While the goal of the National Emissions Inventory (NEI) program is to include all emissions data, there are always challenges meeting this goal. The answer about completeness of coverage of sources depends on the emissions sector and pollutant. In general, the criteria air pollutants and precursors (CAPs) are expected to be more complete, since those pollutants are required to be reported by state, local, and tribal air agencies. The hazardous air pollutants (HAPs) are not required to be reported, but many agencies do report them and the EPA further augments HAP data where it can. Thus, in general HAP emissions are expected to be less complete than the CAPs.
Industrial emissions sectors such as those starting with “Fuel Comb – Electric Generation” will be more complete because they are comprised of large point sources that are permitted by state, local, and tribal air agencies and are required to report emissions. Other examples of such sectors include “Industrial Processes – Cement Manuf”, “Industrial Processes – Petroleum Refineries”, and “Industrial Processes – Pulp & Paper”.
Other non-industrial sectors are estimated by the EPA and those estimates are used to fill in data when not otherwise available. An example of such a sector is “Fuel Comb – Residential Wood.” While estimated by EPA, the emissions for most of these sectors include many more assumptions about activity levels and other inputs used to estimate emissions at a county resolution. So, while these types of sources may be more complete, they may also have additional uncertainties as compared to the point source reported emissions.
Finally, some categories are not currently estimated by the EPA, but they are reported by some states. Historically, examples of such sources include composting, emissions from humans and domestic animals, cigarette smoke, and emissions of backup generators at cell phone towers. The EPA works to assess the significance of such sources to identify those with potentially significant emissions for which an estimation method could be developed in the future.