This chapter is a critical survey of reaction rate coefficient data important in describing high-temperature combustion of H2, CO, and small hydrocarbons up to C4. A recommended reaction mechanism and rate coefficient set is presented. The approximate temperature range for this mechanism is from 1200 to 2500 K, which therefore excludes detailed consideration of cool flames, low-temperature ignition, or reactions of organic peroxides or peroxy radicals. Low-temperature rate-coefficient data are presented, however, when they contribute to defining or understanding high-temperature rate coefficients. Because our current knowledge of reaction kinetics is incomplete, this mechanism is inadequate for very fuel-rich conditions (see Warnatz et al., 1982). For the most part, reactions are considered only when their rates may be important for modeling combustion processes. This criterion eliminates considering many reactions among minor species present at concentrations so low that reactions of these species cannot play an essential part in combustion processes. The philosophy in evaluating the rate-coefficient data was to be selective rather than exhaustive: Recent results obtained with experimental methods capable of measuring isolated elementary reaction rate parameters directly were preferred, while results obtained using computer simulations of complex reacting systems were considered only when sensitivity to a particular elementary reaction was demonstrated or when direct measurements are not available. Theoretical results were not considered.

Rate Coefficients in the C/H/O System

An analysis is made of the various reactions in which ozone and atomic oxygen are involved in the stratosphere. At the present time, hydrogen, nitrogen, and chlorine compounds in the ranges parts per million, parts per billion, and parts per trillion may have significant chemical effects. In the upper stratosphere, above the ozone peak, where there is no strong departure from photochemical equilibrium conditions, the action of hydroxyl and hydroperoxyl radicals of nitrogen dioxide and chlorine monoxide on atomic oxygen and of atomic chlorine on ozone can be introduced. A precise determination of their exact effects requires knowledge of (1) the vertical distribution of the H2O, CH4, and H2 dissociation by reaction of these molecules with electronically excited oxygen atom O(¹D); (2) the ratio of the OH and HO2 concentrations and their absolute values, which depend on insufficiently known rate coefficients; (3) the various origins of nitric oxide production, with their vertical distributions related to latitude and season; and (4) the various sources giving different chlorine compounds that may be dissociated in the stratosphere. In the lower stratosphere, below the ozone peak, there is no important photochemical production of O3, but there exist various possibilities of transport. The predictability of the action of chemical reactions depends strongly on important interactions between OH and HO2 radicals with CO and NO, respectively, which affect the ratio n(OH)/n(HO2) at the tropopause level; between OH and NO2, which lead to the formation of nitric acid with its downward transport toward the troposphere; between NO and HO2, which lead to NO2 and its subsequent photodissociation; between ClO and NO, which also lead to NO2 and become more important than the reaction of ClO with O; and between Cl and various molecules, such as CH4 and H2, which lead to HCl with its downward transport toward the troposphere. All these chemical processes are subject to many changes since they occur in the lower stratosphere where seasonal, latitudinal, and even day-to-day variations of the ozone concentrations are observed in association with advective and dynamic transports, which depend on meteorological conditions as indicated by variations of tropopause heights.

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Stratospheric Ozone: An Introduction to Its Study

Diurnal variations of number densities of oxygen, nitrogen, hydrogen, carbon, and ionic minor constituents in the mesosphere and lower thermosphere are investigated using the simultaneous solution of photochemical-diffusive equations. Special account is taken of the mutual coupling of nitrogen compounds with ions. Quantum yield of N(²D) production in the dissociative recombination of NO+ and the photodissociation of N2, which are the main sources of NO in the lower thermosphere, must be ≳0.5 in order to reproduce the NO density around 100 km deduced from γ band airglow measurements. The NO density shows no substantial diurnal variation between the heights of 75 and 110 km and shows the day-to-night variation of a factor of greater than 2 above 120 km, whereas NO changes almost completely into NO2 at night below ∼70 km. The calculated NO density is smaller than that observed in the height range of 60–80 km, although it explains the observed electron density according to currently known ion chemistry. A larger rate coefficient recently measured for the conversion of NO+ into NO+·N2, which eventually will be hydrated to NO+·H2O, can be used to adequately model the height where the dominant ion changes from hydrated ions to NO+. Disagreement of the calculated atomic oxygen density profiles with the observed ones in the lower thermosphere is discussed in connection with the eddy diffusion coefficient and the solar flux. Dynamical transport of atomic oxygen such as the general circulation may be invoked to resolve this disagreement.

Diurnal variations of odd nitrogen and ionic densities in the mesosphere and lower thermosphere: Simultaneous solution of photochemical‐diffusive equations

A dynamic model of the mesosphere and lower thermosphere

Measurements of atmospheric monoterpene hydrocarbons were made at a site in the Colorado mountains The research was undertaken to examine the influence of the compounds on the photochemistry of the troposphere A sampling technique was developed using Tenax GC porous polymer traps with analysis by capillary gas chromatography and detection by flame ionization and mass spectrometry Positive identification of six monoterpene hydrocarbons, α-pinene, camphene, β-pinene, myrcene, Δ-3-carene, and d-limonene, was obtained, as well as tentative identification of α-thujene and β-phellandrene A definite seasonal trend was evident in the average monoterpene mixing ratios The summertime average was 030 ppbv for the sum of the five major identified monoterpenes, β-pinene, α-pinene, Δ-3-carene, camphene, and d-limonene, with a high degree of constancy in relative ratios of each throughout the summer months Wintertime measurements gave mixing ratios below the detection limits (0001 ppbv of an individual compound) Simultaneous measurements of ozone, NO, NO2, and monoterpene hydrocarbons allowed examination of the contribution of monoterpene photooxidation to ozone production Based on reported modeling studies, monoterpenes were estimated to be a small source of ozone, insufficient to account for the relatively high ozone mixing ratios (>80 ppbv) sometimes observed at this sampling site

W. Wong

Papers

Dye laser flash photolysis kinetics study of the reaction of ground-state atomic oxygen with hydrogen peroxide