Scientists believe that soot (black carbon) is having twice as much of a warming affect than first thought. This would rank it second only behind carbon dioxide and ahead of methane. The particles can be released from burning wood and from diesel engines. The amount of black carbon being released is increasing in many developing countries, but is also significant in many developed countries like the US and Canada. However, many scientists argue that the contribution of black carbon is complicated, because it can also cause cooling
http://www.bbc.co.uk/news/science-environment-21033078