Nobuaki Washida

OH radical‐ initiated photooxidation of isoprene: An estimate of global CO production

The OH radical- initiated photooxidation of isoprene has been investigated experimentally by a 6-m3 photochemical reaction chamber equipped with a long path length Fourier transform infrared spectrometer. In the presence of NOx, the major primary reaction products were methyl vinyl ketone, methacrolein, and formaldehyde. Their yields were in quantitative agreement with previous measurements. In the absence of NOx, the reaction mechanism was found to be quite different from that in the presence of NOx, and major reaction products observed in the infrared spectra were attributed to organic hydroperoxides. Based on the model experiments, the ultimate yield of CO was evaluated to be 60% on the carbon number basis in the presence of NOx and 23% in the absence of NOx. The CO yield in the real atmosphere was evaluated as 30% on the carbon number basis, and global annual CO production from isoprene was estimated to be 105 Tg C yr−1. Together with a previous estimate of the CO production from terpenes, global CO production from natural hydrocarbons was evaluated to be 200±60 Tg C yr−1.

OH-initiated oxidation of benzene

The present work represents a continuation of part I of this series of papers, in which we investigated the phenol yields in the OH-initiated oxidation of benzene under conditions of low to moderate concentrations of NOx, to elevated NOx levels. The products of the OH-initiated oxidation of benzene in 700–760 Torr of N2/O2 diluent at 297u2006±u20064 K were investigated in 3 different photochemical reaction chambers. In situ spectroscopic techniques were employed for the detection of products, and the initial concentrations of benzene, NOx, and O2 were widely varied (by factors of 6300, 1500, and 13, respectively). In contrast to results from previous studies, a pronounced dependence of the product distribution on the NOx concentration was observed. The phenol yield decreases from approximately 50–60% in the presence of low concentrations ( 10u2006000 ppb) NOx concentrations. In the presence of high concentrations of NOx, the phenol yield increases with increasing O2 partial pressure. The rate constant of the reaction of hydroxycyclohexadienyl peroxyl radicals with NO was determined to be (1.7u2006±u20060.6)u2006×u200610−11 cm3 molecule−1 s−1. This reaction leads to the formation of E,E-2,4-hexadienedial as the main identifiable product (29u2006±u200616%). The reaction of the hydroxycyclohexadienyl radical with NO2 gave phenol (5.9u2006±u20063.4%) and E,E-2,4-hexadienedial (3.4u2006±u20061.9%), no other products could be identified. The residual FTIR product spectra indicate the formation of unknown nitrates or other nitrogen-containing species in high yield. The results from the present work also show that experimental studies aimed at establishing/verifying chemical mechanisms for aromatic hydrocarbons must be performed using NOx levels which are representative of those found in the atmosphere.

Laser-induced fluorescence of the CHXCFO (X=F,Cl) radicals

Laser-induced fluorescence spectra of the B←X transition for three new halogen substituted vinoxy radicals were observed in the 306–333 nm region. The spectra of the trans- and cis-CHFCFO radicals were observed in the reactions of CHFCHF and CHFCF2 with atomic oxygen, and the spectrum of the cis-CHClCFO radical was observed in the reactions of CHClCHF and CHClCF2 with atomic oxygen. From an analysis of the laser-induced single vibronic level fluorescence, some of the vibrational frequencies could be determined for the ground electronic states (X); these frequencies are listed in Tables III and V. Some of the vibrational frequencies for the excited state (B) were also determined. These vibrational assignments are supported by ab initio calculations. The experimental results show that the electronic B←X transition energy is increased by halogen atoms, especially by fluorine atoms at either the 1- or 2-positions. The C–C–O skeleton and the spectroscopic characteristics of trans-, cis-CHFCFO, and cis-CH...

Laser-induced fluorescence of radicals produced in reactions of halogenated ethylenes with atomic oxygen

Three new laser-induced fluorescence spectra have been observed when reacting oxygen atoms with halogenated ethylenes such as CHFCF2,u2009CF2CHCl,u2009CH2CCl2,u2009CH2CHCl, and CHClCHCl. These new spectra are similar to those reported previously for the vinoxy CH2CHO and CH2CFO radicals, which suggests that they can be assigned to other halogenated vinoxy-type radicals. Recently Williams and Fleming assigned one of these LIF spectra to the FCO radical; additional experiments have been done to address this conflicting assignment.

Measurements of Rate Constants for HO2

Absolute rate constants for HO2 + NO and NH2 + NO reactions were measured by a photoionization mass spectrometry coupled with a laser flash photolysis. HO2 and NH2 radicals were photoionized by an Ar resonance lamp and were detected as their parent ions ( HO 2 + and NH 2 + ). The rate constants were determined to be K ( HO 2 + NO ) = ( 6.5 ± 2.0 ) 10 − 12 cm 3 molecule − 1 s − 1 K ( NH 2 + NO ) = ( 1.9 ± 0.3 ) 10 − 11 cm 3 molecule − 1 s − 1 Both rates are consistent with those previously reported.