Manabu Shiraiwa

Secondary organic aerosols (SOA) can adopt liquid, amorphous semi-solid, or glassy solid states in the atmosphere. Previous model simulations have revealed that SOA adopt semisolid or glassy solid phase states over dry regions and in the upper troposphere. A highly viscous phase state in SOA particles can cause kinetic limitations in SOA partitioning, challenging traditional SOA treatments with equilibrium partitioning in chemical transport modeling. In this study, we develop a method to simulate kinetic partitioning of SOA accounting for the impacts of SOA phase state by utilizing effective mass accommodation coefficient (α_eff). We initially test this method in box modeling to calculate equilibration timescale of SOA partitioning for a wide parameter space for bulk diffusivity, volatility, and total aerosol mass loading. Comparison with the chemical timestep of 20 min in GEOS-Chem reveals that equilibrium partitioning is still applicable for particles with bulk diffusivity > 10^-15 cm² s^-1, whereas kinetic partitioning is required for partitioning of semi-and low-volatile compounds into highly viscous particles. To reduce computational burden, we developed analytical equations with gas-particle mass transfer coefficient by effectively accounting for gas and bulk diffusion limitations. The application of the new α_eff kinetic partitioning scheme into GEOS-Chem predicts lowers SOA mass over drylands (e.g., Western US) in the boundary layer due to kinetically limited growth of viscous SOA particles, whereas SOA mass can be described well with equilibrium partitioning in most other areas at the surface and at 850 hPa. In the upper troposphere at 500 hPa, the α_eff -kinetic scheme predicts up to 50% higher SOA mass as evaporation is suppressed for glassy particles. The developed method is easily applicable, improving description and treatments of SOA in chemical transport models.

Secondary organic aerosols (SOA) can adopt liquid, amorphous semi-solid, or glassy solid states in the atmosphere. Previous model simulations have revealed that SOA adopt semisolid or glassy solid phase states over dry regions and in the upper troposphere. A highly viscous phase state in SOA particles can cause kinetic limitations in SOA partitioning, challenging traditional SOA treatments with equilibrium partitioning in chemical transport modeling. In this study, we develop a method to simulate kinetic partitioning of SOA accounting for the impacts of SOA phase state by utilizing effective mass accommodation coefficient (α_eff). We initially test this method in box modeling to calculate equilibration timescale of SOA partitioning for a wide parameter space for bulk diffusivity, volatility, and total aerosol mass loading. Comparison with the chemical timestep of 20 min in GEOS-Chem reveals that equilibrium partitioning is still applicable for particles with bulk diffusivity > 10^-15 cm² s^-1, whereas kinetic partitioning is required for partitioning of semi-and low-volatile compounds into highly viscous particles. To reduce computational burden, we developed analytical equations with gas-particle mass transfer coefficient by effectively accounting for gas and bulk diffusion limitations. The application of the new α_eff kinetic partitioning scheme into GEOS-Chem predicts lowers SOA mass over drylands (e.g., Western US) in the boundary layer due to kinetically limited growth of viscous SOA particles, whereas SOA mass can be described well with equilibrium partitioning in most other areas at the surface and at 850 hPa. In the upper troposphere at 500 hPa, the α_eff -kinetic scheme predicts up to 50% higher SOA mass as evaporation is suppressed for glassy particles. The developed method is easily applicable, improving description and treatments of SOA in chemical transport models.

The burden of human health in urban clusters across India is associated with increasing fine particulate matter, PM2.5. However, associating PM2.5 health impacts with its mass burden alone without quantifiable relation to toxicity indicators over polluted regions may cause inaccurate estimations of disease burden resulting from varying chemical composition. Here, we present 17-day measurement results of oxidative potential quantified using hydroxyl radical assay (OP–OH) and environmentally persistent free radicals (EPFRs) in the coastal city of Chennai. We quantify EPFR concentration and OP–OH activity to be 34 ± 21 pmol m–3 and 11 ± 5.4 pmol min–1 m–3, respectively, and observed a weak association between OP–OH and EPFRs. Westerly and mixed wind separation helped characterize the distinct physical and chemical properties of PM2.5. During westerly winds, EPFRs were moderately correlated with particle number concentration 0.68; hydrocarbon-like organic aerosols (HOA), NO2, Fe, Sn, and Ti). During mixed winds, OP–OH exhibited moderate correlations with transition metals (Cr and Zn; R2 > 0.34) and particle number concentration >100 nm (R2 = 0.5) and a clear correlation with EPFRs (R2 = 0.59). Our result underscores the complexity of EPFRs and OP–OH in PM2.5 over polluted urban clusters, suggesting the importance of particle size dependence along with chemistry.

