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 improvedunderstanding of atmospheric photochemical processes. This on-line model determines meteorological processes directlytogether with tracer transport and photochemistry. Usually, o!-line chemistry-transport models are applied which usearchived data from a meteorological model as input information. However, a number of disadvantages result from theseperation of meteorological and photochemical modelling: only a part of the whole meteorological information isavailable in the sampled data sets and only in distinct time intervals. The availability of the whole meteorologicalinformation including subgrid scale dynamical motions like turbulence and moist convection at every model time step isthe main advantage of the on-line procedure. In addition, on-line modelling allows to consider chemical}dynamicalfeedbacks which is not possible in the o!-line mode. For validation and evaluation studies a 10-days simulation ofa summersmog episode over Europe in July 1994 has been carried out with the"rst version of the on-line model. Theresults are compared with observations and with an o!-line model simulation. The on-line model is able to reproducemeasured near-surface concentrations in much better agreement than the o!-line model does. The reason is an improvedrepresentation of tracer transport in convective clouds in the on-line model. It is responsible for the upward mixing ofshort-living precursor substances of photooxidants from the planetary boundary layer into the free troposphere, witha signi"cant impact on photooxidants concentrations in the planetary boundary layer as well as in the free tropo-sphere.(2000 Elsevier Science Ltd. All rights reserve