The modality of particle size distributions of environmental aerosols

Knowledge of the distribution of airborne particulate matter into size frac tions has become an increasing area of focus when examining the effects of air pollution. While total number and mass concentrations may play an impor tant role in exposure and risk assessment analyses, often an understanding of the particle size distributions provides more information on the type of atmospheric processes resulting in the distributions. The modality of the particle size distribution is one such aspect that has been associated with the aerosol formation mechanisms. The aim of this work is to provide a det ailed analysis of the modal characteristics of a large number of particle s ize spectra collected over a period of three years for a range of ambient a erosol types. Measurements of over 6000 size distributions in the size rang e 0.016-30 mu m were made using a scanning mobility particle sizer and an a erodynamic particle sizer for various ambient aerosols including: traffic i nfluenced, urban, vegetation burning influenced, marine, modified backgroun d and suburban. Advanced data analytical procedures were adopted to combine the distributions from the two instruments for the calculation of the volu me size distributions to allow clear interpretation of the modal characteri stics. It was determined that, while in most cases there is a distinct nucl ei mode in the number size distribution, this does not translate to a nucle i mode in the volume size distribution. Furthermore, while many of the numb er size distributions were different for each aerosol studied, the volume d istributions were similar. This finding has serious implications for the se tting of mass-based air quality standards. (C) 1999 Elsevier Science Ltd. A ll rights reserved.