Ultrafine particle loss in aerosol diluters.

Abstract

Measurements of high concentration aerosols typically require some method to reduce the concentration of the aerosol to a measureable method. One common method is to use an aerosol diluter to reduce the aerosol concentration by a known value. Particle loss within the aerosol diluters can result in values of dilution that are particle size dependent and vary from the idealized volumetric dilution value. Experiments were performed to measure the sub-100nm particle loss within aerosol diluters. The diluters tested include the TSI 3302, MSP 1100, a “leaky filter” laboratory diluters and a newly developed Orifice Diluter. Particle loss within aerosol diluters was found to be a significant effect, with up to 50% of the dilution being due to particle loss. Particle loss within the aerosol diluters was found to be primarily diffusion driven and loss within the diluters increased with decreasing particle diameter. A new terms was derived to describe particle loss within the aerosol diluters where diluter penetration describes the percentage of particle loss within an aerosol diluter. Further investigation into particle charge found a significant reduction in particle penetration when the aerosol was singly charged. This can result as much as a 40% enhancement in particle loss. Diluter penetration curves for the TSI 3302,MSP 1100, “leaky filter” and Orifice Diluters were then used to correct measurements of high concentration diesel aerosol. Correction methodology was found to be highly sensitive to the measured size distribution of the diesel aerosol. With accurate size distribution measurements the agreement between diluter and CPC combinations was corrected to less than 3% at a concentration of 1.57E5 cm-3 with a geometric mean diameter (GMD) of 50.0nm . As the GMD was reduced for the aerosol agreement among diluters was reduced, with a resulting agreement of ±4% at a GMD of 36.5 nm and ±25% at a GMD of 16.3nm. The decrease in agreement is due to the accuracy in the size distribution measurements.