Why do the PM2.5 emissions for the electric utilities source category jump up between 1998 and 1999?

The estimation method for PM emissions from fuel combustion sources was enhanced beginning with the 1999 National Emissions Inventory. Prior to 1999, the emissions measurement data methodology only included the filterable portion (which means the solid particle matter at elevated stack temperatures).  Beginning with the 1999 NEI, both the filterable and the condensable portions of PM (gaseous matter at elevated stack temperatures that will condense to form particles once cooled to ambient air temperatures) are included.