AN OBSERVATIONAL STUDY ON THE EFFECTS OF TIME AND SPACE AVERAGING IN PHOTOCHEMICAL MODELS

The effect of time and space averaging in photochemical models is studied using surface and aircraft observations. These measurements relate to concentrations of O3, NO and NO2 sampled in The Netherlands during 2 days in September 1989. In order to use the most suitable technique to estimate the subgrid effects, two different methods are used to calculate the surface mean values over the whole of The Netherlands: a station average and a weighted average. The station average concentration is equal to the arithmetical mean of the observed concentrations at the stations, whereas the weighted average uses a weight function to take into account the representativity of the observed concentration at a station for its surroundings. The subgrid effects, which appear due to time and space averaging of second-order chemical reactions, are described using the intensity of segregation, which is the concentration fluctuations covariance normalized by the product of the average concentrations. The highest values for the intensity of segregation are found at the surface in daytime. These are closely related to the horizontal inhomogeneities in the NO and NO2 emissions. From the aircraft observations values up to 10% are obtained for the intensity of segregation. The energy density spectra of O3 and NO2 from the aircraft observations show high energy contents for wavelengths larger than 5 km. The co-spectrum of O3 and NO is calculated for one flight segment. It shows that these species are mainly negatively correlated for wavelengths between 1 and 50 km. For a model with a typical grid size between 20 and 100 km, this means that the correlation between O3 and NO takes place mainly on subgrid scales and therefore should be parametrized and included on photochemical models.