A mobile lab detected a plume with elevated levels of mercury in the air downwind of the Noranda Alumina plant in St. James Parish last month, according to a state Department of Environmental Quality report. The Mississippi River complex, which converts bauxite ore into alumina, a precursor for aluminum, has been under scrutiny since revelations earlier this year that the facility may have been releasing minute amounts of mercury into the air unexpectedly for decades without a permit.