A quasi--real--time calculation system for the global distribution of ozone (O3) and its precursors, including CO, NOx, and nonmethane hydrocarbons (NMHCs), has been newly developed by using a 3--D chemical--transport model. The model is driven by meteorological data from the National Centers for Environmental Prediction (NCEP), and produces daily 7--day forecast of the distribution of chemical species. The model was used in daily flight planning for the Pacific Exploration of Asian Continental Emission (PEACE)--A and --B aircraft measurement campaigns in January and April--May 2002, respectively. Model--calculated meteorological fields show good agreement with aircraft observations. The model also reproduced events such as polluted air masses in the lower troposphere (LT) corresponding to post--frontal outflow, a high--concentration CO plume in the upper troposphere (UT) in late spring, and the observed plume that was transported by deep convection over central China. The amount of CO transported into the free troposphere (FT) by deep convection was estimated to be about 6 Tg CO over China in May 2002. Meridional and seasonal variations in the long--range transport (LRT) of CO tracers, Asian CO tracers, and CO tracers produced from the oxidation of CH4 and NMHCs were all evaluated using tagged CO tracers. LRT CO comprised about 36% of the total CO budget in December--February 2001/2002, and about 20% in March--May 2002 in the free troposphere over Japan. In late spring, the concentration of Asian CO over southern Japan decreased compared to that in winter because the wind direction shifted from northwesterly to easterly or southerly.