Selected aspects of the chemistry of photochemical air pollution is discussed and some important, unresolved problems dilineated. The reactive species considered include NO,, 0,, O(3P), O( 'D), 02('AJ, OH and HO,. Both the kinetics and mechanisms of the reactions constituting the major tropospheric sources and sinks of these species are treated where available. The application of this information in both computer and smog chamber simulations of photochemical smog is discussed.In this system NO, is the major light-absorbing species[3! At wavelengths less than 430nm NO, dissociates forming NO and ground state oxygen atoms, O('P), (see Section 2):In air this is rapidly followed by: Chem internat. Edit. Vol. 14 (1972) J No. J * I ( 1 ) This is consistent with the NO, bond dissociation energy of 3.1 15 eV (h = 398 nm)[241. At wavelengths greater than 430nm, there is no O(,P) p r o d~c t i o n~~-~~-~~~; only photophysical processes occur.Bond rupture also occurs, however, at wavelengths between 398 and 430 nmr3.25. 26.291, as confirmed by a recent study [28].