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Particle size distribution (PSD) in the downwind plume was analyzed for the study of its impact on ambient particulate matter (PM) monitoring. PSD in the downwind plume varies due to the gravitational settling. Gravitational settling has greater impact on downwind PSD to those source PSDs with larger mass median diameter (MMD). The change of PSD is a function of source PSD of PM emitted, wind speed, and downwind distance. Both MMD and geometric standard deviation (GSD) in the downwind plume decrease with increase of downwind distance and source MMD. The larger the MMD of the source, the greater the change of MMD and GSD in the downwind plume. Also, the longer the distance, the greater change of MMD and GSD in the downwind plume.

Variations of PSD in the downwind plume have significant impact on PM10 sampling errors associated with EPA PM10 sampler. The PM10 over-sampling rate increases with increase in downwind distance caused by the decrease of GSD of PSD in the downwind plume. Gravitational settling of particles does not help reduce over-sampling from problems associated with EPA PM10 sampler. Over-sampling rates decrease with increase of the wind speed.