Molecular Characterization of Composition and Volatility of Ambient Organic Aerosol Sampled by an UAV-Mounted Portable Aethalometer

Aethalometers are commonly used to measure light absorption by brown carbon (BrC) components of organic aerosols (OA), but they offer limited insight into OA composition. In this study, an unmanned aerial vehicle (UAV) was equipped with a microaethalometer and an optical particle counter (OPC) to measure multiwavelength absorption coefficients and aerosol mass concentrations, while collecting ambient OA samples at 15 and 200 m flight altitudes over an urban area. The collected aethalometer samples were analyzed using an advanced analytical chemistry measurement platform that combines temperature-programmed desorption (TPD) with direct analysis in real-time (DART) ionization and high-resolution mass spectrometry (HRMS), enabling combined optical and molecular characterization. Results show significantly higher OA mass concentrations and light absorption near the surface at 15 m compared to 200 m, reflecting strong local ground-level emissions. Apparent enthalpies of evaporation and saturation vapor mass concentrations of OA components derived from TPD-DART-HRMS measurements agreed well with published values. The particle-phase composition, volatility, and viscosity of OA exhibited comparable characteristics between samples collected at two altitudes with a little variation. This study demonstrates the value of combining UAV-based aethalometer sampling with TPD-DART-HRMS for untargeted molecular analysis of ambient OA. This integrated approach enables simultaneous evaluation of light absorption, volatility, and viscosity─offering a powerful tool for advancing the understanding of BrC chemistry and aerosol processes.