US EPA

In the early 1970s, it was understood that combustion particles were formed mostly in sizes below 1 um diameter, and windblown dust was suspended in sizes mostly above 1 um diameter. However, particle size distribution was thought of as a single mode. Particles were thought to form and grow until they became large enough to fall out. When log-probability plots of size distribution were replaced by differential plots, it became clear that both impactor measurement and individual particle-counting techniques yield a bimodal distribution of fine and coarse particles. Somewhat later, particle-counting techniques showed that the fine mode could be divided into nuclei and accumulation modes. Fine and coarse particles have different formation mechanisms, sources and composition, as well as different physical and chemical properties. These differences also lead to differences in toxicologic properties and health effects. Epidemiologic studies that measure fine particles separately tend to show stronger and more statistically significant health effects. The many differences between fine and coarse particles support a separate standard for fine particles and make it essential that new research and monitoring studies measure fine and coarse particles separately rather than combined in PM10.

Wilson, W. The Bimodal Distribution: Development Of The Concept Of Fine And Coarse Particles As Separate And Distinct Components Of Airborne Particulate Matter. Presented at Annual Meeting of the Chinese Assoc. for Aerosol Research, Taiwan, Taipai, October 30-31, 1999.