Gregory W. Vandergrift

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.

Nitrogenous organic aerosol comprises up to 50% of the organic mass in atmospheric particles, but its broader impact remains poorly understood due to limited knowledge of its vertical distribution and molecular composition in the atmosphere. Here, via tethered balloon system flights in urban, agricultural, and mountainous ecosystem atmospheres across the United States, we show a vertical dependence for nitrogenous organic aerosol speciation; up to 30% of total aloft nitrogenous organic aerosol abundance is comprised of uniquely aloft nitrogenous organic aerosol species. We leverage a comprehensive in situ instrumentation (mainly high-resolution mass spectrometry) and modeling suite to explore causative factors, highlighting the particular importance of increased gas to particle phase partitioning of nitrogenous organic aerosol at aloft conditions. These insights are critical for refining atmospheric models, as neglecting the vertical distribution of nitrogenous organic aerosol can result in strong biases in the total organic aerosol budget.

Organic aerosol (OA) particles constitute a substantial fraction of submicron particulate mass in the atmosphere and play a critical role in climate system. OA undergoes dynamic aging processes in the atmosphere, with photolytic aging induced by ultraviolet solar irradiance being an important yet poorly characterized mechanism. Knowledge gaps persist regarding the role of volatility transformations during photolytic aging on the OA mass decay kinetics and the evolution of climate-relevant properties, such as hygroscopicity, hindering the model evaluation of OA spatiotemporal distributions and atmospheric budgets. In this study, we conduct isothermal photolytic aging experiments on both laboratory-generated secondary organic aerosols and ambient-collected particles from urban Atlanta, utilizing a high-sensitivity Quartz Crystal Microbalance. Our results reveal that photolytic aging reduces 40-66% of the low-volatility OA mass with lifetimes ranging from 8 to 200 h under solar irradiance, and 44-92% of the photolytic mass loss is through slow evaporation of semi- or intermediate-volatile products, kinetically limited by their volatility. We observe up to ±50% changes in OA hygroscopicity with the transformation of fresh OA to photorecalcitrant low-volatility products, associated with changes in oxygen-to-carbon ratio and molecular weight. A kinetic model incorporating photolytic volatility transformation provides a cohesive explanation for the observed photolysis-induced changes in mass, volatility, and hygroscopicity. Our results can help constrain model representation of the dynamic evolutions of mass and climate-relevant properties during photolytic aging processes of the ambient OA, improving our understanding of OA atmospheric behavior and climate impact.

Biogenic volatile organic compounds (BVOCs), a dominant source of secondary organic aerosol (SOA) globally, exhibit emission rates and compositions that are plant species-specific and vary with environmental stressors. A common outcome of plant stress is increased emissions of acyclic terpenes. The paucity of information about acyclic terpene SOA chemistry contributes to uncertainties in predictions of SOA global loadings and impacts on Earth’s radiative budget, particularly in a changing climate where acyclic terpene emissions could become more prominent. This study compared properties of SOA derived from hydroxyl radical (OH) oxidation of acyclic and cyclic monoterpenes (β-ocimene, α-pinene) and sesquiterpenes (β-farnesene, β-caryophyllene). Single-particle mass spectrometry was used for assessing shape, density, and evaporation kinetics of size-selected SOA particles, and nanospray desorption electrospray ionization high-resolution mass spectrometry (nano-DESI-HRMS) was used to measure the molecular composition of SOA. Acyclic terpene SOA exhibited higher viscosity and lower volatility compared to cyclic terpene SOA, and had a greater volume fraction remaining (VFR) after ∼24 h of evaporationapproximately 1.3–1.6 times higher VFR than that of cyclic terpene SOA. Additionally, HRMS analysis revealed greater chemical diversity and higher fractions of extremely low-volatility compounds (56–62% ELVOC/LVOC) in acyclic terpene SOA compared to cyclic counterparts (25–37% ELVOC/LVOC). Our findings highlight the potential importance of accounting for acyclic terpene aerosol chemistry under conditions of plant stress to improve predictions of SOA loadings and impacts.

Ambient aerosol vertical profiles are critical for evaluating the role of aerosols in atmospheric chemistry and radiative transfer, but limited data on these profiles hinder our ability to fully assess their impact on the Earth's radiative balance. Here, we investigated the size-, time-, and altitude-resolved composition of individual particles and the bulk molecular composition of particle samples collected by an uncrewed aerial system─ArcticShark over the Southern Great Plains. Single-particle microanalysis shows that the free tropospheric (FT) samples are dominated (56-66%) by carbonaceous sulfate particles, while boundary layer (BL) samples are dominated (57-74%) by carbonaceous particles. Back-trajectory simulations suggest that FT particles are likely influenced by long-range transport and have undergone aqueous-phase processing. Conversely, in situ size distribution data show evidence of particle growth in the upper BL and just below the FT. This observation may indicate vertical transport of particles from an elevated aerosol layer in the FT, possibly linked to a new particle formation event. This observation is further supported by high-resolution molecular composition data, which reveals particle volatility increasing with increasing size, aligning with the growth event. This study aids in a fundamental understanding of the compositional and molecular specificity of vertically resolved organic aerosols to provide insights into particle size evolution for future atmospheric models.

An increasing number of spatial multiomic workflows have recently been developed. Some of these approaches have leveraged initial mass spectrometry imaging (MSI)-based spatial metabolomics to inform the region of interest (ROI) selection for downstream spatial proteomics. However, these workflows have been limited by varied substrate requirements between modalities or have required analyzing serial sections (i.e., one section per modality). To mitigate these issues, we present a new multiomic workflow that uses desorption electrospray ionization (DESI)-MSI to identify representative spatial metabolite patterns on-tissue prior to spatial proteomic analyses on the same tissue section. This workflow is demonstrated here with a model mammalian tissue (coronal rat brain section) mounted on a poly(ethylene naphthalate)-membrane slide. Initial DESI-MSI resulted in 160 annotations (SwissLipids) within the METASPACE platform (≤20% false discovery rate). A segmentation map from the annotated ion images informed the downstream ROI selection for spatial proteomics characterization from the same sample. The unspecific substrate requirements and minimal sample disruption inherent to DESI-MSI allowed for an optimized, downstream spatial proteomics assay, resulting in 3888 ± 240 to 4717 ± 48 proteins being confidently directed per ROI (200 μm × 200 μm). Finally, we demonstrate the integration of multiomic information, where we found ceramide localization to be correlated with SMPD3 abundance (ceramide synthesis protein), and we also utilized protein abundance to resolve metabolite isomeric ambiguity. Overall, the integration of DESI-MSI into the multiomic workflow allows for complementary spatial- and molecular-level information to be achieved from optimized implementations of each MS assay inherent to the workflow itself.