System for Precise Control of Volumetric Flow Rate during Sampling with a Cascade Impactor

摘要

Calculations were made with efficiency curves developed for the micro-orifice impactor (MOI) to estimate errors in mass collected on individual stages due to fluctuations in the flow rate during sampling of submicrometer particles. The sizes of these errors depend on the size distribution of the sampled aerosol and the level of flow rate fluctuation. For a long-normally distributed particle population, mass errors due to flow rate fluctuations were bimodally distributed about stages with cutpoints near the aerosol Mass Median Aerodynamic Diameter (MMAD). The largest errors occurred uniformly on the stage with the smallest cutpoint (here, 0.059 #mu#). These errors were asymmetric with respect to sign, which leads to a net error for a randomly fluctuating flow rate. In general, mass errors increased with decreasing geometric standard deviation (#sigma#_g) and were substantially greater for populations with 0.5 #mu#m MMADs than for those with 0.2 #mu#m MMADs. The largest met errors for the former were 4, 110, and 560 for #sigma#_g of 1.2 and flow rate fluctuations of (+-) 1, (+-)5, and (+-)10, respectively, but decreased to 0.03, 0.9, and 4, respectively, for a #sigma#_g of 1.9. Flow rate fluctuations, therefore, lead to a positive bias in the geometric standard deviation inferred from the measured masses and reduce the user's stability to interpret differences in size distributions. To minimize these effects, we developed and tested a system for controlling the volumetric sampling rate through a MOI at 30 LPM with a precision of 0.06 (600 ms averaging; 0.67 for 5 ms averaging), a level of precision that allows for accurate relative calibration between flow systems and for which errors from flow rate fluctuations are reduced to < 1, even for a very narrow aerosol (#sigma#_g 1.2).Mass errors for an uncontrolled field test were as large as -60, but these were reduced to < 0.22 in a comparable controlled field test. In two replicate tests of the system, agreements between stage masses collected on MOI stage 7 (D_50 = 0.173 #mu#m) of two simultaneously operated flow-controlled impactors sampling 0.2 #mu#m diameter monodisperse test particles were 0.997 and 0.996, although differences as large as 4 were observed for some stages. The system is suitable for use with standard "Federal Reference Method" sampler.