The U.S. Environmental Protection Agency today announced it has ordered the Mystic Station power plant in Everett to take immediate steps to reduce the amount of soot and other particulate pollutants coming out of its smoke stacks. Sithe Mystic, which owns the oil-fired power plant, is cooperating with EPA's compliance order. The order comes after years of complaints by Greater Boston residents about Mystic's smoke, which contains particulate pollution that can trigger asthma and other respiratory illnesses.