Direct measurements of the OH and NO2 evolutions in pulsed photo-oxidation studies of hydrocarbons

A novel method has been developed to study the OH and NO2 evolution in the laser pulse initiated photo-oxidation of hydrocarbons in NOx containing gas mixtures. In this method, long-path laser absorption (LPLA) for the time resolved detection of OH radicals in the (A2∑+ − X2∏) system at 308.417 nm is combined with cw-LIF for the time resolved detection of NO2 after excitation at 488 nm at pressures around 50 mbar. It is shown that simultaneous measurements of these two species represent a detailed and sensitive signature of the elementary processes that occur following pulse initiated oxidation of hydrocarbons with photolytically generated OH radicals. The information contained in such profiles together with detailed numerical simulations permit: (i) to extract and to validate rate coefficients of otherwise difficult to access elementary processes of alkylperoxy and alkoxy radicals, (ii) to determine the extent of chain branching and OH regeneration and (iii) to derive the overall number of NO/NO2 conversions (NOCON factors) in the complete oxidation chain of hydrocarbons. The latter are considered of substantial relevance to the assessment of individual hydrocarbons in the formation of ozone in photochemical smog mechanisms where they may be used to generate lumped oxidation schemes. The present paper describes the operation of the technique as well as the definition and derivation of NOCON factors. Because the species monitored (OH and NO2) are not specific for any individual hydrocarbon, the technique is considered of wide applicability. A first application of the technique to the pulse initiated oxidation of propane by OH radicals at T = 298 K is presented.