INVESTIGATION AND EVALUATION OF THE NOX O-3 PHOTOCHEMICAL STEADY-STATE/

Measurements of O-3, NO, NO2, the photodissociation rate coefficient o f NO2 (j(NO2)), and temperature made during the WAOWE'93 and '94 (Weyb ourne Atmospheric Observatory Winter Experiment) campaigns on the nort h Norfolk coast were used to evaluate the photochemical steady state ( PSS) as a method of deriving peroxy radical (HO2 + RO2) concentrations . Highly polluted air masses containing elevated levels of NOx (NO + N O2) were encountered during WAOWE'93. The PSS ratio, (j(NO2)[NO2])/(kN O + O-3 [NO][O-3]), was found to equal unity within experimental error which resulted in good agreement between predicted and measured O-3 c oncentrations and, in addition, implied the absence of peroxy radicals . Small deviations from the PSS ratio in the apparent absence of perox y radicals indicated the presence of a temperature-dependent perturbat ion. Such a process could lead to artefact or enhanced PSS-derived per oxy radical concentrations at temperatures above 2 degrees C. During t he WAOWE'94 campaign, the PSS ratio reached values of up to two and ca lculated peroxy radical concentrations were found to be positively cor related with [O-3] and negatively correlated with NOx, indicating that the observed perturbations from the PSS were at least partly caused b y peroxy radicals. However, the levels of peroxy radicals predicted by the PSS were up to two orders of magnitude higher than those calculat ed using a simple zero-dimensional model. Previous studies of the NOx/ O-3 PSS are evaluated in conjunction with the results obtained in this study. It is suggested that the PSS method may be used as a text-book case in limited conditions to estimate levels of O-3, NO2, NO or f(NO 2), but is subject to a variety of potential interferences which inval idate its widespread use as a method for deriving atmospheric peroxy r adical concentrations. (C) 1998 Elsevier Science Ltd. All rights reser ved.