Tropospheric gas phase

The direct photolysis of compounds such as HONO, 03, HCHO, and N02 in the tropospheric gas phase is a very important source of reactive species, which are then involved in the transformation of organic compounds. Additionally, some organic molecules including organic pollutants undergo photolysis as a significant or even the main process of removal from the atmosphere. It is for instance the case for nitronaphthalenes, the atmospheric lifetime of which can be as low as a couple of hours because of direct photolysis [11, 12]. 

Huebert, B. J., and A. L. Lazrus (1980a). Tropospheric gas-phase and particulate nitrate measurements. J. Geophys. Res. 85, 7322-7328. 

Emmerson KM, Evans MJ (2009) Comparison of tropospheric gas-phase chemistry schemes for use within global models. Atmos Chem Phys 9 1831-1845... 

Blanco, M.B., R.A. Taccone, S.l. Lane, and M.A. Teruel (2006), On the OH-initiated degradation of methacrylates in the troposphere Gas-phase kinetics and formation of pyruvates, Chem. Phys. Lett., 429, 389-394. 

Example of an analysis of exhaust gas by gas phase chromatography and j relative reactivity of effluents with respect to tropospheric ozone formation. I... 

Considerable attention has been directed to the formation of nitroarenes that may be formed by several mechanisms (a) initial reaction with hydroxyl radicals followed by reactions with nitrate radicals or NO2 and (b) direct reaction with nitrate radicals. The first is important for arenes in the troposphere, whereas the second is a thermal reaction that occurs during combustion of arenes. The kinetics of formation of nitroarenes by gas-phase reaction with N2O5 has been examined for naphthalene (Pitts et al. 1985a) and methylnaphthalenes (Zielinska et al. 1989) biphenyl (Atkinson et al. 1987b,c) acephenanthrylene (Zielinska et al. 1988) and for adsorbed pyrene (Pitts et al. 1985b). Both... 

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

Reactions that simnlate tropospheric conditions have been carried ont in Teflon bags with volumes of ca. 6 m htted with sampling ports for introduction of reactants and snbstrates, and removal of samples for analysis. Substrates can be added in the gas phase or as aerosols that form a surface him. The primary reactants are the hydroxyl and nitrate radicals, and ozone. These mnst be prepared before use by reactions (a) to (c). 

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

Oxidation rate constant k, for gas-phase second order rate constants, kOH for reaction with OH radical, kNC,3 with N03 radical and k(), with 03, or as indicated data at other temperatures see original reference kOH = (7.49 0.39) x 10 11 cm3 molecule-1 s-1 at (298 2) K with a calculated tropospheric lifetime ranging from 1.9 to 2.4 h using a global tropospheric 12-h daytime average OH radical concentration of 2.0 x 10s molecule cm-3 (relative rate method, Phousongphouang Arey 2002)... 

Air t1/2 = 6 h with a steady-state concn of tropospheric ozone of 2 x 10-9 M in clean air (Butkovic et al. 1983) t/2 = 2.01-20.1 h, based on photooxidation half-life in air (Howard et al. 1991) calculated atmospheric lifetime of 11 h based on gas-phase OH reactions (Brubaker Hites 1998). Surface water computed near-surface of a water body, tl/2 = 8.4 h for direct photochemical transformation at latitude 40°N, midday, midsummer with tl/2 = 59 d (no sediment-water partitioning), t,/2 = 69 d (with sediment-water partitioning) on direct photolysis in a 5-m deep inland water body (Zepp Schlotzhauer 1979) t,/2 = 0.44 s in presence of 10 M ozone at pH 7 (Butkovic et al. 1983) calculated t,/2 = 59 d under sunlight for summer at 40°N latitude (Mill Mabey 1985) t,/2 = 3-25 h, based on aqueous photolysis half-life (Howard et al. 1991) ... 

Photolytic. Irradiation of vinyl chloride in the presence of nitrogen dioxide for 160 min produced formic acid, HCl, carbon monoxide, formaldehyde, ozone, and trace amounts of formyl chloride and nitric acid. In the presence of ozone, however, vinyl chloride photooxidized to carbon monoxide, formaldehyde, formic acid, and small amounts of HCl (Gay et al, 1976). Reported photooxidation products in the troposphere include hydrogen chloride and/or formyl chloride (U.S. EPA, 1985). In the presence of moisture, formyl chloride will decompose to carbon monoxide and HCl (Morrison and Boyd, 1971). Vinyl chloride reacts rapidly with OH radicals in the atmosphere. Based on a reaction rate of 6.6 x lO" cmVmolecule-sec, the estimated half-life for this reaction at 299 K is 1.5 d (Perry et al., 1977). Vinyl chloride reacts also with ozone and NO3 in the gas-phase. Sanhueza et al. (1976) reported a rate constant of 6.5 x 10 cmVmolecule-sec for the reaction with OH radicals in air at 295 K. Atkinson et al. (1988) reported a rate constant of 4.45 X 10cmVmolecule-sec for the reaction with NO3 radicals in air at 298 K. 

Chemical/Physical. In the gas phase, cycloate reacts with hydroxyl and NO3 radicals but not with ozone. With hydroxy radicals, cleavage of the cyclohexyl ring was suggested leading to the formation of a compound tentatively identified as C2H5(Cff0)NC(0)SC2H5. The calculated photolysis lifetimes of cycloate in the troposphere with hydroxyl and NO3 radicals are 5.2 h and 1.4 d, respectively. The relative reaction rate constants for the reaction of cycloate with OH and nitrate radials are 3.54 x lO " and 3.29 x 10 cm /molecule-sec, respectively (Kwok et al., 1992). 

In gas-phase tropospheric chemistry, the most common units for concentration, N, are molecules cm-3 and for path length, /, units of cm. The form of the Beer-Lambert law is then... 

In addition to these highly useful data sets, periodically there are reviews directed to the reactions of one particular species (e.g., OH, N03, or 03) or group of compounds (e.g., R02 radicals). These are referenced in the appropriate sections of Chapter 6. For example, a review of the gas-phase tropospheric chemistry of... 

As we shall see in the following sections, these observations are readily understood in terms of the kinetics and mechanisms of oxidation of S02. The oxidation of S02 occurs in solution and on the surfaces of solids as well as in the gas phase. Indeed, under many conditions typical of the troposphere, oxidation in the aqueous phase provided by clouds and fogs predominates, consistent with the observed dependence on these factors. The presence of oxidizers to react with the S02 is, of course, also a requirement hence the dependence on 03 (which is a useful surrogate for other oxidants as well) and sunlight, which is needed to generate significant oxidant concentrations. 

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