Investigation and evaluation of the NOx/O3 photochemical steady state

Measurements of O3, NO, NO2, the photodissociation rate coefficient of NO2 (j(NO2)), and temperature made during the WAOWE’93 and ’94 (Weybourne Atmospheric Observatory Winter Experiment) campaigns on the north 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+NO2) were encountered during WAOWE’93. The PSS ratio, (j(NO2)[NO2])/(kNO+O3 [NO] [O3]), was found to equal unity within experimental error which resulted in good agreement between predicted and measured O3 concentrations and, in addition, implied the absence of peroxy radicals. Small deviations from the PSS ratio in the apparent absence of peroxy radicals indicated the presence of a temperature-dependent perturbation. Such a process could lead to artefact or enhanced PSS-derived peroxy radical concentrations at temperatures above 2°C. During the WAOWE’94 campaign, the PSS ratio reached values of up to two and calculated peroxy radical concentrations were found to be positively correlated with [O3] and negatively correlated with NOx, indicating that the observed perturbations from the PSS were at least partly caused by peroxy radicals. However, the levels of peroxy radicals predicted by the PSS were up to two orders of magnitude higher than those calculated using a simple zero-dimensional model. Previous studies of the NOx/O3 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 O3, NO2, NO or j(NO2), but is subject to a variety of potential interferences which invalidate its widespread use as a method for deriving atmospheric peroxy radical concentrations.