10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Effect of Change in Sheath Air Humidity on Size Distribution Measurements by Scanning Mobility Particle Sizer
B.K. SAPRA, Mariam , Manish Joshi, Arshad Khan, Bhabha Atomic Research Centre, Mumbai
Abstract Number: 1202 Working Group: Instrumentation
Abstract Scanning mobility particle sizer (SMPS) is widely used for measurement of size distribution of sub-micrometer particles in atmospheric and laboratory applications. Sheath air flow inside Differential mobility analyser of SMPS provides laminarity for aerosol particle stream flow. This work investigates the effect of sheath air dryer in the measurement of size distribution of particles with varying hygroscopicities. Aerosol particles of three different materials i.e. NaCl, NaNO3 and nichrome generated by nebulization/electrical heating were sampled through a twin SMPS system from a controlled chamber of volume 40 L. As the particles (of NaCl, NaNO3) generated from nebulization showed geometric mean diameter between 100-150 nm, nichrome nanoparticles were also allowed to grow in the same size range. One of the SMPSs of twin SMPS set-up had modified sheath air path (by keeping dryer in circuit) while the other was kept as such in a T-joint arrangement taking simultaneous sample from the chamber. Three sets of experiments were performed for each type of particle system at 3 different (chamber) relative humidity (RH) levels (15%, 40% and 90%). For the case of NaCl particles, 52.2±0.48% decrease in relative geometric mean (GM) of distribution measured by SMPS with sheath air dryer was seen at 90% RH. No such difference was noted for other RH levels (lesser than Deliquescence RH: 75.3% (Pilinis and Pandis, 1995). NaNO3 particles did not follow any deliquescent behaviour (Hu et. al., 2010, Lee et al., 2011), and showed growth at lower RH (40%, 60%) levels as well. Size distribution of Nichrome metal particles measured by both SMPSs remained unperturbed at all RH levels. Experimentally measured changes in GM diameter for the case of nebulized particles were also found to be reasonably matching with KÖhler theory predicted values. Since there is no clarity on protocols for usage of sheath air dryer for SMPS measurements; correction factors, algorithm modifications, newer protocols are required to be incorporated for size distribution measurements.
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