Molecular Aerosol Composition within the Atmospheric Boundary Layer at the Southern Great Plains: Seasonal Signatures from Surface to Aloft Revealed by High-Resolution Mass Spectrometry

Abstract Although aerosols within the atmospheric boundary layer (ABL) are generally considered well-mixed, how different transport mechanisms and chemical processes affect this assumption is unclear, leading to uncertainties in assessing aerosol’s impacts on radiative forcing. Here, we present multi-season, vertically resolved molecular characterization of organic aerosol within the ABL at the Southern Great Plains (SGP) user facility. Aerosol samples were collected at the ground level and aloft (100–900 m) via a tethered balloon system (TBS) during winter and summer 2024 and analyzed by direct mass spectrometry. While bulk aerosol elemental properties are largely homogeneous throughout the ABL, individual molecular formula abundances reveal persistent altitude-dependent signatures. Orthogonal Partial Least Squares Discriminant Analysis (OPLS-DA) identified 355–384 ground-enriched formulas per season vs. only 6 aloft-enriched formulas, the latter consistent with markers of primary biological particles. The ground-enriched formulas are systematically lower in volatility and enriched in organosulfates (3.9× compared to nondiscriminatory formulas). Vertical profiles on individual flight days reveal that the ground enrichment exhibits as a step-function decrease within approximately the lowest ∼100 m rather than a gradual mixing gradient. These results suggest that continuous chemical processing near the surface sustains a molecular composition gradient even within a thermodynamically well-mixed ABL. Air masses arriving at different altitudes follow distinct transport paths, driving vertical gradients of certain molecular formulas. These findings demonstrate that ground-based molecular measurements systematically overrepresent low-volatility, surface-produced species relative to the ABL column composition, with implications for relating ground-based aerosol composition measurements to aerosol-cloud interaction.