10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Sizing Sub-10 nm Particles from Engine Emissions
JOONAS VANHANEN, Jenni Alanen, Kati Lehtoranta, Sanna Saarikoski, Minna Väkevä, Topi Rönkkö, Airmodus Ltd.
Abstract Number: 716 Working Group: Combustion
Abstract It has been shown that a big fraction of the particle number concentration of an engine emission can be in sub – 10 nm size range (Rönkkö et al. 2017). The measurements conducted by Alanen et al. (2015) have indicated that some of these particles have a solid core as small as 2 nm in diameter. In this study, we measured nanoparticle number concentration and mean diameter from a natural gas engine using a modified Airmodus A20 butanol Condensation Particle Counter. In our measurements the particle mean sizes from engine emission were often between 3-6 nm. The mean diameters measured with the modified A20 bCPC agreed with mean particle diameters from SMPS measurement.
The A20 is a laminar, one flow instrument that has 5 nm lower cut-point with factory settings of 1 lpm flow rate, 39oC saturator temperature and 15oC condenser temperature. We have increased the flow rate up to 2.2 lpm, the saturator temperature up to 44oC and lowered the condenser temperature down to 10oC. The increase of flow rate lowers the diffusional losses especially for the smallest particles and the higher temperature difference between the saturator and the condenser increases the highest supersaturation for the n-butanol vapor inside the instrument. This makes it possible to activate and grow smaller particles with smaller losses. The grown particles are detected using light scattering. Every particle creates a pulse that is, in some extent relative to the initial size of the sampled particle (Marti et al. 1996). Smaller particles activate later inside the instrument than the bigger ones and thus have less time to grow. This creates smaller droplets and smaller measured pulses. The instrument was calibrated using silver nanoparticles in the size range of 2.3 nm to 40 nm. The 50% cut-off for silver particles was 3.1 nm, but the instrument was still capable of counting 20% of the 2.3 nm. Airmodus A20 measures the mean pulse width of the optical pulse created by the grown particle inside the CPC within the sampling time of 1 second. It was found that this mean pulse width corresponds nicely with the sample particle size from 3 to 10 nm. If there is no significant signal from particles larger than 10 nm, this information can be used to measure mean size of particles. This new method of using the mean pulse width enables the measurement of particle size and concentration with 1 Hz sample rate. Other instruments capable of measuring size distributions down to 3 nm usually have sample rate of a couple of minutes. In the future this method will be developed further to include full pulse width spectra for aerosol size distribution measurements.
Rönkkö, T. et al.: Traffic is a major source of atmospheric nanocluster aerosol. Proc. Natl. Acad. Sci. U.S.A. Published online July 3, 2017.
Alanen, J., et al.: The formation and physical properties of the particle emissions from a natural gas engine. Fuel, vol 162, 155-161, 2015.
Marti, J. J., Weber, R. J., Saros, M. T., Vasiliou, J. G. & McMurry, P. H. Modification of the TSI 3025 Condensation Particle Counter for Pulse Height Analysis, Aerosol Science and Technology, 25:2, 214-218, 1996.