Atmospheric degradation of volatile organic compounds

A model for the photochemistry of the global troposphere constrained by observed concentrations of H2O, O3, CO, CH4, NO, NO2, and HNO3 is presented. Data for NO and NO2 are insufficient to define the global distribution of these gases but are nonetheless useful in limiting several of the more uncertain parameters of the model. Concentrations of OH, HO2, H2O2, NO, NO2, NO3, N2O5, HNO2, HO2NO2, CH3O2, CH3OOH, CH2O, and CH3CCl3 are calculated as functions of altitude, latitude, and season. Results imply that the source for nitrogen oxides in the remote troposphere is geographically dispersed and surprisingly small, less than 107 tons N yr−1. Global sources for CO and CH4 are 1.5 × 109 tons C yr−1 and 4.5 × 108 tons C yr−1, respectively. Carbon monoxide is derived from combustion of fossil fuel (15%) and oxidation of atmospheric CH4 (25%), with the balance from burning of vegetation and oxidation of biospheric hydrocarbons. Production of CO in the northern hemisphere exceeds that in the southern hemisphere by about a factor of 2. Industrial and agricultural activities provide approximately half the global source of CO. Oxidation of CO and CH4 provides sources of tropospheric O3 similar in magnitude to loss by in situ photochemistry. Observations of CH3CCl3 could offer an important check of the tropospheric model and results shown here suggest that computed concentrations of OH should be reliable within a factor of 2. A more definitive test requires better definition of release rates for CH3CCl3 and improved measurements for its distribution in the atmosphere.

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Tropospheric chemistry: A global perspective

This overview compiles the actual knowledge of the biogenic emissions of some volatile organic compounds (VOCs), i.e., isoprene, terpenes, alkanes, alkenes, alcohols, esters, carbonyls, and acids. We discuss VOC biosynthesis, emission inventories, relations between emission and plant physiology as well as temperature and radiation, and ecophysiological functions. For isoprene and monoterpenes, an extended summary of standard emission factors, with data related to the plant genus and species, is included. The data compilation shows that we have quite a substantial knowledge of the emission of isoprene and monoterpenes, including emission rates, emission regulation, and biosynthesis. The situation is worse in the case of numerous other compounds (other VOCs or OVOCs) being emitted by the biosphere. This is reflected in the insufficient knowledge of emission rates and biological functions. Except for the terpenoids, only a limited number of studies of OVOCs are available; data are summarized for alkanes, alkenes, carbonyls, alcohols, acids, and esters. In addition to closing these gaps of knowledge, one of the major objectives for future VOC research is improving our knowledge of the fate of organic carbon in the atmosphere, ending up in oxidation products and/or as aerosol particles.

Biogenic Volatile Organic Compounds (VOC): An Overview on Emission, Physiology and Ecology

. The last decade has seen tremendous advances in atmospheric aerosol particle research that is often performed in the context of climate and global change science. Biomass burning, one of the largest sources of accumulation mode particles globally, has been closely studied for its radiative, geochemical, and dynamic impacts. These studies have taken many forms including laboratory burns, in situ experiments, remote sensing, and modeling. While the differing perspectives of these studies have ultimately improved our qualitative understanding of biomass-burning issues, the varied nature of the work make inter-comparisons and resolutions of some specific issues difficult. In short, the literature base has become a milieu of small pieces of the biomass-burning puzzle. This manuscript, the second part of four, examines the properties of biomass-burning particle emissions. Here we review and discuss the literature concerning the measurement of smoke particle size, chemistry, thermodynamic properties, and emission factors. Where appropriate, critiques of measurement techniques are presented. We show that very large differences in measured particle properties have appeared in the literature, in particular with regards to particle carbon budgets. We investigate emissions uncertainties using scale analyses, which shows that while emission factors for grass and brush are relatively well known, very large uncertainties still exist in emission factors of boreal, temperate and some tropical forests. Based on an uncertainty analysis of the community data set of biomass burning measurements, we present simplified models for particle size and emission factors. We close this review paper with a discussion of the community experimental data, point to lapses in the data set, and prioritize future research topics.

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A review of biomass burning emissions part III: intensive optical properties of biomass burning particles

https://www.engr.colostate.edu/~marchese/combustion10/3.pdf

A Comprehensive Modeling Study of iso-Octane Oxidation

The kinetics and mechanism of the self-reaction of cyclopentylperoxy radicals: 2 c-C5H9O2→ 2 c-C5H9O + O2(1a), → c-C5H9OH + c-C5H8O + O2(1b), have been studied using both time-resolved and end-product-analysis techniques. Determination of the product yields from the photolysis of Cl2–c-C5H10–O2–N2 mixtures using FTIR spectroscopy demonstrates that ring-opening of the cyclopentoxy radical formed in channel (1a): c-C5H9O + M → CH2(CH2)3CHO + M (3) dominates over reaction with oxygen: c-C5H9O + O2→ c-C5H8O + HO2(2), under atmospheric conditions. Flash photolysis-UV absorption experiments were used to obtain the UV spectrum of the cyclopentylperoxy radical and the kinetics of reaction (1). The spectrum of c-C5H9O2 is similar to those of other alkylperoxy radicals, with a maximum cross-section of (5.22 ± 0.20)× 10–18 cm2 molecule–1 at 250 nm, measured relative to a value of 4.55 × 10–18 cm2 molecule–1 for CH3O2 at 240 nm. The observed second-order rate constant, kobs(–d[c-C5H9O2]/dt= 2kobs[c-C5H9O2]2), for removal of cyclopentylperoxy radicals was dependent on the oxygen partial pressure. Experiments as a function of temperature from 243 to 373 K gave limiting minimum and maximum values of kobs at low ( 50 Torr) oxygen partial pressures, respectively: kmin/cm3 molecule–1 s–1=(1.3 ± 0.4)× 10–14 exp[(188 ± 83)K/T] and kmax/cm3 molecule–1 s–1=(2.9 ± 0.8)× 10–13 exp[–(555 ± 77)K/T]. At low oxygen partial pressures, the only effective removal channel for cyclopentylperoxy radicals is the molecular channel (1b) and kmin can be equated to k1b. Simulations suggest that kmax represents an upper limit on k1 and is at most 25% greater. In light of the present results on the cyclopentylperoxy radical, further experiments were performed on the cyclohexylperoxy radical self-reaction: 2 c-C6H11O2→ 2 c-C6H11O + O2(16a), → c-C6H11OH + c-C6H10O + O2(16b) at low oxygen partial pressures, giving k16b/cm3 molecule–1 s–1=(1.3 ± 0.3)× 10–14 exp[(185 ± 15) K/T] and an estimated k16/cm3 molecule–1 s–1= 7.7 × 10–14 exp(–184 K/T). The above errors are 1σ and represent experimental uncertainty only.

