This study aimed to characterize the aerosol emission from motorcycle engine, and to explore the feasibility and necessity of adopting the PM2.5 size-selective sampling convention. A dynamometer was used to control a four-stroke motorcycle engine to generate stable exhaust aerosols. Several aerosol size spectrometers including a scanning mobility particle sizer, an electrical low pressure impactor, an engine exhaust particle sizer, and a multi-orifice uniform deposition impactor, were used in this work to measure the aerosol number concentration and size distribution. A PM2.5 cyclone was employed as the size-selective device. An aerodynamic particle sizer was used to verify the cut-off size of this pre-separator. A sampling chamber was used as an adaptor connecting to the tailpipe. This system allowed the emitted aerosol particles to coagulate for up to 2 seconds when the engine was set at idle condition, so that the number concentration could be reduced to 106 particle/cm3 to lower the effect of aerosol coagulation. A dilution system was made and mainly used reduce the relative humidity of cooled exhaust air to avoid the waster condensation. This diluter also helped to bring the aerosol concentration down to the working range of some aerosol instruments.
The results showed that the aerosol number concentration decreased from 1.5×107 #/cm3 at tailpipe exit with the distance downstream of the sampling chamber to 106 #/cm3. However, the mass concentration remained unchanged, indicating that the decrease in number concentration was mainly due to aerosol coagulation. The water content in the exhaust air was higher than the saturated vapor pressure when the exhaust air cooled down along the exhaust pipe. Therefore, the use of diluter was essential to eliminate the contamination and interference caused by waster condensation. The use of SMPS and EEPS was limited because of the pre-impactor equipped upstream of the sampling line. The ELPI appeared to be an ideal aerosol instrument for investigating the PM2.5 of motorcycle engine exhaust from the working size range manufacturer claimed, but the mass concentration converted from ELPI’s number concentration did not match with the filter weighing data. Therefore, the ELPI performance needs to be re-validated. For the time being, the use of TEOM with a PM2.5 pre-separator was recommended. The use of opacity together with PM2.5 separator is also a potential option if the device is well designed and engineered. The filter samples showed that PM2.5 accounted for over 90% of total aerosol mass concentration.