AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Inter- and Intra-model Comparisons of Water-based Condensation Particle Counters near a Major Freeway with Significant Heavy-duty Diesel Traffic
EON LEE, Yifang Zhu, Andrea Polidori, Michael Koch, Philip Fine, Ahmed Mehadi, Donald Hammond, Jeffery Wright, Antonio H. Miguel, Alberto Ayala, University of California, Los Angeles
Abstract Number: 93 Working Group: Instrumentation and Methods
Abstract Accurate and precise measurement of ultrafine particle (UFP) number concentration is challenging because of uncertain variability among the currently available condensation particle counters (CPCs). This study investigated the performance of three TSI water-based CPC models by measuring atmospheric UFP levels in close proximity (15 m) to the I-710, a major Southern California freeway with significant heavy-duty diesel traffic. The three TSI models (3781, 3783, and 3785) were operated in triplicate (nine units in total) in parallel with two sets of Scanning Mobility Particle Sizer (SMPS) spectrometers for the concurrent measurement of particle size distributions for one month. Inter-model comparisons using one-minute data showed that all three CPC models agreed well during upwind conditions (UFP concentration range between 10$^3 and 10$^4 particles cm$^(-3)). However, significant differences were observed under downwind conditions (particle count above 10$^4 particles cm$^(-3)). We also evaluated the effect of using longer averaging time intervals in an effort to increase inter-model linear correlations. An averaging time of at least 15-minutes achieved R$^2 values of 0.95 or higher when comparing all three models. Similar results were also observed for intra-model comparisons. Sensitivity analysis on concentration ranges found that TSI 3783 recorded the highest average particle count which was 6% higher than that measured by TSI 3785 under high concentration conditions (above 10$^4 particles cm$^(-3)). TSI 3781 measured an average UFP level that was 18% lower than that observed by TSI 3783 during the same time period. Our analysis showed that all three TSI 3781 units underestimated particles with a count median diameter less than 45 nm. The same problem was also observed for one TSI 3783 and one TSI 3785 units. Subsequently, we quantified inter-model bias using exponential regression analysis and proposed correction equations to adjust the data from TSI 3781 and 3785 to the most recently developed TSI 3783 model.