Assessing the size-resolved chemical composition of 10-50 nm particles with an online DMA-VIA-MION-Orbitrap setup

Measuring the molecular composition of small aerosol particles that have not yet grown to sizes of several 10 nm is difficult, predominately because of the little mass constituted by the particles. At the same time, composition measurements of the early particles would directly reveal the condensable species. Additionally, the chemical composition of these early particles controls the fate of the aerosol particles as it controls their robustness against evaporation. The vaporization inlet for aerosols (VIA), coupled to an Eisele-type chemical ionisation (NO3-) inlet and a time-of-flight mass spectrometer, has been successfully demonstrated previously (Häkkinen et al 2023, Zhao et al 2023, Zhao et al 2023). Particles from a sample gas are first size selected by a differential mobility analyzer. Trace gases present in the gas phase are stripped by an active charcoal denuder. The particles are then evaporated in a sulfinert-coated stainless steel tube (OD ¼”) at temperatures up to 300 °C. The evaporated molecules are then delivered to a chemical ionization mass spectrometer. The concentration of particles entering the setup are monitored by a scanning mobility particle sizer. Amending the experimental setup of the previous studies by a DMA for size selection before analysis, in this study we investigate how the aerosol composition differs between different particle sizes. Here, particles are first size-selected in a DMA, then vaporized in VIA. The resulting gases are ionized in a MION atmospheric pressure interface chemical ionization inlet using both positively and negatively charged reagent ions and detected in a polarity-switching high-resolution Orbitrap mass spectrometer. We demonstrate the general feasibility of the experimental approach in laboratory measurements using ammonium sulfate and a-pinene derived particles. In ambient measurements, we show the ability to reach mass closure between the detected concentration of vaporized trace gases even at low particle sizes and low atmospheric particle concentration. We further present first results from a deployment of the novel approach to Mace Head, Ireland, where marine VOC emissions and the composition of 10-20 nm particles were targeted. Coordinated measurements of the gas and particle phase are used to constrain what species contribute to particle formation under the local conditions. The work presents a step towards closing the measurement gap of nano-particle composition and contributes to a more complete understanding of aerosol formation from more complex gas mixtures.