Combustion of coal for power generation and industrial processes are the largest sources of energy-related SO2 emissions in Southeast Asia today, together accounting for around three-quarters of the total 3.9 million tonnes (Mt) of sulfur dioxide (SO2) emissions. Almost half of the SO2 emissions from coal-fired power generation occur in Indonesia alone – roughly matching its share of the regional coal-fired power generation fleet. Energy-related nitrogen oxides (NOX) emissions were 5.7 Mt in Southeast Asia in 2015, of which around two-thirds were the result of oil combustion, primarily in the transport sector. The number of private cars in the region more than doubled from around 10 million in 2000 to more than 20 million today, reflecting income growth, although two- and three-wheelers and public buses remain important sources of mobility and, hence, urban NOX emissions (Box 9.1). Fine particulate matter (PM2.5) emissions from the energy sector in Southeast