This book deals with a detailed study of atmospheric air pollution problems using different numerical models. Comprehensive time dependent two dimensional advection-diffusion numerical models for primary and secondary pollutants of an urban area source are presented in this book. Primary pollutants are emitted from the area source of urban city extending upto l = 6 km and source free region beyond l i.e X0=12 km. Secondary pollutants are formed by means of first order chemical reaction of primary in droplet phase. The different removal mechanisms of the primary and secondary pollutants have been taken into account through different parameters in these models. These removal mechanisms include (i) removal by chemical reaction of chemically reactive primary pollutant (ii) removal by means of gravitational settling velocity of larger particles (iii) removal by wet deposition by means of rainout/washout (iv) removal by dry deposition which is due to absorption by earth’s surface terrain (v) leakage of some amount of pollutants through the mixing height of atmospheric boundary layer.