Combustion systems such as motor vehicles and power plants are major sources of fine particulate matter.  This talk describes some of the changes in fine particle emissions that occur as exhaust from combustion systems mix with background air.  This mixing cools and dilutes the exhaust which influences gas-particle partitioning of semi-volatile species, the aerosol size distribution, and the fine particle mass.  Dilution sampling is used to characterize fine particle emissions from combustion systems because it simulates the rapid cooling and dilution that occur as exhaust mixes with the atmosphere.  Results from dilution sampler experiments conducted using exhaust from a diesel engine, wood stove and full-scale coal boiler will be presented.  An absorptive partitioning modeling will be used to interpret some of the experimental data.  This model will then be used to examine the influences of background aerosol mass and temperature on gas-particle partitioning of emissions in the atmosphere, and the ultimate contribution of emissions to ambient aerosol loading.  The talk will conclude with a discussion of the implications of the results on approaches used to measure aerosol emissions and the treatment of these emissions in air quality models.