Organic aerosol volatility and its drivers in realistic urban air replicas

An important property of the compounds comprising organic aerosol is volatility, typically described in terms of saturation vapor pressure or saturation concentration (measured in µg/m3). The volatility of aerosol constituents can be estimated based on their molecular formula using different parametrizations or measured experimentally by using a chemical ionization mass spectrometer (CIMS) coupled to a filter inlet for gases and aerosols (FIGAERO). In this technique, aerosol sample is collected semi-online and evaporated via gradually heated nitrogen flow desorbing organic constituents to be measured by CIMS. From the temperature at which detected chemical species reach their maximum signal, it is possible to determine the respective compositions’ volatility.In 2024, the FIGAERO-CIMS was deployed at the CHANEL (household chemicals amplifying urban aerosol pollution) measurement campaign at the SAPHIR chamber at Jülich Research Centre, Germany. During the campaign, complex reactive mixtures representing urban air scenarios were injected into the chamber, and exposed to both day- and night-time oxidation via opening or closing the roof to natural sunlight. We developed a multi-peak fitting algorithm to fully fit each composition’s thermogram (signal vs. desorption temperature), resulting in multiple nominal saturation concentrations per detected composition. We interpret these as combinations of simple volatility-driven desorption and decomposition (typically at higher temperatures) of larger compounds, such as accretion products.We tracked the chemical composition and volatility of secondary organic aerosol throughout its formation and subsequent aging in the chamber over several hours. The chemical composition measured by FIGAERO-CIMS was compared with other co-located online mass spectrometric techniques, e.g., CIMS following online aerosol evaporation by a heated sheath flow (WALL-E). Our initial results show how aerosol volatilities typically decreased with age, as more oxygen was incorporated. Further, night-time conditions resulted in both increased organonitrate formation and lower product volatility relative to day-time conditions.