A number of factors cause differences in emissions from year to year.

First, the number of pollutants in the emission inventories has increased, particularly for nonmetal compounds (excluding polycyclic aromatic hydrocarbons or PAHs). The number of nonmetal compounds with emissions in 1999 (131) was about twice that in 1996 and 1997 (58 and 59, respectively).

Second, the coverage of area source categories has been expanded. There are 25 area source categories (see Area sources for more detail) in the 1999 inventory, up from 16 in 1996.

Third, emission estimation methods, emission factors, and activity data have been improved. For example, the chromium emission factor for residential natural gas combustion used in the 1996 inventory is 45 times higher than the revised one used in the 1997 and 1999 inventories. The sum of emission factors for PAHs in the 1999 inventory is 35% of the value in the 1997 and 1996 inventories for Residential Wood Burning - Certified, Catalytic Stoves.

Finally, different emission models were used for mobile sources. For example, MOBILE 6.2 was used for onroad vehicles in 1999 inventory, but MOBILE 5b was used in 1996 and 1997 inventories. MOBILE 6.2 was developed with an updated understanding of emissions. It also incorporated new and improved data and new regulations. Consequently, MOBILE 6.2 provided results different from MOBILE 5b.

Therefore, the emission inventory results should not be used as a trend analysis.