The atmosphere is an extremely thin shell of atmosphere around the Earth. Nevertheless, there's plenty of air mass out there (5 x 10¹⁵ metric tons) that to produce significant pollution concentrations globally, you would need gigatonnes of pollutant to produce ppm mixing ratios (which is actually easier to achieve than it sounds, which is one reason why we have a global air pollution problem). Since air pollution is usually emitted by point sources and the pollutants must disperse away from these points, it is easy to create significant pollution concentrations on the local scale, which is all that matters initially because we breathe the local air supply, not the air on the other side of the Earth.