Abstract View
The Particles We Don’t Count Can Significantly Bias Optical Measurements
CHUN-NING MAO, Akua Asa-Awuku, James Radney, Christopher Zangmeister, University of Maryland
Abstract Number: 91
Working Group: Aerosol Standards
Abstract
A Cavity Ring Down Spectroscopy (CRDS) coupled with a Differential Mobility Analyzer (DMA) and a Condensational Particle Counter(CPC), is an experimental technique frequently used in lab to retrieve the aerosol extinction cross section. However, the extinction cross section is a size dependent parameter. It can be underestimated by the Mie Scattering theory when large, multiply charged particles exist in the CRDS measurement. Our experimental results show that even when an impactor was installed before the DMA, a low number density (~10 #/ccm) of large particles passed through the impactor and caused errors in extinction cross section measurement. Since the size of multiply charged particles are usually much larger than singly charged particles, 1% of the multiply charged particles can cause more than 5% relative errors in CRDS measurement. To obtain the correct extinction cross section, Tandem DMA measurement was applied to acquire the fraction of doubly charged and triply charged particles. Particles carrying more than three charges are usually out of the DMA size limitation, and their number density could be too low to measure. Since some multiply charging errors from DMA are inevitable, a theoretical method is developed to predict the fraction of the multiply charged particles in CRDS measurement. In this work, we test equations to predict the fraction of multiply charged particles and graphically identify the fraction of the multiply charged particles that would cause 5% relative errors in extinction cross section measurement. Results indicate that for 200nm size selected particles, 0.2% fraction of the particles carrying more than 3 charges could cause a 5% relative errors.