Export of Air Pollution from Southern Asia and its Large-Scale Effects

Abstract

Southern Asia is one of the most heavily populated regions of the world. In contrast to much of the rest of the world, biofuel and biomass burning account for a large fraction of the total energy production in southern Asia compared to fossil fuel burning. This results in air masses which are relatively rich in carbon-containing aerosols, carbon monoxide, and hydrocarbons. The export of these polluted air masses is characterized by two main seasonal circulation patterns: the summer and winter monsoon. During summer, the heavy rains in this region effectively remove soluble gases and aerosols. Less soluble species, on the other hand, are lifted to the upper troposphere in deep convective clouds, and are then transported rapidly away from the region by the strong upper tropospheric winds, particularly towards northern Africa and the Mediterranean in the tropical easterly jet. During winter, the near-surface flow is mostly northeasterly, and the regional pollution forms a thick haze layer in the lower troposphere which spreads out over thousands of km between southern Asia and the Intertropical Convergence Zone (ITCZ). This chapter reviews the many observational and modeling studies which have examined the southern Asian outflow and its large scale effects.