Ultraviolet absorption spectrum and self-reaction of cyclopentylperoxy radicals

The reaction paths for the extrusion of a Z  H or Cl atom from an alkoxy radical CXYZO where X and Y are H, Cl and F in all possible combinations have been studied using the MNDO/CI method. Radicals of this type have been implicated in the upper atmosphere reactions that are thought to be involved in the decomposition of the ozone layer. The results are discussed using Marcus theory and compared to existing experimental observations. Correlation diagrams are used to illustrate the effect of the position along the reaction coordinate of a surface crossing upon the activation energy for the extrusion of Cl reaction, due to the presence of a p-type orbital on the departing atom.

A theoretical study of the decomposition of halogenated alkoxy radicals. I. Hydrogen and chlorine extrusions

The temperature dependence of the rate coefficients for β‐hydroxyperoxy radical self reactions

The kinetics and mechanism of the gas-phase reaction of the cyclohexadienyl radical c-C6H7 with O2 have been investigated using both experimental and theoretical approaches. The rate constant has been measured using conventional flash photolysis in the temperature range 302–456 K, 1 atm pressure. c-C6H7 radicals were produced by reacting Cl atoms with 1,4-cyclohexadiene. The rate expression is k1=(1.4±0.26)×10−13 exp[−(300±74) K/T] cm3 molecule−1 s−1
(2σ error bars). The reaction can proceed either by association, yielding a peroxy radical RO2 or by H-abstraction, yielding benzene+HO2, the two reaction channels involving two distinct transition states. In contrast to what is observed for the c-C6H6OH radical, no equilibrium with the peroxy radical could be characterised. The theoretical approach, involving both DFT and ab initio methods, was used to determine if the measured rate constant should be assigned to the association or to the H-abstraction channel. Comparison of experimental and theoretical results shows that H-abstraction must be the only significant reaction channel.

Kinetics and mechanism of the gas-phase reaction of the cyclohexadienyl radical c-C6H7 with O2

The kinetics of reaction 1, Cl{sub 3} + O{sub 2} + M {yields} CCl{sub 3}O{sub 2} + M, has been investigated in detail as a function of temperature and over a large pressure range. At low pressure, 0.8-12 Torr, the reaction was investigated by laser photolysis and time-resolved mass spectrometry, while at high pressure (760 Torr), flash photolysis with UV absorption spectrometry was employed. At the low-pressure limit, the rate expression, k{sub 1}(0) = (1.6 {+-} 0.3) {times} 10{sup {minus}30}(T/298){sup {minus}(6.3 {+-} 0.5)} cm{sup 6}/molecule{sup 2} sec (M = N{sub 2}), exhibits a quite strong negative temperature coefficient. The obtained strong collision rate expression, 7.0 {times} 10{sup {minus}30}(t/298){sup {minus}4.3} cm{sup 6}/molecule{sup 2} sec, using either RRKM calculations of Troe's factorized expression, is unable to reproduce the experimental temperature dependence, unless an unreasonably strong temperature dependence is assigned to the collisional efficiency factor: {beta}{sub c} = 0.23(t/298){sup {minus}2.0} (M = N{sub 2}). Similar results are obtained for other chlorofluoromethyl radicals. The falloff curves were constructed by using RRKM calculations obtained by adjusting {beta}{sub c} and the transition-state model, in order to reproduce the experimental data. The rate expression at the high-pressure limit was derived from these calculations k{sub 1}({infinity}) = (3.2more » {+-} 0.7) {times} 10{sup {minus}12}(T/298){sup {minus}(1.2 {+-} 0.4)} cm{sup 3}/molecule sec. All the parameters to be used in Troe's analytical expression for calculating the biomolecular rate constant at any pressure and temperature are given. The rate constant at the low-pressure limit k{sub 1}(0) is more than an order of magnitude lower than for the CF{sub 3} radical. The RRKM calculations show that this arises from a large difference in the C-O bond dissociation energies in the corresponding peroxy radicals.« less

Robert Lesclaux

Papers

Ultraviolet absorption spectrum and self-reaction of cyclopentylperoxy radicals

A theoretical study of the decomposition of halogenated alkoxy radicals. I. Hydrogen and chlorine extrusions

The temperature dependence of the rate coefficients for β‐hydroxyperoxy radical self reactions

Kinetics and mechanism of the gas-phase reaction of the cyclohexadienyl radical c-C6H7 with O2

Kinetic study of the reaction trichloromethyl + oxygen + M .fwdarw. trichloromethylperoxo + M from 1 to 760 Torr and from 233 to 333 K