Intercomparison of Capture and Standard Vaporizers in the Aerodyne Aerosol Mass Spectrometer (AMS)
Abstract:Aerosol mass spectrometers (AMS) and Aerosol Chemical Speciation Monitors (ACSM) commercialized by Aerodyne Research Inc. are used widely to measure 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 requires the use of collection efficiency (CE) for correcting the loss of particles due to bounce on the vaporizer. However, CE depends on aerosol phase, and thus can vary with location, airmass, and season, and typically contributes the most uncertainty to quantification. 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, one with the standard vaporizer and one with the capture vaporizer were operated side by side in the lab and field during the Southern Oxidant and Aerosol Study (SOAS) campaign in Alabama, 2013. Scanning mobility particle sizer (SMPS) size distribution measurements were made in parallel. 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), verifying that CE~1 in the capture vaporizer. However, compared to the standard vaporizer, fragmentation of organic and inorganic species was shifted to smaller fragments on the capture vaporizer, suggesting additional thermal decomposition arising from the increased residence time and surface collisions of molecules in the vaporize and 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 limited in field studies since ambient distributions are typically quite broad. The influence of vaporizer temperature on the shape of the size distributions was also investigated. Relative ionization efficiencies were recalibrated in the capture vaporizer. Positive matrix factorization (PMF) was performed on both data sets to compare the results of this source apportionment technique.