Inertial impaction of particulates upon bodies of simple geometry

Comparing pollen and spore counts collected with the Rotorod Sampler and Burkard spore trap

BACKGROUND

The Rotorod Sampler and Burkard spore trap are air-sampling instruments commonly used by allergists in the United States. Although both devices are volumetric, their principles of operation and particle recoveries differ.

OBJECTIVE

This review will develop some guidelines for interpreting and comparing pollen counts obtained with these instruments.

DATA SOURCES

Investigations examining particle recovery by each device will be reviewed. Five studies where the Rotorod and Burkard were operated in parallel will also be assessed.

RESULTS

The Rotorod's theoretical and empirical collection efficiencies are low for particles <10 microm but typically exceed 80% for particles above this threshold. This instrument has traditionally been considered insensitive to wind; experimental data present a mixed picture. The Burkard offers high collection efficiencies, particularly for small particles, when an aerosol's velocity is low. Bi-directional errors in collection efficiency occur as a function of increasing wind speed and particle size. Parallel trapping investigations demonstrated that the Burkard yielded a higher estimate of the atmospheric particle concentration for all particle sizes. Differences were widest for small fungus spores but narrowed for pollen-sized particles. Some recovery differences are readily explained by sampling theory. Other disparities may reflect over-sampling, under-sampling or each device's principles of operation.

CONCLUSIONS

Both instruments appear to record the same relative changes in airborne particle concentrations. The Burkard appears to be a superior instrument for sampling particles <10 microm. The Rotorod appears to be equal or superior to the Burkard for collecting particles >10 microm. A rough empirical means for comparing differences in particle recovery is presented.