Tropospheric photolytic reactions

In this case, N20 (called nitrous oxide or laughing gas) has natural sources, such as emissions from swamps and other oxygen-free ( anoxic ) waters and soils. The oxygen atoms in this reaction can come from several tropospheric photolytic reactions involving OH or OOH. Another source of NO is the thermal reaction between N2 and 02 ... 

The supply of radicals is dependent on photolytic reactions, so that most significant gas-phase chemistry only occurs during the daytime. The supply of radicals is also linked to the availability of water vapor (H2O) through Equation (6). Both the photochemical production and loss of pollutants are slower in winter, due to lack of sunlight and lower H2O. Photochemical loss rates are also slower in the upper troposphere, where temperatures are lower and mixing ratios of H2O are much smaller. 

Figure 9-6 summarizes our current understanding of the chemical reactions involving nitrogen oxides in the troposphere. Photolytically induced... 

In the calculation for atmospheric photodissociation reactions, how to calculate the effective solar intensity is a major issue, because not only direct irradiation from the sun, but light from all directions reflected and scattered by the ground surface, clouds, atmospheric molecules, and aerosols can contribute to photolysis. Furthermore, in the troposphere for example, only solar radiation that has not been absorbed by atmospheric molecules in the higher atmosphere, the stratosphere and above, can cause photolytic reactions. The spherically integrated solar intensity after considering those many atmospheric processes is called the actinic flux F (X) (photons cm s ), which means solar irradiation valid for photochemical effect. In atmospheric chemistry, jp is often used instead of kp for representing photolysis rate constant. Photodissociation rate constant in the atmosphere can be expressed using these parameters as... 

Hydrogen peroxide H2O2 is formed by the radical termination reaction HO2 + HO2, and exists in the troposphere generally at the mixing ratio in order of ppbv. Since H2O2 is water soluble, it is removed by the dissolution into cloud and fog water, while photolytic reaction is another important removal process. Methyl hydroperoxide CH3OOH also exists in the whole region of the troposphere in natural atmosphere as an oxidation product of methane. Its photolytic reaction is important as its removal process, and also as a radical source in the upper troposphere. 

The rate constants for the reaction of l,2-dibromo-3-chloropropane with ozone and OH radicals in the atmosphere at 296 K are <5.4 x 10 ° and 4.4 x lO cm /molecule-sec (Tuazon et al., 1986). The smaller rate constant for the reaction with ozone indicates that the reaction with ozone is not an important atmospheric loss of l,2-dibromo-3-chloropropane. The calculated photolytic half-life and tropospheric lifetime for the reaction with OH radicals in the atmosphere are 36 and 55 d, respectively. The compound l-bromo-3-chloropropan-2-one was tentatively identified as a product of the reaction of l,2-dibromo-3-chloropropane with OH radicals. 

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. 

Reactions 9 and 10 are thought to dominate HO production in the cleanest regions of the troposphere. Thus any approach that utilizes intense laser radiation at wavelengths shorter than about 320 nm should be undertaken with some care. In addition to ozone, H202 and HONO photolyze in the UV to produce HO and must be considered photolytic precursors to spurious HO. Nevertheless, 03 has been the only significant known photolytic parent of spurious HO to date. The earliest measurements of tropospheric HO contained significant contributions from spurious HO for which corrections were applied (71). 

Rowland and Molina and Stolarski and Cicerone proposed that halocarbons 11 and 12 (CFCh and CF2CI2) have a long lifetime in the troposphere, can accumulate there, and diffuse upwards to the stratosphere. Although not photolysed in the troposphere, the two halocarbons absorb in the solar radiation window between 190 and 210 nm, which can occur in the stratosphere at similar altitudes to those where the ozone concentration is reasonably high. Photolytically formed chlorine atoms, either from man-made sources or some natural sources such as combustion of vegetation, may then react via a rapid chain-reaction scheme, giving the overall stoicheiometry O + O3 -> 2O2 as the combination of reactions (31)... 

The photolytic lifetimes for average tropospheric conditions were calculated on the basis of the cited quantum yields 4 days for MEK, 14 h for MEK, 22 h for MACR and 35 min for MGLY. These results indicate that photolysis processes in the troposphere dominate (in the case of MGLY) or are in competition with removal reactions initiated by OH radicals. 

Other reaction pathway which split into three product species, e.g., are also energetically possible within the wavelength range of tropospheric solar actinic flux. The photolytic quantum yields of HO2NO2 are unity at the wavelength longer than 200 nm measured by MacLeod et al. (1988), Roehl et al. (2001), and Jimenez et al. (2005). Since the quantum yields of HO2 and NO2 are 0.8 and those of OH and NO3 are 0.2 (Sander et al. 2011), (HO2-b NO2) = 0.8, and 4> (OH -b NO3) = 0.2, if they are ascribed to reactions (4.39) and (4.40), respectively. 

Other than NO and N2O, NO2, NO3, N2O5, HNO3, HO2NO2 are also important oxides of nitrogen in the stratosphere. Since their photolyses are also all important in the troposphere, their absorption spectra, cross sections, and photolytic processes have been described in the previous section including the stratospheric actinic flus region. H2O2 and CH4 also exist in the stratosphere in considerable concentration as they are formed by the mutual chain terminatimi reaction of HO2, and in the oxidation process of CH4, respectively, as in the troposphere. Their absorption spectra, cross sections, and photolytic processes have already been described in the previous section to be referred. 

Chlorine monoxide CIO, bromine monoxide BrO, and iodine monoxide lO are the main chain carrier of stratospheric and tropospheric halogen chain reactions. CIO is partially photolyzed only in the stratosphere, and the photolytic rates of BrO and lO are large also in the troposphere. Their photolysis rates have to be considered in the evaluation of ozone depleting chain reactions. 

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