Estimation of diffusion losses when sampling diesel aerosol: A quality assurance measure

Under the sponsorship of the Coordinating Research Council (CRC), the University of Minnesota (UMN) formed an international research team to investigate the physical and chemical nature of diesel emissions from heavy-duty vehicles while operating on highways (CRC Project E-43). These ambient measurements of vehicle emissions following their release into and dilution by the atmosphere guided the development of dilution and sampling procedures for laboratory test cells to simulate on-highway conditions. The importance, visibility, and potential implications of the project prompted the adoption of a quality assurance (QA) plan by an independent implementation team. Because exhaust aerosol characterization for mobile sources lacks prescribed sampling methodologies, standard operating procedures were developed as part of the QA effort to ensure the consistency and validity of the data collected. To verify the daily protocols used, the QA team made surveillance visits to observe UMN team performance on project tasks. Evaluation of instrument performance using aerosols of known size was also done as part of QA system audits conducted to assess the accuracy of particle size measurements. QA for particle concentration measurement was hampered by the lack of a concentration standard, which is a problem common to aerosol science investigations today. Thus, the standard practice in aerosol work is to verify particle concentration by comparing results from two identical condensation particle counters (CPC) and rely on instrument manufacturer calibrations. A fundamental component of QA for assessment of instruments and sampling system performance was investigation of particle losses. Along an aerosol sample pathway from source to collection media or measuring instrument, some particles are lost to surfaces. The magnitude of these losses as a function of particle size was determined experimentally by challenging the sampling trains with mono-disperse particles in the sub-50 nm aerodynamic diameter size range (nm=10^-9 meters). Since the most probable loss mechanism was diffusion for sub-50 nm sized particles, theoretical calculations of diffusion loss for 100 nm particles and smaller were also made. Results indicated average sampling train total losses of approximately 50% and 20% for 10 nm and 17 nm size particles, respectively. Measured Scanning Mobility Particle Sizer (SMPS) instrument internal losses were approximately 70% for a 10 nm size mono-disperse aerosol. Measured sample line losses were better predicted by theory if the flow in the sampling lines is considered turbulent. However, the flow in the lines was laminar. This noted discrepancy may be the result of local turbulence created by valves and bends in the sample lines and is an area recommended for additional investigation. Finally, application of a particle loss correction to particle size distributions from the study increased SMPS number concentrations at 10 nm by approximately a factor of 5 and at 20 nm by a factor of 2.