Distribution of optical properties among atmospheric submicrometer particles of given electrical mobilities

Abstract

With the serial combination of a differential mobility analyzer (DMA) and an optical particle counter (OPC) the optical state of mixture in the suburban aerosol at Tsukuba, Japan, was explored. At mobility diameters between 200‐ and 400‐nm broad optical signal distributions were measured which indicated optically external mixtures, i.e., particles with a given electrical mobility exhibiting different optical properties. On average ∼50% of the analyzed particles were classified as optically external mixtures. The average measured distribution of optical signals was combined with the average size distribution to calculate an estimate of the integral scattering properties of the submicrometer aerosol for the measured state of optical mixture. This estimate yields 71% of the scattering coefficient of a size distribution in which all particles of a given size have the optical properties of ammonium sulfate. However, for extended periods of time this percentage was as low as 41%. With a nonspherical particle model number fractions of nonspherical particles have been retrieved from the OPC signal distributions. The results of the present study demonstrate that the combination of DMA and OPC can quantify the optical state of mixture of submicrometer particles with strong consequences for the quantification of their integral scattering coefficients. Conversely, the study indicates the limitations of conventional DMA size distributions measured in urban aerosols with number fractions of nonspherical particles on the order of 50%. DMA penetrations are dependent on particle shape. Thus information on particle shape appears to be a necessary prerequisite for interpreting DMA measurements.