Photodissociation simplified models

A detailed analysis of the dynamics of polyatomic photodissociation presents a formidable problem. For this reason a number of simplified models have been introduced. Based on these models, one can obtain a qualitative description of the phenomenon, and of some of the important aspects. 

Uncertainties in Photochemical Models. The ability of photochemical models to accurately predict HO concentrations is undoubtedly more reliable in clean vs. polluted air, since the number of processes that affect [HO ] and [H02 ] is much greater in the presence of NMHC. Logan et al (58) have obtained simplified equations for [HO ] and [HO2 ] for conditions where NMHC chemistry can be ignored. The equation for HO concentration is given in Equation E6. The first term in the numerator refers to the fraction of excited oxygen atoms formed in R1 that react to form HO J refers to the photodissociation of hydrogen peroxide to form 2 HO molecules other rate constants refer to numbered reactions above. 

State of the art models of photodissociation regions today involve hundreds of chemical species, thousands of chemical reactions, and tens of physical and chemical processes in order to accurately describe the physical and chemical structure of a piece of gas illuminated by FUV photons. This degree of sophistication cannot yet be implemented in 3D dynamical models. Most PDR models are therefore steady-state and one dimensional, with ongoing developments towards simplified three... 

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