Photolysis in the Troposphere

In this section, among the chemical species that are photodecomposed in the troposphere, important molecules in atmospheric chemistry are discussed, with the exception of inorganic halogens, which are described in Sect. 4.4. 

The production of OH radicals by the reaction of H2O with the electronically excited oxygen atoms 0( D) formed in the photolysis of ozone (O3) is the most important reaction in the natural atmosphere to trigger tropospheric photochemistry. Here, the absorption spectrum and the 0( D) production quantum yields in the photolysis in the troposphere are described in detail. 

In addition, O3 has weak absorption bands called the Chappuis bands in the visible range and the Wulf bands in further longer wavelengths as shown in Fig. 4.2. These bands corresponds to the forbidden transitions to the lower electronically excited states that cross with repulsive potential curve dissociating into the ground states of an O atom and O2 molecule. 

Photodissociation Quantum Yields The wavelength thresholds below which photodissociation is energetically possible in the photolysis of O3 are shown in Table 4.3 (Okabe 1978). From the table, energetically possible photodissociation processes by the solar flux reaching to the troposphere are the following five processes, 

The 0( D) atoms formed in the photolysis of O3 reacts with water vapor, 

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The surface albedo can have a large effect on the total light available for photolysis in the troposphere. Calculate the factor by which the photolysis of H202 would increase at a solar zenith angle of 60° on December 1 over snow with a surface albedo of 80% compared to a normal best estimate surface albedo. 

Volatilization volatilization t,/2 4.0 h from a model river 1 m deep flowing 1 m/s with a wind velocity of 3 m/s, based on estimated Henry s law constant (estimated-P/C, Lyman et al. 1982 quoted, Howard 1990). Photolysis will not undergo direct photolysis in the troposphere (quoted, Howard 1990). 

CH3I -2 2/ 0.05 0.014 Ocean, marine biota Photolysis in the troposphere ... 

Unfortunately, the product quantum yields for cyclohexanone photodecomposition are insufficient to allow reasonable estimates of the y-values for its photolysis in the troposphere. However, it can be concluded that the unsaturated aldehydes, affected very little by oxygen addition, probably will be important products under these conditions. 

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