Determination of the Coarse Mode of the Atmospheric Aerosol Using Data from a Forward-Scattering Spectrometer Probe

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Abstract
The coarse mode of the atmospheric aerosol, containing mostly particles larger than 1 μm in diameter, can conveniently be measured by means of an optical-forward-scattering spectrometer probe mounted on an airplane. Although the instrument is able to count single particles, at least 10,000 particles have to pass the sensitive volume in order to reduce errors due to statistical fluctuations of the counts, especially in the bins of the larger particles. The fitting of a lognormal curve to the measured particle counts is possible by means of a least-squares technique, described in this paper. The quality of the fit can be examined by determination of the best fit for the particle number distribution and the particle volume distribution, and an intercomparison of the two. Data for atmospheric samples show geometric mean diameters for the number size distribution between 1 and 2.5 μm and standard deviations between 1.7 and 2.5. Aerosols dominated by one source have a small standard deviation. Standard deviations larger than 2.5 are an indication of an aerosol coming from several sources and in many cases a good fit can be obtained by using two lognormal distributions.