Secondary organic aerosols (SOA) can adopt liquid, amorphous semi-solid, or glassy solid states in the atmosphere. Previous model simulations have revealed that SOA adopt semisolid or glassy solid phase states over dry regions and in the upper troposphere. A highly viscous phase state in SOA particles can cause kinetic limitations in SOA partitioning, challenging traditional SOA treatments with equilibrium partitioning in chemical transport modeling. In this study, we develop a method to simulate kinetic partitioning of SOA accounting for the impacts of SOA phase state by utilizing effective mass accommodation coefficient (α_eff). We initially test this method in box modeling to calculate equilibration timescale of SOA partitioning for a wide parameter space for bulk diffusivity, volatility, and total aerosol mass loading. Comparison with the chemical timestep of 20 min in GEOS-Chem reveals that equilibrium partitioning is still applicable for particles with bulk diffusivity > 10^-15 cm² s^-1, whereas kinetic partitioning is required for partitioning of semi-and low-volatile compounds into highly viscous particles. To reduce computational burden, we developed analytical equations with gas-particle mass transfer coefficient by effectively accounting for gas and bulk diffusion limitations. The application of the new α_eff kinetic partitioning scheme into GEOS-Chem predicts lowers SOA mass over drylands (e.g., Western US) in the boundary layer due to kinetically limited growth of viscous SOA particles, whereas SOA mass can be described well with equilibrium partitioning in most other areas at the surface and at 850 hPa. In the upper troposphere at 500 hPa, the α_eff -kinetic scheme predicts up to 50% higher SOA mass as evaporation is suppressed for glassy particles. The developed method is easily applicable, improving description and treatments of SOA in chemical transport models.

Oxidative stress induced by reactive oxygen species (ROS) is a key process for adverse health effects upon respiratory deposition of particulate matter (PM). Alveolar macrophages can release superoxide upon phagocytosis of PM through a process called the respiratory burst. The release of cellular ROS should depend on both PM composition and dose, but the dose–response relationship for various types of PM remains poorly quantified. Here we measure cellular superoxide production by macrophage cells exposed to secondary organic aerosols (SOA) derived from oxidation of limonene, toluene, and naphthalene, as well as PM generated from biomass burning and flaming combustion of five different types of plastics. We quantify cellular superoxide production over the span of 4 h after an initial exposure of macrophages to PM suspensions by applying a chemiluminescence assay combined with electron paramagnetic resonance. We then define the activation threshold as the dose at which cells exhibit a clear increase in superoxide production. Limonene SOA is found to induce the largest amount of superoxide release (∼32 μM) but has the largest activation threshold of 300 μg mL–1. Naphthalene and toluene SOA show smaller enhancement of superoxide production (∼2.5–7 μM) but have lower activation thresholds from 0.02 to 0.1 μg mL–1. Biomass burning PM does not show a clear activation threshold. Plastic burning PM shows a varied response depending on the type of plastic, with PM derived from poly(ethylene terephthalate) (PET) having the highest superoxide production (∼12 μM). The activation thresholds for polystyrene (PS), low-density polyethylene (LDPE), and high-density polyethylene (HDPE) burning PM are very low (∼0.01 μg mL–1), while those for poly(vinyl chloride) (PVC) and PET burning PM are substantially higher (50–100 μg mL–1). Our findings highlight the importance of employing dose–response relationships and superoxide activation thresholds, rather than single-dose comparisons, to evaluate and compare the oxidative stress potential of different PMs more accurately.

