The estimation method for PM emissions from fuel combustion sources was enhanced beginning with the 1999 National Emissions Inventory (NEI). Prior to 1999, the PM estimates for stationary fuel combustion sources were only including the filterable portion of the PM (the solid particle matter at elevated stack temperatures), due to the available stack emission measurement data at the time.  The 1999 NEI (and beyond) includes both that filterable portion of PM and also the condensible portion of PM (gaseous matter at elevated stack temperatures that will condense to form particles once cooled to ambient air temperatures).