AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
Abstract View
Intercomparison of Capture and Standard Vaporizers in the Aerodyne Aerosol Mass Spectrometer (AMS)
WEIWEI HU, Pedro Campuzano-Jost, Douglas Day, Philip Croteau, Manjula Canagaratna, John Jayne, Jose-Luis Jimenez, CIRES, University of Colorado, Boulder
Abstract Number: 112 Working Group: Instrumentation and Methods
Abstract Aerosol mass spectrometers (AMS) and Aerosol Chemical Speciation Monitors (ACSM) commercialized by Aerodyne Research Inc. are used widely to measure the mass concentrations and size distributions of non-refractory species in submicron-particles. With the “standard” vaporizer that is installed in all commercial instruments to date, the quantification of ambient aerosol mass concentration requires the use of a collection efficiency (CE) for correcting the loss of particles due to bounce on the vaporizer. However, CE depends on aerosol phase and chemical composition, and thus can vary with location, airmass, and season of sampling, and typically contributes the most uncertainty to the quantification of aerosol mass. To address this limitation, a new “capture” vaporizer has been designed to reduce or eliminate particle bounce and thus the CE correction.
To test the performance of the capture vaporizer, two AMS instruments, one with the standard vaporizer and one with the capture vaporizer were operated side by side in the lab and multiple field studies. In four ambient or flow reactor datasets, good agreement was observed between the time series of mass concentration of the main species between the capture (without use of CE) and standard vaporizers (with the composition-dependent CE correction from Middlebrook et al., AS&T 2012), verifying that CE~1 in the capture vaporizer. However, compared to the standard vaporizer, the observed mass spectra were shifted to smaller fragments with the capture vaporizer, suggesting additional thermal decomposition arising from the increased residence time and surface collisions of the molecules in the vaporizer. The additional decomposition is also consistent with the observed longer particle vaporization times which lead to artificially broadened particle size distribution measurements. The impact of the size distribution broadening is significant for lab studies using monodisperse particles, but less important for field studies since ambient distributions are typically quite broad. The influence of vaporizer temperature on fragment pattern and the shape of the size distributions was also investigated.