Factors Causing Differences in Emissions Among Years

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 calculated in 1999, 2002, and 2005 (163, 172, and 180, respectively) was about twice that in 1996 and 1997 (86 and 87, respectively).

Second, the coverage of nonpoint source categories has been expanded. There are 38 nonpoint source categories (see Nonpoint Sources for more detail) in the 2005 inventory, up from 33 in 2002.

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 emission factor used in the inventories since 1997. The sum of emission factors for 16 PAHs in the 1999 inventory and after are 24% and 35% of the value in the 1997 and 1996 inventories for residential wood burning - certified, catalytic stoves, respectively.

Fourth, different emission models were used for mobile sources. For example, MOBILE 6.2 was used for onroad vehicles in the 1999 inventory and after, but MOBILE 5b was used in the 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.

Finally, it would be extremely difficult and the MPCA does not have the capacity to attempt to revise past-year emission inventories to conform with newer factors and assumptions.

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