In recent years several fine dust monitors that are based on single-particle light-scattering have achieved equivalency certifications for the measurement of PM2.5 in ambient air. Some of them show a lower particle size limit of 0.3 µm, others even claim 0.12 µm (without showing an independant validation). I'm wondering how these devices perform in heavily traffic dominated environments, where SMPS and related submicrometer sizing instruments typically show particle size distributions that peak between 50 and 150 nm. I obvioulsy understand the diameter to the power of 3 relationship between particle number and mass, but wouldn't they miss most of the PM2.5 anyway? Has anyone done a detailed comparison of these comparatively affordable PM monitors against reference methods?
We have done similar studies in heavy traffic regions. As to the correlation between mass and number it is fine at r 2 = 0.80 in ambient regions. but for traffic regions, correlation is weaker at r 2 = 0.45. We did not find a problem with the monitors in traffic regions.
There is no "reference method" for an optical measurement of PM, which is a challenge in reporting results of these devices. Overall, the Particle Number Concentration (PNC) is not well correlated with PM2.5 mass concentration in traffic-dominated areas. It is better correlated with NOx. This is likely due to small particles with little mass being emitted from combustion (which also emits NOx). However, all of this is very sensitive to meteorology and the nature of local emissions. See these papers:
Article Land use regression modeling of ultrafine particles, ozone, ...
Article Two-Years of Fine and Ultrafine Particles Measurements in Rome, Italy
Dear Oliver, we used optical analyzers (based on light scattering) in Rome, in order to carry out studies related to emissions and diffusion of traffic pollutants, for particulate matter.
The instrument is a MetONE which can misure and record concentrations of PM1- 2.5 - 7- 10 and TSP (total suspended particulate from about 0.1 micrometers to 200 micrometers). You have to consider that these category instruments are based on empirical priciples. In detail, the manifacturer build an equation that correlates variations in light absorbance/scattering of the air sample to concentration of particles. The problem is that all the calibrations/references are done with particles with a average density. This means that if you are going to measure particles characterized by a specific weight different from the reference particles calibration you can have errors in your data, sometime significant.
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