Troposphere hydrocarbons

For most organic compounds that enter the troposphere, reactions with HO govern their disappearance (Carter and Atkinson, 1985). For example, methane, which is by far the most abundant tropospheric hydrocarbon, is virtually inert to atmospheric reactions except for its reaction with OH (4.38), which initiates a series of free-radical reactions that lead to its conversion to oxidized forms ... 

Reactions of peroxy radicals in the tropospheric hydrocarbon/NO chain, in P.M. Borrell, P. Borrell, T. CvitaS, W. Seiler (eds), Proc. EUROTRAC Symp. 90, SPB Academic Publ., The Hague 1991, pp. 413-416. 

The chemical transformations occurring in the atmosphere are best characterized as oxidation processes. Reactions involving compounds of carbon (C), nitrogen (N), and sulfur (S) are of most interest. The chemical processes in the troposphere involve oxidation of hydrocarbons, NO, and SO2 to... 

NMHC. A large number of hydrocarbons are present in petroleum deposits, and their release during refining or use of fuels and solvents, or during the combustion of fuels, results in the presence of more than a hundred different hydrocarbons in polluted air (43,44). These unnatural hydrocarbons join the natural terpenes such as isoprene and the pinenes in their reactions with tropospheric hydroxyl radical. In saturated hydrocarbons (containing all single carbon-carbon bonds) abstraction of a hydrogen (e,g, R4) is the sole tropospheric reaction, but in unsaturated hydrocarbons HO-addition to a carbon-carbon double bond is usually the dominant reaction pathway. 

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... 

When NMHC are significant in concentration, differences in their oxidation mechanisms such as how the NMHC chemistry was parameterized, details of R02-/R02 recombination (95), and heterogenous chemistry also contribute to differences in computed [HO ]. Recently, the sensitivity of [HO ] to non-methane hydrocarbon oxidation was studied in the context of the remote marine boundary-layer (156). It was concluded that differences in radical-radical recombination mechanisms (R02 /R02 ) can cause significant differences in computed [HO ] in regions of low NO and NMHC levels. The effect of cloud chemistry in the troposphere has also recently been studied (151,180). The rapid aqueous-phase breakdown of formaldehyde in the presence of clouds reduces the source of HOj due to RIO. In addition, the dissolution in clouds of a NO reservoir (N2O5) at night reduces the formation of HO and CH2O due to R6-RIO and R13. Predictions for HO and HO2 concentrations with cloud chemistry considered compared to predictions without cloud chemistry are 10-40% lower for HO and 10-45% lower for HO2. 

The transformation of arenes in the troposphere has been discussed in detail (Arey 1998). Their destruction can be mediated by reaction with hydroxyl radicals, and from naphthalene a wide range of compounds is produced, including 1- and 2-naphthols, 2-formylcinnamaldehyde, phthalic anhydride, and with less certainty 1,4-naphthoquinone and 2,3-epoxynaphthoquinone. Both 1- and 2-nitronaphthalene were formed through the intervention of NO2 (Bunce et al. 1997). Attention has also been directed to the composition of secondary organic aerosols from the photooxidation of monocyclic aromatic hydrocarbons in the presence of NO (Eorstner et al. 1997) the main products from a range of alkylated aromatics were 2,5-furandione and the 3-methyl and 3-ethyl congeners. 

Finlayson-Pitts BJ, IN Pitts (1997) Tropospheric air pollution ozone, airborne toxics, polycyclic aromatic hydrocarbons, and particles. Science 276 1045-1052. 

Air pollution in cities can be considered to have three components sources, transport and transformations in the troposphere, and receptors. The sources are processes, devices, or activities that emits airborne substances. When the substances are released, they are transported through the atmosphere, and are transformed into different substances. Air pollutants that are emitted directly to the atmosphere are called primary pollutants. Pollutants that are formed in the atmosphere as a result of transformations are called secondary pollutants. The reactants that undergo the transformation are referred to as precursors. An example of a secondary pollutant is troposphere ozone, O3, and its precursors are nitrogen oxides (NO = NO + NO2) and non-methane hydrocarbons, NMHC. The receptors are the person, animal, plant, material, or urban ecosystems affected by the emissions (Wolff, 1999). 

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). 

On a global scale the air layers within a few kilometers of the earth s surface are rapidly mixed by wind action. This region is called the troposphere. Natural and manmade sources of chemicals such as CH4 and other hydrocarbons, CO, SO, NO, ozone, and chlorine are emitted into the troposphere. Most of these are removed or reacted away to form harmless products by dissolving in rain, adsorption on solids, and chemical reactions. 

Historically, organics in the troposphere have been measured as non-methane hydrocarbons (NMHC). As... 

Finlayson-Pitts, B. J., and J. N. Pitts, Jr., Tropospheric Air Pollution Ozone, Airborne Toxics, Polycyclic Aromatic Hydrocarbons, and Particles, Science, 27b, 1045-1052 (1997). 

Singh, H. B M. Kanakidou, P. J. Crutzen, and D. J. Jacob, High Concentrations and Photochemical Fate of Oxygenated Hydrocarbons in the Global Troposphere, Nature, 378, 50-54 (f995). 

The first thing that stands out in Table 6.2 is that the OH-CH4 rate constant, 6.2 X 10 15 cm3 molecule 1 s-1, is much smaller than those for the higher alkanes, a factor of 40 below that for ethane. This relatively slow reaction between OH and CH4 is the reason that the focus is on non-methane hydrocarbons (NMHC) in terms of ozone control in urban areas. Thus, even at a typical peak OH concentration of 5 X 106 molecules cm 3, the calculated lifetime of CH4 at 298 K is 373 days, far too long to play a significant role on urban and even regional scales. Clearly, however, this reaction is important in the global troposphere (see Chapter 14.B.2b). 

As seen in Table 6.1, the reactions of the nitrate radical with the simple aromatic hydrocarbons are generally too slow to be important in the tropospheric decay of the organic. However, one of the products of the aromatic reactions, the cresols, reacts quite rapidly with NO,. o-Cresol, for example, reacts with N03 with a room temperature rate constant of 1.4 X 10 " cm3 molecule-1 s-1, giving a lifetime for the cresol of only 1 min at 50 ppt N03. This rapid reaction is effectively an overall hydrogen abstraction from the pheno-... 

Rockmann, T C. A. M. Brenninkmeijer, P. Neeb, and P. J. Crutzen, Ozonolysis of Nonmethane Hydrocarbons as a Source of the Observed Mass Independent Oxygen Isotope Enrichment in Tropospheric CO, J. Geophys. Res., 103, 1463-1470 (1998). 

Rudolph, J., A. Khedim, and B. Bonsang, Light Hydrocarbons in the Tropospheric Boundary Layer over Tropical Africa, . /. Geophys. Res., 97, 6181-6186 (1992). 

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