High temperature sample and low particle concentration: challenge for measurements

In engine exhaust particle measurements, the sample is usually warm. Sample temperature needs to decrease before it can be measure with real-time aerosol measurement instruments. By decreasing the sample temperature, there is possibility to have condensation and/or nucleation of water, sulphuric compound and VOC. When the aim of the research is to study solid exhaust particles this problem is typically handled with a dilution system that decreases the temperature and vapor pressures in two dilution stages including thermal treatment. However with low-emission vehicles, this might also lower the particle concentration below instrument detection limit. Also, despite the controlled temperature profile dilution might still affect the size, composition and structure of particles, and uncertainty in dilution factor might decrease the measurement accuracy. To avoid these issues a new version of the Electrical Low Pressure Impactor ELPI+T was developed for high temperature, low concentration measurement. In this system, the detector part, impactor, is heated to a sample temperature up to 180C, see Figure 1. This allows high temperature tailpipe aerosol measurement without dilution, without sample cooling and without VOC condensation. In other words, the particles are measured as they exist in the tailpipe. Figure 1. Operating principle of the High Temperature ELPI+T. Impactor is heated to be able to operate with high temperature sample The system was tested with a 2-litre passenger car engine that was modified for natural gas fuel, known to produce very low particle emissions. In the test setup, the exhaust gas composition was made adjustable by adding methane, propane, ethane and ethene to tailpipe-extracted side flow to mimic large bore engine (ship / power plant) emission composition. The developed High Temperature ELPI+T was used to measure the particle size distribution and concentration from tailpipe environment before catalysts. A bimodal particle number size distribution was detected, one mode at below 10 nm particle size range and the other mode at around 30 nm, see Figure 2. A volume-weighed size distribution also shows a large particle mode at around 750 nm. In this presentation, we describe the HT-ELPI+T and the engine and exhaust conditioning setup and present the particle measurement results from the system. We also discuss about the detected particles and their origin and present comparison results from other instruments (EEPS, SMPS and gravimetric filter measurement) that simultaneously measured diluted exhaust. Figure 2. Number size distribution measured with HT-ELPI+T.