Characterization of the Newly Designed Wall-Free Particle Evaporator (WALL-E) for Online Measurements of Atmospheric Particles

Abstract. Organic aerosols (OA) play a critical role in the atmosphere by directly altering human health and climate. Understanding their formation and evolution as well as their physicochemical properties requires a detailed characterization of their chemical composition. Despite advanced analytical techniques developed within the last decades, real-time online measurement of atmospheric particles remains challenging and suffers from different artifacts. In this work, we introduce the newly designed wall-free particle evaporator (WALL-E) coupled with a chemical ionization mass spectrometer (CIMS) using bromide (Br^–) as the reagent ion. We comprehensively evaluate the performance of the WALL-E system, demonstrating its ability to evaporate particles while maintaining the integrity of the compounds composing the particles (i.e., minimal thermal decomposition). To demonstrate WALL-E’s performance, the composition of aerosol particles formed from α-pinene ozonolysis in the presence of SO₂ is characterized. In addition, by applying the scan declustering method, we can now provide a quantification of the different species present in the condensed phase, e.g., C₁₀H₁₆O₄ 84 ng m^-3, C₁₉H₂₈O₇ 7 ng m^-3 for a total SOA mass of 1 µg·m^-3. While dimers exhibit higher sensitivities, they account for only 14–18 % of the total particle masses, which is considerably lower than their signal fractions (23–29 %). This suggests a potential overestimation of the dimer contributions when relying solely on signal fractions. In addition, volatility analysis using thermograms reveals a clear relationship between T₅₀ and compound saturation vapor pressure (C^*), with lower-volatility species desorbing at higher temperatures. In addition, measured T₅₀ for α-pinene-derived SOA products agree well with theoretical volatility estimation models (e.g., SIMPOL). Overall, this study demonstrates that WALL-E system coupled to a CIMS is a promising technique for real-time particle characterization (i.e., composition, quantification, and volatility) of atmospheric aerosols.