Collection efficiency of α-pinene secondary organic aerosol particles explored via light-scattering single-particle aerosol mass spectrometry

Abstract. We investigated the collection efficiency and effective ionization efficiency for secondary organic aerosol (SOA) particles made from α-pinene + O₃ using the single-particle capabilities of the aerosol mass spectrometer (AMS). The mean count-based collection efficiency (CE_p) for SOA across these experiments is 0.30 (±0.04 SD), ranging from 0.25 to 0.40. The mean mass-based collection efficiency (CE_m) is 0.49 (±0.07 SD). This sub-unit collection efficiency and delayed vaporization is attributable to particle bounce in the vaporization region. Using the coupled optical and chemical detection of the light-scattering single-particle (LSSP) module of the AMS, we provide clear evidence that delayed vaporization is somewhat of a misnomer for these particles: SOA particles measured as a part of the AMS mass distribution do not vaporize at a slow rate; rather, they flash-vaporize, albeit often not on the initial impact with the vaporizer but instead upon a subsequent impact with a hot surface in the vaporization region. We also find that the effective ionization efficiency (defined as ions per particle, IPP) decreases with delayed arrival time. CE_p is not a function of particle size (for the mobility diameter range investigated, 170–460 nm), but we did see a decrease in CE_p with thermodenuder temperature, implying that oxidation state and/or volatility can affect CE_p for SOA. By measuring the mean ions per particle produced for monodisperse particles as a function of signal delay time, we can separately determine CE_p and CE_m and thus more accurately measure the relative ionization efficiency (compared to ammonium nitrate) of different particle types.