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

Abstract

Abstract. We investigated the collection efficiency and effective ionization efficiency for secondary organic aerosol (SOA) particles made from α-pinene + O 3 using the singleparticle 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.

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

“…Differences in the spectra of prompt and delayed particles were also recently reported for ammonium sulfate and laboratory-generated secondary organic aerosols (SOAs), both with a substantial fraction of delayed particles (Robinson et al, 2017). Indeed, the pattern of sulfate ions between prompt and delayed particles in their study is similar to what is shown in Fig.…”

“…The broadening of the traditional AMS mass distribution to larger sizes due to delayed particles was also observed during the Mexico City and Bakersfield field studies (Cross et al, 2009;Liu et al, 2013) and the alpha-pinene SOA laboratory study (Robinson et al, 2017). Thus, d va-LS instead of d va-MS is a more reliable parameter to represent particle size in the AMS.…”

“…Because the spectra were saved every 32 µs and vaporization event lengths in the AMS are on the order of 30-60 µs and constant for pure ammonium sulfate particles at temperatures between 400 and 800 • C, the data collected in this brief lab study could not be used to validate the possibility of a less drastic change in vaporization temperature. Equivalent vaporization timescales between prompt and delayed particles along with the same issue of insufficient precision were also reported for the laboratory study of SOA particles (Robinson et al, 2017). The increased fragmentation with a hotter vaporizer could be due to changes in the vaporization process itself or to evolved gas molecules hitting a hotter surface, or both.…”

“…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.…”

“…The delayed particle mass could be lower on average than the prompt particle mass because the evolved gas from the delayed particles was produced in a region where the vapors are not as efficiently ionized by the electron beam. This explanation was recently proposed based on a laboratory study of monodisperse, alpha-pinene SOAs that had a low numberbased CE (0.30), a significant fraction of delayed particles (53 % of all LS particles with MS signals), and a clear trend of more ions per particle for prompt particles than for delayed particles (Robinson et al, 2017). Since the SENEX data are from a range of particle sizes, we normalized the total LSSP ion signals to the cube of their d va-LS and plotted the averages as a function of delay time up to 3.5 ms. Figure 6 shows the results from two flights on 6 July and 3 July along with results from ammonium sulfate particles and confirms that delayed particles have a slightly lower total ion signal per unit volume than prompt particles.…”