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Mount Diablo Fire 3:40_MG_0239.JPG
Black or brown smoke means that something other than natural, untreated, wood and grass are  burning.  Something petroleum or chemical based, like paint or treated wood, is burning.  Here, smoke, both black and white, rises from the Mount Diablo fire, viewed from a city street in San Ramon, CA, seven miles away at 3:40 PM, Monday, Sept. 9, 2013.