Tropospheric photolytic

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 ... 

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

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. 

Demerjian, K. L K. L. Schere, and J. T. Peterson, Theoretical Estimates of Actinic (Spherically Integrated) Flux and Photolytic Rate Constants of Atmospheric Species in the Lower Troposphere, Adv. Environ. Sci. Technol., 10, 369-459 (1980). 

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). 

Although the gas phase provides major pathway for hydroxyl radical and hydrogen peroxide production in the atmosphere, there is overwhelming evidence [158-168] that aqueous phases in the troposphere also provides a significant medium for the photolytic production of these important oxidants. 

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 4. Spectrai thresholds of photolytic radical sources in the troposphere and flie spectral distribution of sunlight a) above flie ozone layer, b) near hie surface.

Demerjian, K. L., Schere, K. L., and Peterson, J. T., Theoretical estimates of actinic (spherically integrated) flux and photolytic rate constants of atmospheric species in the lower troposphere. Adu. Environ. Sci. Technol. 10, 369 (1980). 

From Figure 6.8 we see that at the Earth s surface (288 K) the lifetime of PAN against thermal decomposition is about 3h, whereas that against photodissociation is about 13 days. Because the photolytic loss of PAN is approximately independent of altitude and the rate of thermal decomposition is strongly temperature dependent, a point is reached, at about 7 km, where the two rates become equal above that altitude, photolysis is the more important loss process. At the temperature of the upper troposphere, PAN is an effective reservoir for NO because PAN is transported in the upper troposphere, this amounts to a mechanism for long-range transport of NO. ... 

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. 

Spectroscopic, kinetic, photolytic and mechanistic studies have been carried out on simple peroxy radicals such as HO2, CH3O2, C2H5O2 and CH3C(0)02 using a variety of laboratory techniques. The photo-oxidation studies of selected carbonyl compounds was performed and quantum yields established in order to assess their photolytic lifetime in the troposphere. The product distribution of the ozonolysis of selected alkenes was determined in the presence of water vapour. 

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