National Park Service's Stack Filter Unit Network (NPS-SFU) 

This network was the predecessor to IMPROVE.  The network consisted of 80 monitoring sites which collected particulate samples in Class I areas throughout the United States.  The network operated from 7/79 to 11/93 with monitoring sites coming on and off line throughout this time period.  The network used two stage stacked filter samplers collecting fine (< 2.5 mm) and coarse (> 2.5 mm) particulate samples over a 72 hour sampling period from 7/79 -5/86 and 24 hour sampling period from 6/86 - 11/93.  The samples were analyzed for PM2.5 mass and its elemental constituents and light absorption and PM coarse mass and its elemental constituents.