An Instrumental Comparison of Mobility and Mass Measurements of Atmospheric Small Ions

• Published in: Aerosol science and technology Vol. 45; no. 4; pp. 522 - 532
• Main Authors: Ehn, Mikael, Junninen, Heikki, Schobesberger, Siegfried, Manninen, Hanna E., Franchin, Alessandro, Sipilä, Mikko, Petäjä, Tuukka, Kerminen, Veli-Matti, Tammet, Hannes, Mirme, Aadu, Mirme, Sander, Hõrrak, Urmas, Kulmala, Markku, Worsnop, Douglas R.
• Format: Journal Article
• Language: English
• Published: New York Taylor & Francis Group 01.04.2011; Taylor & Francis Ltd
• Subjects: Accuracy; Aerosols; Atmospheric aerosols; Atmospherics; Channels; Chemical compounds; Comparative analysis; Complement; Ions; Mass spectra; Mass spectrometry; Mathematical analysis; Measuring instruments; Spectrometers; Transfer functions; Finland
• ISSN: 0278-6826; 1521-7388
• DOI: 10.1080/02786826.2010.547890

Ambient, naturally charged small ions (<2000 Da) were measured in Hyytiälä, Finland, with a mass spectrometer (atmospheric pressure interface time-of-flight, APi-TOF) and two mobility spectrometers (air ion spectrometer, AIS, and balanced scanning mobility analyzer, BSMA). To compare these different instrument types, a mass/mobility conversion and instrumental transfer functions are required to convert high-resolution mass spectra measured by the APi-TOF into low-resolution mobility spectra measured by the AIS and BSMA. A modified version of the Stokes-Millikan equation was used to convert between mass and mobility. Comparison of APi-TOF and BSMA results showed good agreement, especially for sizes above 200 Da (Pearson's R = 0.7-0.9). Below this size, agreement was fair, and broadening BSMA transfer functions improved the correlation. To achieve equally good agreement between APi-TOF and AIS, AIS results needed to be shifted by 1-1.5 mobility channels. The most likely cause was incorrect sizing in the AIS. In summary, the mass and mobility spectrometers complement each other, with the APi-TOF giving superior chemical information, limited to relatively small ions (<2.5 nm diameter), whereas the mobility spectrometers are better suited for quantitative concentration measurements up to 40 nm. The BSMA and AIS were used to infer a transmission function for the APi-TOF, making it possible to give quantitative estimates of the concentrations of detected chemical ions.