The relationship between secondary pollutants and their formation during long-range transport (LRT) and its contribution to air quality in East Asia were investigated through field observations and numerical simulations. At first, it was found that the tropospheric ozone level in this area was highest in the world with maximum values occurring in the spring. This was suggested to have been caused by two mechanisms: the stratospheric ozone intrusion into the troposphere, and the regional-scale LRT of anthropogenic pollutants and the tropospheric ozone formation caused by them. This was found to be associated with yellow sand events and to cause a simultaneous increase in spm and ozone concentrations. The elevated ozone concentration was predicted well by an Eulerian transport / chemistry / deposition model (STEM-II model, Carmichael et al., 1986), which quantitatively suggested the important role of the latter mechanism. In addition, a clear insight into chemical processed during LRT was presented, based upon the concentration changes of secondary pollutants and radicals (OH and RO2) along the route of LRT in central Japan.