Numerical modelling of regional scale transport and photochemistry directly together with meteorological processes

A three-dimensional regional scale atmosphere-chemistry model has been developed to contribute to an improved understanding of atmospheric photochemical processes. This on-line model determines meteorological processes directly together with tracer transport and photochemistry. Usually, off-line chemistry-transport models are applied which use archived data from a meteorological model as input information. However, a number of disadvantages result from the seperation of meteorological and photochemical modelling: only a part of the whole meteorological information is available in the sampled data sets and only in distinct time intervals. The availability of the whole meteorological information including subgrid scale dynamical motions like turbulence and moist convection at every model time step is the main advantage of the on-line procedure. In addition, on-line modelling allows to consider chemical–dynamical feedbacks which is not possible in the off-line mode. For validation and evaluation studies a 10-days simulation of a summersmog episode over Europe in July 1994 has been carried out with the first version of the on-line model. The results are compared with observations and with an off-line model simulation. The on-line model is able to reproduce measured near-surface concentrations in much better agreement than the off-line model does. The reason is an improved representation of tracer transport in convective clouds in the on-line model. It is responsible for the upward mixing of short-living precursor substances of photooxidants from the planetary boundary layer into the free troposphere, with a significant impact on photooxidants concentrations in the planetary boundary layer as well as in the free troposphere.