Characterization of the newly designed wall-free particle evaporator (WALL-E) for online measurements of atmospheric particles

Abstract

Organic aerosols (OAs) play a critical role in the atmosphere by directly altering human health and the climate. Understanding the formation and evolution of OAs as well as their physicochemical properties requires a detailed characterization of their chemical composition. Despite advanced analytical techniques developed within the last decades, the real-time online measurement of atmospheric particles remains challenging and is affected by different artifacts (i.e., thermal decomposition, fragmentation, wall loss). 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 secondary organic aerosol (SOA) mass of 1 µg m−3. While dimers exhibit higher sensitivities, they account for only 14 %–18 % of the total particle mass, 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, a 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, the measured T50 (the temperature at which 50 % of a compound evaporates) for α-pinene-derived SOA products agree well with theoretical volatility estimation models (e.g., SIMPOL). Overall, this study demonstrates that the WALL-E system coupled with a CIMS is a promising technique for real-time particle characterization (i.e., composition, quantification, and volatility) of atmospheric aerosols.