Evaluation of the new capture vapourizer for aerosol mass spectrometers (AMS) through laboratory studies of inorganic species

Abstract

Abstract. Aerosol mass spectrometers (AMSs) and Aerosol Chemical Speciation Monitors (ACSMs) commercialized by Aerodyne are widely used to measure the non-refractory species in submicron particles. With the standard vapourizer (SV) that is installed in all commercial instruments to date, the quantification of ambient aerosol mass concentration requires the use of the collection efficiency (CE) to correct for the loss of particles due to bounce. A new capture vapourizer (CV) has been designed to reduce the need for a bouncerelated CE correction.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, NH 4 NO 3 , NaNO 3 , (NH 4 ) 2 SO 4 and NH 4 Cl, which typically constitute the majority of the mass of ambient submicron inorganic species, are studied. The effect of vapourizer temperature (T v ∼ 200-800 • C) on the detected fragments, CE and size distributions are investigated. A T v of 500-550 • C for the CV is recommended. In the CV, CE was identical (around unity) for more volatile species (e.g. NH 4 NO 3 ) and comparable to or higher than the SV for less-volatile species (e.g. (NH 4 ) 2 SO 4 ), demonstrating an improvement in CE for laboratory inorganic species in the CV. The detected relative intensities of fragments of NO 3 and SO 4 species observed with the CV are different from those observed with the SV, and are consistent with additional thermal decomposition arising from the increased residence time and multiple collisions. Increased residence times with the CV also lead to broader particle size distribution measurements than with the SV. A method for estimating whether pure species will be detected in AMS sizing mode is proposed. Production of CO 2 (g) from sampled nitrate on the vapourizer surface, which has been reported for the SV, is negligible for the CV for NH 4 NO 3 and comparable to the SV for NaNO 3. . We observe an extremely consistent fragmentation for ammonium compared to very large changes for the associated anions. Together with other evidence, this indicates that it is unlikely that a major fraction of inorganic species vapourizes as intact salts in the AMS.

Field characterization of the PM<sub>2.5</sub> Aerosol Chemical Speciation Monitor: insights into the composition, sources, and processes of fine particles in eastern China

“…Although the CV produces different fragmentation patterns of organic and inorganic compounds compared with those of a standard vaporizer (SV), it reduces the particle bouncing effect at the vaporizer and hence improves the quantitative uncertainties caused by collection efficiency (CE). The recent evaluation of the CV for inorganic species measurements showed overall agreement with those by co-located measurements (Hu et al, 2017b). The PM 2.5 -AMS equipped with a SV was deployed once in China, which provided new insights into composition and sources of PM 2.5 in Beijing and Xi'an (Elser et al, 2016).…”

“…The particle lens of the PM 2.5 -Q-ACSM operates at a higher pressure than that of the PM 1 -Q-ACSM (Liu et al, 2007;Ng et al, 2011b) and transmits larger particles (Peck et al, 2016;Xu et al, 2017a). Additionally, the SV is replaced with the CV to eliminate the effect of particle bounce which can lead to a fraction of the particle mass not being detected, an effect which increases at larger particle diameters (Hu et al, 2017a;Xu et al, 2017a). The PM 1 and PM 2.5 Q-ACSM mass spectra were analyzed using the Q-ACSM local toolkit (version 1.5.11.2), a data analysis software written in WaveMetrics Igor Pro ™ .…”

Single-particle measurements of bouncing particles and in situ collection efficiency from an airborne aerosol mass spectrometer (AMS) with light-scattering detection

“…Lower vaporizer temperatures increase the ions at m/z 46 relative to m/z 30 for ammonium nitrate and at m/z 80 and 81 relative to the other ions for ammonium sulfate (Hu et al, 2017). Spectra from most but not all organic compounds have more fragmentation when vaporized at 600 • C than at 200 • C (Canagaratna et al, 2015).…”

“…The lost kinetic energy for the particles that bounced may have been converted into plastic deformation and fracturing (Miyakawa et al, 2013). Alternatively, the delays in appearance of the mass spectral signals could have been due to multiple bounces prior to vaporization (Robinson et al, 2017;Hu et al, 2017). Particles without detectible chemical ion signals could be those that bounced far away from the ionization source cage region and could not be vaporized and ionized efficiently.…”

“…Gas molecules from particles vaporizing from other surfaces, such as the top of the vaporizer or the ionization chamber, would experience fewer wall collisions where they could decompose. For both ammonium nitrate and ammonium sulfate particles, there is far more fragmentation when the vapors are contained in a capture vaporizer prior to ionization compared to the conical vaporizer at the same temperature (Hu et al, 2017).…”