Abstract
Ultrafine (< 100 nm) and nanoparticles (< 30 nm) are found in high concentrations on and near urban roadways. These particles are emitted by both Diesel and spark ignition (SI) engines. Recent on-road measurements made in Minnesota using a mobile aerosol laboratory and related engine laboratory measurements will be described. Size distributions measured on urban freeways show structures similar to those measured in the laboratory. High-speed urban freeway traffic produces very small particles and high number concentrations sometimes exceeding 106 part./cm3. On-road fuel specific emission factors for mixed gasoline and Diesel traffic are in the range of 10 14 1015 part./kgfuel . Measurements on urban roadways and in the laboratory show that roadway aerosol consists of particles in three distinct but overlapping size modes, a nuclei mode 3 to 30 nm diameter range, an accumulation mode 30 to 500 nm range, and a course mode consisting particles > 500 nm. These modes are formed by different mechanisms and at different times. Most of the particle mass is found in the accumulation mode. For a Diesel engine, this mode is formed early in the combustion process and consists mainly of carbonaceous agglomerates that adsorb hydrocarbons and sulfates as the exhaust cools and dilutes. For a Diesel or SI engine most of the particle number is found in the nuclei mode. It is not usually formed within the engine but instead forms from volatile precursors as the exhaust dilutes and cools. Nuclei mode particles are volatile, consisting primarily of heavy hydrocarbons and sulfuric acid. Size distribution measurements made with a thermal denuder or catalytic stripper upstream of the sizing instrument show that the nuclei mode is reduced in size suggesting that these particles are composed primarily of volatile material. In the laboratory, the magnitude of the nuclei mode is very to sensitive to dilution and operating conditions, prior exhaust system condition and fuel and lube oil composition. Changing test conditions can result in nuclei modes that differ in number count by orders of magnitude. Use of laboratory mini-dilution systems that allow systematic variation of parameters like dilution air temperature, residence time and dilution ratio show that the number of particles in the nuclei may change by two orders of magnitude with only modest changes in dilution conditions. On the other hand the accumulation mode is relatively insensitive to dilution conditions. The sulfur content of the fuel of influences the formation of the nuclei mode under some but not all engine conditions. It was found with a modern, large (300 kW) heavy-duty Diesel engine the number in the nuclei mode increases strongly with fuel sulfur at higher loads, but at lighter loads the mode is nearly independent of fuel sulfur cont ent. The accumulation mode did not show a significant dependence upon fuel sulfur. The independence of the nuclei mode of fuel sulfur at light load along with results from growth rate calculations; volatility measurements and TDPBMS studies suggest that hydrocarbons are a major constituent of the nuclei mode.