Also known as the "Great Pall Of China", the brown cloud is extensively fueled by China's coal fire problem.
Each year in China, an estimated 100-200 million tons of coal is lost to spontaneous combustion - five to ten times the amount of Chinese coal export. The fires are very difficult to put out, as they are spread over a 5000 km by 700 km area. Some of the fires are estimated to date back to the Pleistocene Era.
The fires are thought to contribute 2-3% of the total world carbon dioxide production due to fossil fuels and significantly add to the brown cloud.
Source: International Institute for Geo-Information Science and Earth Observation, http://www.itc.nl/