AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
Investigation of Poissonian Sampling Behavior for Nanometer-Sized Aerosols
BRIAN DAMIT, Chang-Yu Wu, Meng-Dawn Cheng, University of Florida
Abstract Number: 136 Working Group: Aerosol Physics
Abstract The Poisson distribution is traditionally believed to describe how aerosols are sampled. Given a constant aerosol concentration, it is assumed that a Poisson process describes the fluctuation in the measured concentration because aerosols are stochastically distributed in space. Practically, the Poisson distribution relays confidence in a concentration measurement. Recent studies, however, have demonstrated that sampling of micrometer-sized aerosols has non-Poissonian behavior with positive correlations. The validity of the Poisson assumption for nanometer-sized aerosols has not been examined and thus was tested in this study. Four particle sizes - 10 nm, 25 nm, 50 nm and 100 nm - were first sampled from indoor air with a DMA-CPC setup to obtain a time series of particle counts. Truncated time series which showed constant underlying concentration were found and five metrics were calculated from the data: pair-correlation function (PCF), time-averaged PCF, coefficient of variation, probability of measuring a concentration at least 25% greater than average, and posterior distributions from Bayesian inference. To identify departures from Poissonian behavior, these metrics were also calculated for 1,000 computer-generated Poisson time series with the same mean as the experimental data and then compared with the experimental metrics.
For nearly all comparisons, the experimental data fell within the range of 80% of the Poisson-simulation values. Essentially, the metrics for the experimental data were indistinguishable from a simulated Poisson process thus supporting the validity of the Poisson assumption for nanometer-sized aerosol sampling. Brownian motion, which is more influential for nanometer-sized aerosols, may explain the Poissonian behavior observed here and not for larger aerosols. Although the Poisson assumption was confirmed to be valid in this study, it must be carefully applied as the results here do not definitively prove its use in all aerosol sampling situations.