Nitrogen Oxides (NO_x) are indirect greenhouse gases. Fuel combustion activities are the most significant anthropogenic source of NO_x. Within fuel combustion, the most important categories are Energy industries and Road transportation. Generally two different formation mechanisms can be distinguished:

• Formation of "fuel NO" from the conversion of chemically bound nitrogen in the fuel
  
• Formation of "thermal NO" from the fixation of the atmospheric nitrogen in the combustion process

Table 1-9. IPCC NO_x Default Emission Factors

Carbon monoxide (CO) is an indirect greenhouse gas. The majority of CO emissions from fuel combustion come from motor vehicles. Small residential and commercial combustion activities are also large contributors to CO emissions. Carbon monoxide is an intermediate product of the combustion process.

The formation mechanism of CO is directly influenced by usage patterns, technology type and size, vintage, maintenance, and operation of the technology. Emission rates may vary by several orders of magnitude for facilities that are poorly operated or improperly maintained, such as might be the case of older units.

Table 1-10. IPCC CO Default Emission Factors