Air-quality modeling using off-line or on-line techniques

A long tradition when performing air-quality simulations is to use already generated fields from a meteorological model. This means that interpolation has to be done in time and often also in space before the meteorological parameters are used for calculating the transport and mixing of the chemical species. This interpolation leads to errors, more interpolation – larger errors.

In this study, results are presented from a high-resolution mesoscale model with photochemistry included and everything calculated on-line. The model is applied for the highly polluted Athens area in Greece. The simulations date is September 14, 1994; one of the days during MEDCAPHOT-TRACE when the air-quality and meteorological field was monitored with about 15 meteorological and 20 chemical observational sites. The meteorological situation is characterized with a number of sea-breeze circulations.

The results are compared with simulations of the air-quality part of the code with interpolated meteorological fields. The interpolation intervals used are 1h, 3h and 6 h; no interpolation in space is done, the air-quality code is run on the same grid. Statistical calculations show that there are not very large differences in the correlation coefficients obtained for the off-line and the on-line results. But, very large differences are found when the number of hours over a limit and the area exposed to a certain concentrations. The results are examined using the EU regulations for ozone.