicated that the temperature dependence of water effects on HO 2 self-reaction kinetics was mainly controlled by that of the equilibrium constant of the HO 2 -H 2 O complex formation. Altitude profiles of HO 2 -H 2 O complexes and HO 2 self-reaction rate were also estimated in the northwest Pacific Ocean. The importance of HO 2 -H 2 O complexes in the lower troposphere was suggested.

The kinetics and absolute rate constants of the gas-phase reaction of the hydroxymethyl radical (CH2OH) with molecular oxygen have been studied using laser photolysis/near-IR absorption spectroscopy. The reaction was tracked by monitoring the time-dependent changes in the production of the hydroperoxy radical (HO2) concentration. For sensitive detection of HO2, two-tone frequency modulation absorption spectroscopy was used in combination with a Herriott-type optical multipass absorption cell. Rate constants were determined as a function of temperature (236 K

Vacuum ultraviolet single-photon ionization time-of-flight mass spectrometry (VUV-SPI-TOFMS) has been applied to the detection of volatile organic compounds (VOCs), including aromatic, chlorinated, and oxygenated compounds. Photoionization mass spectra of 23 VOCs were measured using SPI-TOFMS at 10.5 eV (118 nm). The limits of detection of VOCs using SPI-TOFMS at 10.5 eV were estimated to be a few ppbv. The mass spectra of 20 VOCs exhibit only the parent ion and its isotopes' signals. The ionization processes of the VOCs were discussed on the basis of the reaction enthalpies predicted by the quantum chemical calculations. Absolute photoionization cross-sections for 23 VOCs, including 12 newly measured VOCs, at 10.5 eV were determined in comparison to the reported absolute photoionization cross-section of NO.