Photolysis, atmosphere

With the addition of CO caused by photochemical oxidation of methane, a significant flux enters the atmosphere annually, but the principal global contributions are terrestrial, anthropogenic and from atmospheric photolysis of methane. 

Photolysis atmospheric photolysis t,/2 = 2808-16152 h, based on measured photolysis half-lives in deionized water (Hustert et al. 1981 Howard et al. 1991) ... 

Some molecules in this group (HONO, NC j 0, HONC ) have been extensively studied because the photofragments OH and NO can be probed by tunable lasers. These molecules are important minor constituents in the earth atmosphere and their photochemistry plays a major role in air pollution. Atmospheric pollutants N0X (NO, NO2, NO3) are formed from combustion of fuel and subsequent chemical reactions in the atmosphere. Photolysis of alkyl oxides produces NO and NO2 that can be probed by LIF the internal energy distribution provides an important clue to the mechanism of photodissociation. 

Photolysis both aqueous and atmospheric photolysis half-lives are infinite (Howard et al. 1991). 

Photolysis aqueous photolysis tA = 67-200 d, based on measured photolysis rate constant in distilled water under midday sun at 40°N latitude (Simmons Zepp 1986 Howard 1989 Howard et al. 1991) atmospheric photolysis tA = 67-200 d, based on measured photolysis rate constant in distilled water under midday sun at 40°N latitude (Simmons Zepp 1986 quoted, Howard 1989 Howard et al. 1991) rate constant k = 2.37 x 10-3 Ir1 with H202 under photolysis at 25°C in F-l 13 solution and with HO- in the gas (Dilling et al. 1988). 

Photolysis atmosphere photolysis t,/2 = 288-864 h, based on observed photolysis on soil TLC plates under summer sunlight (Helling 1976 quoted, Howard et al. 1991) and adjusted for relative winter sunlight intensity (Lyman et al. 1982 quoted, Howard et al. 1991) aqueous photolysis t,/2 = 288-864 h, based on observed photolysis on soil TLC plates under summer sunlight (Helling 1976 quoted, Howard et al. 1991) and adjusted for relative winter sunlight intensity (Lyman et al. 1982 quoted, Howard et al. 1991). 

It we compare the simulated cross sections in Figures 8.1 and 8.2, we find that at energies higher than required tor photolysis via Eq. (1) ( 14000 cm ) the cross section trom vibrational transitions is very small. Similarly, in the region where there are photons in the stratosphere, up to 55900 cm , the cross section trom electronic transitions is also very low. Both these regions are likely to contribute to the atmospheric photolysis ot H2SO4, the region that dominates will depend on the actual cross sections, the altitude (photon flux) and the dynamics ot the dissociation reactions. 

Under atmospheric conditions where a range of wavelengths at A, > 290 nm are present then the atmospheric photolysis rate coefficient is best described by the following expression ... 

In many atmospheric chemistry models quantum yields are assumed to be unity, which, as shown in Table 1, is seldom the case. Thus it appears that die importance of photolysis in die degradation of the aldehydes and their contribution to radical formation in the atmosphere is often over-estimated. However, details concerning the production of radicals fi om the atmospheric photolysis of aldehydes are best obtained from mechanistic studies. 

Rattigan O.V., D.E. Shallcross and R.A. Cox UV absorption cross-sections and atmospheric photolysis rate of CF3I, CH3I, C2H5I and CHjIcl, J. Chem. Soc. Faraday Trans. 93 (1997) 2839-2846. 

As shown by Johnston and Selwyn (1975), the cross section of N2O varies strongly with temperature. The quantum yield for photodissociation is unity, and the products are N2 and 0(1D). The atmospheric photolysis rate comes predominantly from the absorption of solar radiation in the 02 Herzberg continuum and Schumann Runge bands. Jn2o 9 x 10-7s-1 for A > 175 nm. 

Formaldehyde is a first-generation oxidation product of CH4 and, it turns out, of many other hydrocarbons. Indeed, the chemistry of formaldehyde is common to virtually all mechanisms of tropospheric chemistry. Formaldehyde undergoes two main reactions in the atmosphere, photolysis... 

Formaldehyde undergoes two main reactions in the atmosphere, photolysis (see Table -T3). 

Atmospheric photolysis provides the predominant source of free radicals in the atmosphere. Photolysis of NO2 produces O atoms which form ozone ozone photolysis in the near UV produces 0( D) which reacts with H2O to produce OH radicals. A number of organic species absorb UV light and dissociate to yield organic peroxy and HO2 radicals in the presence of O2,... 

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