Advances in the Vaporization Inlet for Aerosols (VIA) for Online Measurements of Particulate Highly Oxygenated Organic Molecules (HOM)

Particulate matter significantly influences global climate and human health, necessitating accurate measurement techniques for understanding its composition. Many methods, both offline and online, have been employed over the years to achieve this goal. One of the most recent developments is the Vaporization Inlet for Aerosols (VIA) coupled to a nitrate Chemical Ionization Mass Spectrometer (NO₃-CIMS). Despite advancements, a thorough understanding of the VIA–NO₃-CIMS system remains incomplete.

In this work, we ran a series of tests to assess the impacts of different systems and sampling parameters on the detection efficiency of highly oxygenated organic molecules (HOM) in the VIA–NO₃-CIMS. Our findings indicate that the current VIA system, including an activated carbon denuder and a vaporization tube, efficiently transmits particles (>90% for particles larger than 50 nm) while removing gaseous compounds (>97% for tested volatile organic compounds). One of the main differences between the VIA and traditional thermal desorption (TD) techniques is the very short residence time in the heating region, on the order of 0.1 s. This short residence time and the corresponding short contact with heated surfaces is likely one of the main reasons why relatively reactive or weakly bound, such as peroxides, were observable using the VIA. However, the VIA requires much higher temperatures to fully evaporate the aerosol components. For example, the evaporation temperature of ammonium sulfate particles using the VIA was found to be 100-150 ^oC higher than in typical TD systems.

Optimizing the VIA–NO₃-CIMS interface to minimize gas-phase wall losses was critical. Introducing a dedicated sheath flow unit between the VIA and the NO₃-CIMS markedly reduced wall losses, improving sensitivity compared to earlier VIA work. This unit also facilitated sample cooling and provided the NO₃-CIMS with the necessary high flow (10 L min^-1). Our results indicate that most organic molecules observable by the NO₃-CIMS can evaporate and be transported efficiently in the VIA system, but upon contact with the hot walls of the VIA, the molecules are instantaneously lost. This loss potentially leads to fragmentation of products that are not observable by the NO₃-CIMS.

Thermograms, obtained by scanning the VIA temperature, proved invaluable for both quantification purposes and estimating the volatility of the evaporating compounds. We developed a simple one-dimensional model to account for the evaporation of particles and the temperature-dependent wall losses of the evaporated molecules, allowing estimation of HOM concentration in organic particles. Finally, we applied this system to study four different monoterpenes, and compared HOM distribution between the gas and particle phase. Overall, our results provide much-needed insights into the key processes underlying the VIA–NO₃-CIMS method. Although hardware improvements are needed to address certain limitations, the VIA–NO₃-CIMS system emerges as a promising tool for fast online measurements of HOM in the particle phase, contributing to our understanding of particulate matter composition and its broader implications.