Non-Methane Volatile Organic Compounds (NMVOCs) are indirect greenhouse gases. Emissions of NMVOCs (e.g., olefins, ketones, and aldehydes) are the product of incomplete combustion. The most important sources of NMVOCs from fuel combustion activities are mobile sources and residential combustion, especially biomass combustion (e.g., firewood).

NMVOC emission levels are directly influenced by fuel used, usage patterns, technology type, and size, vintage, maintenance, and operation of the technology. The emissions are very low for large-combustion plants. NMVOC emissions tend to decrease with increases in plant size and increasing efficiency of the combustion process. Emission rates may also vary by several orders of magnitude for facilities that are poorly operated or improperly maintained, such as might be the case with older units.

Table 1-11. IPCC NMVOC Default Emission Factors

Sulfur dioxide (SO₂) is an aerosol precursor, and its presence in the atmosphere may have a cooling effect on the climate. SO₂ can react with a variety of photochemically produced oxidants to form sulphate aerosols. The concentration of these particles increases with the burning of fossil fuels that contain sulfur. Emissions of SO₂ are closely related to the sulfur content of fuels.

Table 1-12. IPCC Sample and Default Values of of Sulfur Content
in Fuel