The OFIS model (Ozone Fine Structure) belongs to the European Zooming Model (EZM) system, a comprehensive model system for simulations of wind flow and pollutant transport and transformation. OFIS was developed to allow authorities to assess urban air quality by means of a fast, simple and still reliable model, capable for estimating the urban subgrid effect on pollution levels.
OFIS being closely related to the 3D photochemical dispersion model MARS/MUSE, simulates concentration changes due to the advection of species and chemical reactions in each cell of the computational domain. The concentration values outside this domain are assumed to coincide with the regional background concentrations used for the calculation of the boundary conditions. The computational domain of the model consists of a two-layer gridded strip with a length of 240 km and a width defined by the city size, with the city in the centre. The strip is oriented along the prevailing wind direction, altering direction when the meteorological input is modified (every 3 hours). The model simulates separately each day of, typically, one year.
From the overall concept of OFIS it is apparent that the model aim is not to accurately reproduce concentration patterns in any location over the whole year, but rather to correctly report when and to what extent high concentration levels or a regulated threshold exceedance occur at an urban location or a spot downwind the city.