PASSIVE AEROSOL SAMPLER
Jeff Wagner and David Leith
A miniature, passive aerosol sampler has been developed. The device can sample for periods of hours to weeks, is inexpensive and easy to operate, and has potential utility as a personal sampler. Scanning electron microscopy (SEM) and automated image analysis are used to count and size collected particles with dp > 0.1 micrometers. Alternatively, more advanced microscopy techniques can be used for ambient-pressure analysis or elemental characterization. The measured flux and a deposition velocity model are used to estimate the average mass concentration and size distribution over the sampling period. The deposition velocity model consists of a theoretical component and an empirical component. The theoretical component can be approximated by the simple terminal settling velocity in many cases.
Wind tunnel experiments were performed on the passive sampler using a high-output aerosol generator and an eight-stage impactor as a reference sampler. The empirical portion of the deposition velocity model, ?m, was determined as a function of particle size by minimizing the sum-of-squares difference between impactor and passive sampler across all size bins and experiments. The relatively simple correlation is a function of the particle Reynolds number only. Precision was assessed by running three passive samplers in each experiment, yielding CVPM2.5 = 18.1% and CVPM10 = 32.2%. If SEM is used, the passive sampler will exhibit some error when sampling volatile aerosols.
Field tests were conducted in a well-ventilated occupational
environment with coarse, high-concentration aerosols. Measured
friction velocities were less than 0.4 m/s, a range in which passive
sampler performance does not depend on turbulence. Passive sampler
results correlated well with those of eight-stage impactors, with R2 =
0.80 and 0.93 for PM2.5 and PM10. Average disagreement between the
passive samplers and the impactors was –31 and 41% for PM2.5 and PM10.
These discrepancies were attributed to the small amount of fine
particles present, hygroscopic particles, and particle bounce in the
impactors. The average CVPM10 for all samples was 20%. The average
CVPM2.5 for non-hygroscopic samples was 16%. The average CVPM2.5 for
hygroscopic samples was much higher, 59%; water losses in these samples
created ill-defined particle boundaries which led to imprecision.
|last updated on: 10/27/08|