A NUMERICAL SCHEME FOR THE INTEGRATION OF THE GAS-PHASE CHEMICAL RATE-EQUATIONS IN 3-DIMENSIONAL ATMOSPHERIC MODELS

A numerical integration scheme, designed for the gas-phase chemistry m odule in a 3D regional tropospheric chemistry model, is presented. Spe cies in the chemical system are partitioned into ''slow'' and ''fast'' species, according to their lifetime under typical atmospheric condit ions. A Newton-Raphson iterative scheme (implicit) is used for the fas t species, while an explicit scheme is used for the slow ones. The hyb rid implicit/explicit approach is tested against the Gear integration scheme for several rural and urban conditions (with and without emissi on/deposition). Comparisons with an exponential approximation scheme a re also presented. The present integration scheme is shown to be accur ate and efficient. With a 30 min integration step, the implicit/explic it scheme is at least 10-times faster than the Gear integration (with an equivalent integration step).