The modality of particle size distributions of environmental aerosols

Knowledge of the distribution of airborne particulate matter into size fractions has become an increasing area of focus when examining the effects of air pollution. While total number and mass concentrations may play an important 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 detailed analysis of the modal characteristics of a large number of particle size spectra collected over a period of three years for a range of ambient aerosol types. Measurements of over 6000 size distributions in the size range 0.016–30 μm were made using a scanning mobility particle sizer and an aerodynamic particle sizer for various ambient aerosols including: traffic influenced, urban, vegetation burning influenced, marine, modified background and suburban. Advanced data analytical procedures were adopted to combine the distributions from the two instruments for the calculation of the volume size distributions to allow clear interpretation of the modal characteristics. It was determined that, while in most cases there is a distinct nuclei mode in the number size distribution, this does not translate to a nuclei mode in the volume size distribution. Furthermore, while many of the number size distributions were different for each aerosol studied, the volume distributions were similar. This finding has serious implications for the setting of mass-based air quality standards.