SEM stub monitor for particulate matter
In field testing, these samplers have been demonstrated to vary (CV) only 11.6% from Federal Reference Methods, when measuring course particulates (PM10-2.5), making this the most precise published passive particle monitor design [7]. Public Lab is evaluating the deployment of this technology.
SEM Stub Monitor Documentation:
Capabilities
Passive particle monitors must be deployed for 7-day periods to get accurate averages, unless airborne concentrations are above ~40μm/m3. Roughly 300 particles must be collected of a given size range to get an accurate count [8].
We are focusing development on analysis with visible light microscopy, which has demonstrated variation from FRMs of 11.6% for PM10-2.5 [7], but methods for measuring PM2.5 and ultrafine particles with a Scanning Electron Microscope (SEM) have been demonstrated [10, 12].
Current Progress
Next steps
Background
Developed originally for indoor dust monitoring at the University of North Carolina by Jeff Wagner and David Leith, this tiny monitor (in the middle of the housing, below) consists of a fine mesh cap over top of a Scanning Electron Microscopy (SEM) pin stub, a small aluminum object that looks like a pin. Thomas Peters and Darrin Ott at the University of Iowa added a wind-and-rain housing so the monitors can be used outside. They also added a glass microscope slide cover on top of the stub, allowing lower-cost analysis with a standard visible-light microscope.
Function
Early testing by Wagner, et al. demonstrated that there was a high correlation between results from the passive sampler and impactor sampling in indoor environments [2, 4]. The wind tunnel used is described in [4]. Windy environments would require wind speed measurements to correct against.
Enclosures for outdoor operation
Thomas Peters and his team were interested in using passive monitors for outdoor testing that would obviate the need for wind speed data. They developed a housing [6] and rigorously tested it in relation to optical and filter-based dichotomous samplers in wind tunnels and field tests [6, 7, 8]. The friction between the two plates slows air down and normalizes the deposition of particles.
Wagner adapted a version of this flat plate system [10]:
Bibliography
Wagner, Jeff. Passive Aerosol Sampling. PhD Thesis. UNC Chapel Hill (2000).
Jeff Wagner and David Leith. Field tests of a passive aerosol sampler Aerosol Science 32 33-48 (2001)
Jeff Wagner and David Leith. Passive Aerosol Sampler. Part I: Principle of Operation. Aerosol Science and Technology 34: 186– 192 (2001)
Jeff Wagner and David Leith. Passive Aerosol Sampler. Part II: Wind Tunnel Experiments. Aerosol Science and Technology 34: 193– 201 (2001)
Darrin K. Ott, William Cyrs, Thomas M. Peters. Passive measurement of coarse particulate matter, PM10-2.5. Aerosol Science 39 156 – 167 (2008)
Darrin K. Ott, Naresh Kumar, Thomas M. Peters. Passive sampling to capture spatial variability in PM10–2.5. Atmospheric Environment 42 746–756 (2008)
Jeff Wagner, Kinnery Naik-Patel, Stephen Wall, Martha Harnly. Measurement of ambient particulate matter concentrations and particle types near agricultural burns using electron microscopy and passive samplers. Atmospheric Environment 54 260-271 (2012)
Peters, Grassian et al. Single-Particle SEM-EDX Analysis of Iron-Containing Coarse Particulate Matter in an Urban Environment: Sources and Distribution of Iron within Cleveland, Ohio. Environmental Science and Technology 46 4331−4339 (2012)
Maiko Arashiro and David Leith. Precision of PM measurements with the UNC passive aerosol sampler. Journal of Aerosol Science 57 181–184 (2013)
Jeff Wagner and Gary Casuccio. Spectral imaging and passive sampling to investigate particle sources in urban desert regions. Environmental Science: Processes & Impacts 16 1745 (2014)