The Michigan Department of Environmental Quality announced last week that it has approved a permit for Marathon Oil Corp. to begin a $1.9 billion expansion of a Detroit refinery. Marathon agreed to install, operate, and maintain at least four air monitoring stations located in and around the refinery to chart emissions. Marathon plans to install equipment that will enable the refinery to process a new source of crude oil — “heavy oil” from Canadian tar sands — and increase capacity by about 15 percent.