The emission of ambient particulate matter that is less than 2.5 mum in aerodynamic diameter from commercial coal combustion sources may represent a greater risk of inhalation into human and animal respiratory systems than emission of larger particles. In addition, there is lower removal efficiency in flue gas particle collection equipment for these smaller particles that may also increase deposition in the downwind environment and subsequent. migration into the water table. Recent results suggest that pulverized coal fly ash particle formation is best described as a tri-modal particle size distribution that includes a submicron fume region, a fine fragmentation region centered at approximately 2.0 mum diameter, and a bulk fragmentation region. A fundamental understanding of the mechanisms leading to the formation of the fine fragmentation region and of how this formation influences toxic trace metal partitioning is an important step to mitigating the environmental impact of coal combustion. Results are presented related to some of the factors related to this issue. An extensive SEM examination of fly ash particles in the fine fragmentation region indicates that these particles appear to have a much larger effective surface area compared to supermicron particles due to irregularities such as fractures, stretching, and shedding. These particles also appear to be more reactive with oxy-anion trace elements, such as arsenic and selenium, which may be important in understanding the dominant mechanism related to trace element partitioning during pulverized coal combustion. (C) 2003 Elsevier Science B.V All rights reserved.