Summary

Atmospheric brown cloud (ABC), a frequently occurring phenomenon in many regions of the world, is a regional scale plume of air pollution that consists of a mixture of anthropogenic sulfate, nitrate, organics, black carbon, dust and fly ash particles and natural aerosols, such as sea salt and mineral dust. Greater awareness of the ABC problem was generated by the 1999 Indian Ocean Experiment (INDOEX) initiative over the North Indian Ocean region. Global concern came about following the first preliminary report on ABC, which was based on INDOEX. The global implications of ABCs were highlighted by the United Nations Environment Programme in 2002 (UNEP and C 4 , 2002). In-situ measurements revealed that the main sources of ABCs are anthropogenic (for example, biomass open burning, biofuels and fossil fuel combustion).