Emissions from home firewood burning were the largest source of PM2.5 in 2013, representing 165 kt (54% of the national total excluding open sources = 306 kt). Industrial sources excluding oil and gas industry (Other industries) emitted the next-largest proportions of national PM2.5 emissions, representing 18% (54 kt), followed by transportation (road, rail, air, marine) with 11% (33 kt). Between 1990 and 2013, industrial sources excluding oil and gas industry had the largest reduction (140 kt) in emissions.