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.