A new chemical kinetics mechanism for simulating urban and regional photochemistry has been developed and evaluated. The mechanism, called the Carbon Bond Mechanism IV (CBM‐IV), was derived by condensing a detailed mechanism that included the most recent kinetic, mechanistic, and photolytic information. The CBM‐IV contains extensive improvements to earlier carbon bond mechanisms in the chemical representations of aromatics, biogenic hydrocarbons, peroxyacetyl nitrates, and formaldehyde. The performance of the CBM‐IV was evaluated against data from 170 experiments conducted in three different smog chambers. These experiments included NOx‐air irradiations of individual organic compounds as well as a number of simple and complex organic mixtures. The results of the evaluation indicate substantial improvement in the ability of the CBM‐IV to simulate aromatic and isoprene systems with average overcalculation of ozone concentrations of 1% for the aromatic simulations and 6% for the isoprene simulations. The mechanism also performed well in simulating organic mixture experiments. Maximum ozone concentrations calculated for 68 of these experiments were approximately 2% greater than the observed values while formaldehyde values were low by 9%.

Dark-phase experiments between isoprene and 0 3 are discussed. UNC outdoor chamber experiments have shown that in high-concentration systems of isoprene and 0 3 (5 ppm C and 1 ppm) approximately 75% of the reacted carbon can be observed in the product formation of HCHO, CO, methacrolein, methylvinylketone, methylglyoxal, acetaldehyde, and propylene.Mechanisms were developed which gave reasonable fits to dark-phase chamber experiments of MACR, MVK, isoprene, and 03. Experimental data and modeling results were used to generate 0 3 rates of attack on MVK and MACR. An isoprene-03 rate of 1.67 X ppm-l-rnin-l was used and is consistent with other rates reported in the literature. Dark isoprene-03 systems appear to form homogeneously nucleated aerosol. Most of these particles appear and remain a t diameters well below the optical cutoff region (0.3-0.5 Fm), as opposed to the particles from similar a-pinene-03 systems, which also form at smaller sizes hut then grow into the optical size range (>0.5 Fm). Lower concentrations of a-pinene and O3 (0.2 ppm C and 0.12 ppm) still generated substantial aerosol, hut by comparison, rapid CN nucleation was not observed during a similar side-by-side system of isoprene and 03.

A continuous stirred tank reactor (CSTR) was used to study the gas-phase reaction between H O and toluene. HO' was generated by the in situ photolysis of nitrous acid.Flow reactor operation at steady-state conditions with a residence time of 20 min allowed investigation of primary and very rapid secondary reactions. CSTR and batch reactor experiments were also performed with selected products. Both gas-phase and aerosol products were identified by chromatography and mass spectroscopy, with total product yields between 55 and 75% of reacted carbon. Toluene reaction products included cresols, nitrocresols, nitrotoluenes, 3,5-dinitrotouluene, benzaldehyde, benzyl nitrate, nitrophenols, methyl-p-benzoquinone, glyoxal, methylglyoxal, formaldehyde, methyl nitrate, PAN, and CO. The fraction of HO' methyl hydrogen abstraction was calculated to be 0.13 2 0.04. The ratio of reaction rate constants for nitrotoluene versus cresol formation from the HO-adduct was calculated to be about 3.3 x lo4. Also, the ratio of cresol formation versus 0, addition to the HO-adduct was estimated to be 20.5 for atmospheric conditions. Comparisons of these measurements with previous values and the implications with respect to photochemical kinetics modeling of the atmosphere are discussed.

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