The USFS Missoula Technology and Development Center's Watershed Soil and Air Program conducted a collocation study of real-time particulate monitors in the Missoula and Bitterroot Valleys during the large Montana/Idaho wildfires of 2000. The real-time particulate instruments use particle light-scattering (nephelometer) and light-absorption (aethalometer) principles to estimate particulate concentrations in real-time. Results from the 5 different real-time instruments were compared to gravimetric results from a collocated Federal Reference Method PM 2.5 Sampler.

Results from the collocation study indicate that the real-time instruments tend to overestimate particulate concentrations, sometimes by more than 2 times. However, the overestimated results from the real-time instruments were linear over the entire range of particulate concentrations (from less than 10 ug/m3 to more than 400 ug/m3 as calculated from the FRM sampler) so correction curves or equations can be established for each instrument.