A closure study of aerosol mass concentration measurements: comparison of values obtained with filters and by direct measurements of mass distributions

We compare measurements of aerosol mass concentrations obtained gravimetrically using Teflon coated glass fiber filters and by integrating mass distributions measured with the differential mobility analyzer-aerosol particle mass analyzer (DMA-APM) technique (Aerosol Sci. Technol. 36 (2002) 227). The DMA-APM technique measures the distribution of particle mass as a function of mobility size for particles of arbitrary shape and composition (Relationship between particle mass and mobility, and between aerodynamic and mobility size distributions for diesel exhaust particles, Environ. Sci. Technol., 2003). Because DMA-APM measurements are made on particles suspended in the air, data are not affected by volatilization or adsorption that can affect the accuracy of the filter measurements. We show that the average ratio of the filter to DMA-APM mass concentrations for laboratory-generated dioctyl sebacate (DOS) and sodium chloride (NaCl) aerosols is 1.14 +/- 0.28, and they are well correlated (R-2 > 0.97). For diesel exhaust aerosols from an engine operating at 75% load, the two techniques agreed well with the average ratio of 0.98 +/- 0.20. When the engine was operated at a low (10%) load, mass concentrations measured with the filter were 2.13 +/- 0.54 times higher than values measured with the DMA-APM. We believe that the higher filter loading may be due to the adsorption of condensable vapors, which are emitted at higher rates under low engine load conditions. Measurements in which the condensable organics were removed with a catalytic stripper show much better agreement between the filter and DMA-APM, which support the hypothesis that vapor adsorption leads to artificially high filter data for low-load measurements. We conclude that the DMA-APM technique can be used to evaluate the accuracy of filter samples that may be affected by sampling artifacts, and to measure mass distributions with high time resolution for sub-0.5 mum aerosols. (C) 2003 Elsevier Science Ltd. All rights reserved.