A zero-dimensional model (box model) was developed for the simulation of diurnal variations of minor constituents in the atmosphere, using a new technique well adapted to the integration of stiff ordinary differential equations. The aim of developing this code is the easy coupling with any multi-dimensional dynamical model, especially 3D models using the on-line time-splitting method. The integration scheme is a semi-implicit symetric method (SIS) which is twenty times faster than Gear's scheme and has a comparable accuracy for stiff problems. In a closed system, this method strictly conserves the number of atoms, avoiding the need for any corrections due to long integration. The code accepts any photochemical system as input. In a first application, the box model is coupled with a steady-state one-dimensional model, including NOY, HOX, Cl(Y), O(X) and Br(Y) families to study the effect of increases in bromine species on stratospheric ozone. To this end, the ground-mixing ratio of the source species CH3Br is increased from the present level 20 pptv to 100 pptv. The maximum decrease of ozone abundance then occurs at 19 km. This decrease is 9% if the expected low value of HBr concentration is assumed, but it could be reduced to 4% if the concentration of HBr is higher than expected, as recently measured. A possible explanation of the high HBr concentration is that HBr and O3 are products of the reaction BrO + HO2.