Evaluation of the new capture vaporizer for Aerosol Mass Spectrometers (AMS)
through laboratory studies of inorganic species
Authors: Hu Weiwei , Campuzano-Jost Pedro , Day Douglas A. , Croteau Philip , Canagaratna Manjula R. , Jayne John T. , Worsnop Douglas R. , Jimenez Jose L. 
Published: November 2016

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 (SV) 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 SV. However, CE depends on aerosol phase, and thus can vary with location, airmass, and season of sampling. Although a composition-dependent parameterization of CE in the SV for ambient data has been successful, CE still contributes most of the estimated uncertainty to reported concentrations, and is also an important uncertainty in laboratory studies. To address this limitation, a new "capture" vaporizer (CV) has been designed to reduce or eliminate particle bounce and thus the need for a CE correction. To test the performance of the CV, two high-resolution AMS instruments, one with a SV and one with a CV were operated side by side in the laboratory. Four standard species NH4NO3, NaNO3, (NH4)2SO4 and NH4Cl, which typically constitute the majority of the mass of ambient submicron inorganic species, are studied. The effect of vaporizer temperature (Tv ~ 200–800 ℃) on the detected fragments, CE and size distributions are investigated. A Tv of 500–550 ℃ for the CV is recommended based on the observed performance. In the CV, CE was identical (around unity) for more volatile species and comparable or higher compared to the SV for less volatile species, demonstrating a substantial improvement in CE of inorganic species in the CV. The detected fragments of NO3 and SO4 species observed with the CV are different than those observed with the SV, suggesting additional thermal decomposition arising from the increased residence time and hot surface collisions. Longer particle detection times lead to broadened particle size distribution measurements made with the AMS. The degradation of CV size distributions due to this broadening is significant for laboratory studies using monodisperse particles, but minor for field studies since ambient distributions are typically quite broad. A method for estimating whether pure species will be detected in AMS sizing mode is proposed. Production of CO2(g) from sampled nitrate on the vaporizer surface, which has been reported for the SV, is negligible for the CV for NH4NO3 and comparable to the SV for NaNO3. Adjusting the alignment of aerodynamic lens to focus particles on the edge of the CV results in higher resolution size distributions, which can be useful in some laboratory experiments. We observe an extremely consistent detection of ammonium from different inorganic ammonium salts, independent of the vaporizer types and/or the Tv. This contradicts a recent suggestion by Murphy (2016) that inorganic species evaporate as intact salts in the AMS.