AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
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Characterization of the Particle Wall Loss in the UCR Collapsible FEP-Teflon Chamber
CHEN LE, Don Collins, David R. Cocker III, University of California, Riverside
Abstract Number: 751 Working Group: Aerosol Physics
Abstract Understanding and correcting for particle wall-loss is critical for accurate determination of aerosol yield from environmental chamber studies. The most commonly applied methods for accounting for wall-loss are use of i) size-dependent corrections, ii) volume-averaged corrections, iii) inert tracer corrections, and iv) coagulation-corrected number-averaged corrections (Nah et al., 2017). Analysis of data from the UCR CE-CERT 90 m3 indoor chambers relies on corrections determined from the observed average rate of decay of the particle number concentration during the final few hours of each experiment when particle formation and growth are minimal. A size-independent correction has generally been assumed in part because it could explain the observed evolution of the size distribution of the late-experiment polydisperse aerosol. The magnitude of the loss rate and resulting correction is observed to vary from experiment to experiment and, especially because of the relatively long experimental time span (6~14 hours), the corrected aerosol yield is very sensitive to the average decay rate.
This study evaluates particle wall-loss characteristics in the CE-CERT chambers through a series of seed-only experiments in which one or more monodisperse ammonium sulfate particle modes were monitored over several hours. The average decay rate was found to increase with increasing particle size over the 50 nm to 400 nm diameter range examined. This pattern is believed to reflect a significant contribution of electrostatic loss of charged particles. This interpretation is further supported by the observation that the loss rate differed for injections of positively charged, negatively charged, and neutral particles. Size dependent particle wall-loss rates were evaluated as both a function of experimental time and as a function of the estimated ratio of the chamber wall surface area to the chamber volume (A/V). The sensitivity of particle wall-loss to chamber temperature was also investigated. In addition, a sensitivity analysis of SOA yields to corrections within the range of historical variations in daily particle wall-loss rates and a systematic comparison of the “size-dependent correction” and the “number-averaged correction” will be presented.