Aerosol particles contain complex mixtures of polar and non-polar species that can undergo organic-inorganic phase separation. In phase-separated aerosol particles, the phase state of the outer organic coating can modulate heterogeneous chemistry. Heterogeneous chemistry leading to isoprene epoxydiol (IEPOX)-derived secondary organic aerosol (IEPOX-SOA) is encoded in the Community Multiscale Air Quality (CMAQ) model and has been the focus of previous aerosol phase separation and phase state work. In a previous study, a constant ratio of water in the organic coating ( w _s ) was assumed in modeling phase separation and state. Recent studies, however, have highlighted w _s as an important modulator of phase state. This work uses a later CMAQ version (version 5.3.2) with capabilities to model dynamic water uptake to the organic coating – to better predict w _s and its impact on the organic coating phase state. In addition, new parameterizations for estimating organic aerosol phase state were encoded into CMAQ, and were compared with respect to their impacts on phase state and IEPOX-SOA predictions. These evaluations were completed simulating a summertime episode over the continental United States. Simulated diurnal profiles of aerosol phase state agreed within one standard deviation of observationally-derived field measurements. The implementation of phase separation and phase state parameterizations resulted in times and grid cells where IEPOX reactive uptake is completely suppressed. While modelled positive bias in 2-methyltetrol concentrations were decreased with phase separation and phase state updates, modelled methyltetrol sulfates and total IEPOX-SOA concentrations further underpredicted field observations in comparison to Base CMAQ.

Secondary organic aerosols (SOA) can adopt liquid, amorphous semi-solid, or glassy solid states in the atmosphere. Previous model simulations have revealed that SOA adopt semisolid or glassy solid phase states over dry regions and in the upper troposphere. A highly viscous phase state in SOA particles can cause kinetic limitations in SOA partitioning, challenging traditional SOA treatments with equilibrium partitioning in chemical transport modeling. In this study, we develop a method to simulate kinetic partitioning of SOA accounting for the impacts of SOA phase state by utilizing effective mass accommodation coefficient (α_eff). We initially test this method in box modeling to calculate equilibration timescale of SOA partitioning for a wide parameter space for bulk diffusivity, volatility, and total aerosol mass loading. Comparison with the chemical timestep of 20 min in GEOS-Chem reveals that equilibrium partitioning is still applicable for particles with bulk diffusivity > 10^-15 cm² s^-1, whereas kinetic partitioning is required for partitioning of semi-and low-volatile compounds into highly viscous particles. To reduce computational burden, we developed analytical equations with gas-particle mass transfer coefficient by effectively accounting for gas and bulk diffusion limitations. The application of the new α_eff kinetic partitioning scheme into GEOS-Chem predicts lowers SOA mass over drylands (e.g., Western US) in the boundary layer due to kinetically limited growth of viscous SOA particles, whereas SOA mass can be described well with equilibrium partitioning in most other areas at the surface and at 850 hPa. In the upper troposphere at 500 hPa, the α_eff -kinetic scheme predicts up to 50% higher SOA mass as evaporation is suppressed for glassy particles. The developed method is easily applicable, improving description and treatments of SOA in chemical transport models.

Personal care products spread on permeable materials generate a reservoir for sustained limonene emissions, potentially increasing indoor limonene concentrations and influencing indoor air quality.

Photochemical oxidation in the aqueous phase is critical for chemical evolution of secondary organic aerosols (SOA), but it represents one of the most uncertain processes in aerosols and clouds. Here, we show that photoirradiation of SOA derived from biomass burning precursors leads to substantial formation of superoxide. The combination of laboratory measurements and kinetic modeling reveals that superoxide generation is driven by photosensitization reactions in aromatic SOA, while the dominant source of superoxide in biogenic SOA is photoinduced decomposition of carbonyls with minor contributions from peroxide decomposition. These processes serve as a substantial source of reactive oxygen species (ROS) including hydrogen peroxide and hydroxyl radical in cloud droplets and deliquesced particles, competing with traditional sources such as uptake from the gas phase. This previously unrecognized source of ROS will enhance the impact of aqueous-phase processing on composition and properties of aerosols and clouds.

Mass accommodation is a critical process, controlling uptake coefficients of gaseous species in aerosols and clouds. The effective mass accommodation coefficient (αeff) was developed to account for kinetic limitations of bulk diffusion and reactions with a simple analytical equation. In this work, αeff is extended by including surface reactions and bulk accommodation. Systematic simulations over a wide range of kinetic parameters demonstrate excellent agreement between the αeff method and a benchmark kinetic multilayer model once quasi-equilibrium is achieved in the effective penetration depth. The αeff method can quantify the contributions of surface and bulk processes to overall uptake. Comparisons with two other models (Wilson et al., 2022; Prophet and Wilson, 2025) show that the αeff method more accurately predicts uptake coefficients when uptake is limited by bulk diffusion of volatile species or when surface reactions dominate under high surface coverage conditions. The developed method provides a useful tool for analyzing experimental measurements and developing uptake parametrizations for large-scale models.