Power Spectral Densities of Atmospheric Aerosol Particle Counts

Report No. ARL-TR-5064
Authors: Chatt C. Williamson, Steven C. Hill, Dennis M. Garvey, Michael L. Larsen, and Cheryl L. Klipp
Date/Pages: January 2010; 32 pages
Abstract: The velocity components of the atmospheric wind typically have a power spectral density (PSD) with a slope of -5/3 in an "inertial subrange" of frequencies, roughly from 0.01 to >10 Hz depending on the wind speed and stability. As passive scalars, atmospheric temperature and humidity have a relatively small direct effect on the winds, and they also typically have a -5/3 slope in this frequency range. Atmospheric aerosol particles are also expected to behave as passive scalars, and PSD slopes of -5/3 have been reported for some atmospheric aerosol measurements with a volume-based light-scattering instrument. Herein, we present aerosol measurements obtained with a single-particle optical counter over two four-day periods with concurrent sonic anemometer measurements of wind speed and temperature. PSDs of the aerosol particle counts, temperature, and winds are presented and compared in the frequency range 10-4 to 10 Hz for 5 min subintervals. As expected, the PSDs of the velocity components and of the temperature decrease with a slope near the predicted -5/3. But the slopes of the PSDs of the aerosol particle counts are near -5/3 for only a small fraction of the subintervals; generally the magnitude of the slopes is much smaller than -5/3 and often close to zero. The discrete nature of the particle counts results in a shot-noise floor, which limits the slopes in PSD that can be measured at low particle concentrations and high sample rates. Interestingly, an overall slope of -7/6 is found over a large frequency range when the entirety of both of the four day periods is considered.
Distribution: Approved for public release
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Last Update / Reviewed: January 1, 2010