Power Spectral Densities of Atmospheric Aerosol Particle Counts
Final rept. 2008-2009
ARMY RESEARCH LAB ABERDEEN PROVING GROUND MD COMPUTATIONAL AND INFORMATION SCIENCES DIR
Pagination or Media Count:
The velocity components of the atmospheric wind typically have a power spectral density PSD with a slope of -53 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 -53 slope in this frequency range. Atmospheric aerosol particles are also expected to behave as passive scalars, and PSD slopes of -53 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 10exp-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 -53. But the slopes of the PSDs of the aerosol particle counts are near -53 for only a small fraction of the subintervals generally the magnitude of the slopes is much smaller than -53 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 -76 is found over a large frequency range when the entirety of both of the four day periods is considered.
- Atmospheric Physics
- Physical Chemistry
- Atomic and Molecular Physics and Spectroscopy