Troposphere organic compounds

In addition to reactions with HO, tropospheric organic compounds may be oxidized by ozone (via ozonation of non-aromatic carbon/carbon double bonds, Atkinson 1990) and in some cases by reaction with nitrate radical, described below. Table I gives representative trace-gas removal rates for these three processes. In spite of these competing reactions, HO largely serves as... 

HO oxidation of CO is much faster than the reaction with methane, resulting in a mean CO lifetime of about two months, but considerably slower than reaction with the majority of the nonmethane hydrocarbons. Table I gives representative removal rates for a number of atmospheric organic compounds their atmospheric lifetimes are the reciprocals of these removal rates (see Equation E4, below). The reaction sequence R31, R13, R14, R15 constitutes one of many tropospheric chain reactions that use CO or hydrocarbons as fuel in the production of tropospheric ozone. These four reactions (if not diverted through other pathways) produce the net reaction... 

There are a number of important reasons for discussing the reactions of organic compounds in the troposphere ... 

Considerable attention has been given to the persistence and fate of organic compounds in the troposphere, and this has been increasingly motivated by their possible role in the production of ozone by reactions involving NO. ... 

Atkinson R (1990) Gas-phase troposphere chemistry of organic compounds a review. Atmos Environ 24A 1-41. 

Atkinson, R. (1997) Gas-phase tropospheric chemistry of volatile organic compounds l. Alkanes and alkenes. J. Phys. Chem. Ref. Data 26, 215-289. 

Chlorine-containing organic compounds, which are not destroyed in the troposphere, are photolyzed in the stratosphere ... 

Jenkin, M. E., Saunders, S. M., and Pilling, M. J. The tropospheric degradation of volatile organic compounds A protocol for mechanism development, Atmos. Environ., 31, 81-104, 1997. 

Volatile organic compounds (VOC) contribute to the formation of tropospheric ozone (summer smog). Certain halogenated hydrocarbons (e.g. CFCs) also destroy the stratospheric ozone layer. Chlorinated solvents are hazardous to water and, if disposed of incorrectly (e.g. burning), may emit highly toxic substances (e.g. dioxins). 

The chemistry of the troposphere (the layer of the atmosphere closest to earth s surface) overlaps with low-temperature combustion, as one would expect for an oxidative environment. Consequently, the concerns of atmospheric chemistry overlap with those of combustion chemistry. Monks recently published a tutorial review of radical chemistry in the troposphere. Atkinson and Arey have compiled a thorough database of atmospheric degradation reactions of volatile organic compounds (VOCs), while Atkinson et al. have generated a database of reactions for several reactive species with atmospheric implications. Also, Sandler et al. have contributed to the Jet Propulsion Laboratory s extensive database of chemical kinetic and photochemical data. These reviews address reactions with atmospheric implications in far greater detail than is possible for the scope of this review. For our purposes, we can extend the low-temperature combustion reactions [Equations (4) and (5)], whereby peroxy radicals would have the capacity to react with prevalent atmospheric radicals, such as HO2, NO, NO2, and NO3 (the latter three of which are collectively known as NOy) ... 

Alfassi, Z. B S. Padmaja, P. Neta, and R. E. Huie, Rate Constants for Reactions of NO, Radicals with Organic Compounds in Water and Acetonitrile, J. Phys. Chem., 97, 3780-3782 (1993). Allen, H. C., J. M. Laux, R. Vogt, B. J. Finlayson-Pitts, and J. C. Hemminger, Water-Induced Reorganization of Ultrathin Nitrate Films on NaCI—Implications for the Tropospheric Chemistry of Sea Salt Particles, J. Phys. Chem., 100, 6371-6375 (1996). Allen, H. C., D. E. Gragson, and G. L. Richmond, Molecular Structure and Adsorption of Dimethyl Sulfoxide at the Surface of Aqueous Solutions, J. Phys. Chem. B, 103, 660-666 (1999). Anthony, S. E R. T. Tisdale, R. S. Disselkamp, and M. A. Tolbert, FTIR Studies of Low Temperature Sulfuric Acid Aerosols, Geophys. Res. Lett., 22, 1105-1108 (1995). 

As discussed in other chapters of this book and summarized in Chapter 16, the formation of tropospheric ozone from photochemical reactions of volatile organic compounds (VOC) and oxides of nitrogen (NC/) involves many reactions. Concentrations are therefore quite variable geographically, temporally, and altitudinally. Additional complications come from the fact that there are episodic injections of stratospheric 03 into the troposphere as well as a number of sinks for its removal. Because 03 decomposes thermally, particularly on surfaces, it is not preserved in ice cores. All of these factors make the development of a global climatology for 03 in a manner similar to that for N20 and CH4, for example, much more difficult. In addition, the complexity of the chemistry leading to O, formation from VOC and NOx is such that model-predicted ozone concentrations can vary from model to model (e.g., see Olson et al., 1997). 

R. Atkinson. Gas-phase tropospheric chemistry of organic-compounds — a review, Atmos. Environ. Part A - Gen. Top., 24 1 1 (1990). 

Nonfluorine CFC substitutes have been considered, but few are fully satisfactory. For example, we could go back 50 years to the use of anhydrous ammonia as a refrigerant, but NH3 is as toxic now as it ever was. Cyclopentane could be used as a foam-blowing agent, but it is less effective than HCFC-141b and besides would contribute to the volatile organic compound load in the troposphere, which is the root cause of ozone pollution (Section 8.3.2). On the other hand, supercritical CO2 is emerging as an alternative to CFCs in various steps in the preparation of fluorocarbon polymers (Section 8.1.3). 

Figure 16.7 Second-order rate constants and half-lives for reaction of HO radicals in the troposphere at 298 K for a series of organic compounds. For calculation of the half-lives a HO" steady-state concentration of 10 6 molecule cm 3 has been assumed. Data from Atkinson (1989), Atkinson (1994), Anderson and Hites (1996), Brubaker and Hites (1997).

Atkinson Gas-Phase Tropospheric Chemistry of Volatile Organic Compounds 1. Alkanes and Alkenes [15]... 

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