For criteria air pollutants and precursors as well as for hazardous air pollutants, the methods are described in the technical support document (TSD) for the latest National Emissions Inventory (NEI) available from the main NEI page.

For greenhouse gas emissions, you can find greenhouse gas inventory methods from the latest “Inventory of U.S. Greenhouse Gas Emissions and Sinks,” which is available on the Sources of Greenhouse Gas Emissions page.