The MOGUNTIA Model.

Main working tool of our research is the 'MOGUNTIA' (Model of the Global Universal Tracer transport In the Atmosphere, Zimmermann, 1988). As one of the first time dependent global 3D chemical tracer model (CTM) it was designed in the 1970s to numerically simulate the transport of trace constituents in the global atmosphere from surface up to 100 hPa pressure level which represents an altitude of about 16 km. The model domain is resolved in a 10°´10°´100 hPa grid and the integration time step of 2 hr gives stable results. The large scale model transport is based on monthly mean observed temperature and wind fields (Oort, 1983; resp. ECMWF analysed meteorological data). Turbulent diffusion is parameterized proportional to the day by day deviations of the winds. Deep convection is performed explicitly according to observational occurrence of cumulus clouds (Feichter and Crutzen, 1989). The transport is routinely tested against observations using chemically inert tracers with different lifetimes such as F11, F12, 85Kr, and 222Rn (Zimmermann et al., 1988).

In the 1980s parts of the photochemistry module of the 2-dimensional CTM (Crutzen and Gidel, 1983) were coupled to the transport with the aim to simulate background air chemistry processes. In context with tropospheric ozone studies the transport simulation of the reactive species such as CH4, CO, NOx (= NO + NO2 ), HNO3, O3, H2O2, and CH3O2H is required while extremely short lived species ( e.g. OH) are assumed in photochemical steady state. The photochemistry part of the model includes the basic gas phase reactions as well as eventual dry and wet deposition parameterisations which these species undergo in the troposphere and lower stratosphere (Crutzen and Zimmermann, 1992).

In the early 1990s a scheme of heterogeneous night time reactions was developed (Crutzen and Dentener, 1992) and implemented into the MOGUNTIA Model. An economic version was put to use in the model release with which this study was performed, namely a precalculated parameterised pseudo reaction rate khet combining NO2 and O3 to 2 HNO3 during night-time in the presence of clouds. The khet field varies with time and in space and is based on cloud statistics (Lelieveld et al., 1989).

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