The Influence of Relative Humidity on Nanoparticle Concentration and Particle Mass Distribution Measurements by the MOUDI

SHENG-CHIEH CHEN (1), CHUEN-JINN TSAI (1)

(1) National Chiao Tung University

     Abstract Number: 822
     Last modified: July 30, 2010

Abstract
A humidity control system was operated upstream of two collocated MOUDIs (Micro-orifice Uniform Deposit Impactors) for sampling ambient aerosol particles. One MOUDI used silicone grease coated aluminum foils (ALs) as the impaction substrates and was considered as the reference impactor while the other used uncoated ALs or uncoated Teflon filters (TFs) as the impaction substrates for quantifying the effect of different controlled RHs (relative humidities) and impaction substrates on the PM0.1 concentrations and mass distributions of ambient PMs. Test results showed that decreasing RH in general increased particle bounce from uncoated substrates with the bounce from uncoated ALs being more severe than that from uncoated TFs. Particle bounce did not influence the overall mass distribution of ambient fine particles when RH ranged between 40 to 80 %, whereas it undersampled particles greater than 2.5 micrometer in aerodynamic diameter severely. Oversampling of PM0.1 occurred by as much as 95-180 % or 25-55 % when the MOUDI used uncoated ALs or TFs, respectively, as RH was reduced from 50 to 25%. Particle bounce was largely reduced and PM0.1 was found to be sampled accurately with less than 5 % error by the MOUDI with uncoated ALs or TFs at the RH of 75-80% or 65-80%, respectively.