Chi-Tsan Wang

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.

The presence of dense forest canopies significantly alters the near-field dynamical, physical, and chemical environment, with implications for atmospheric composition and air quality variables such as boundary layer ozone (O3). Observations show profound vertical gradients in O3 concentration beneath forest canopies; however, most chemical transport models (CTMs) used in the operational and research community, such as the Community Multiscale Air Quality (CMAQ) model, cannot account for such effects due to inadequate canopy representation and lack of sub-canopy processes. To address this knowledge gap, we implemented detailed forest canopy processes – including in-canopy photolysis attenuation and turbulence – into the CMAQv5.3.1 model, driven by the Global Forecast System and enhanced with high-resolution vegetation datasets. Simulations were conducted for August 2019 over the contiguous US. The canopy-aware model shows substantial improvement, with mean O3 bias reduced from +0.70 ppb (Base) to −0.10 ppb (Canopy), and fractional bias from +9.71 % to +6.37 %. Monthly mean O3 in the lowest model layer (∼ 0–40 m) decreased by up to 9 ppb in dense forests, especially in the East. Process analysis reveals a 75.2 % drop in first-layer O3 chemical production, with daily surface production declining from 673 to 167 ppb d−1, driven by suppressed photolysis and vertical mixing. This enhances NOx titration and reduces O3 formation under darker, stable conditions. The results highlight the critical role of canopy processes in atmospheric chemistry and demonstrate the importance of incorporating realistic vegetation-atmosphere interactions in CTMs to improve air quality forecasts and health-relevant exposure assessments.

Aerosol particles contain complex mixtures of polar and non-polar species that can undergo organic-inorganic phase separations. 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 CMAQ version (version 5.3) 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 implemented in CMAQ, and the subsequent model predictions were used to compare 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, on average, decreased IEPOX reactive uptake by up to 99.99 % compared to Base CMAQ, resulting in mixed model performance. While 2-methyltetrol performance improved with phase separation and phase state updates, methyltetrol sulfates and total IEPOX-SOA concentrations further underpredicted field observations in comparison to Base CMAQ.

Aerosol particles contain complex mixtures of polar and non-polar species that can undergo organic-inorganic phase separations. 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 CMAQ version (version 5.3) 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 implemented in CMAQ, and the subsequent model predictions were used to compare 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, on average, decreased IEPOX reactive uptake by up to 99.99 % compared to Base CMAQ, resulting in mixed model performance. While 2-methyltetrol performance improved with phase separation and phase state updates, methyltetrol sulfates and total IEPOX-SOA concentrations further underpredicted field observations in comparison to Base CMAQ.

Aerosol particles contain complex mixtures of polar and non-polar species that can undergo organic-inorganic phase separations. 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 CMAQ version (version 5.3) 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 implemented in CMAQ, and the subsequent model predictions were used to compare 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, on average, decreased IEPOX reactive uptake by up to 99.99 % compared to Base CMAQ, resulting in mixed model performance. While 2-methyltetrol performance improved with phase separation and phase state updates, methyltetrol sulfates and total IEPOX-SOA concentrations further underpredicted field observations in comparison to Base CMAQ.

The presence of dense forest canopies significantly alters the near-field dynamical, physical, and chemical environment, with implications for atmospheric composition and air quality variables such as boundary layer ozone (O₃). Observations show profound vertical gradients in O₃ concentration beneath forest canopies; however, most chemical transport models (CTMs) used in the operational and research community, such as the Community Multiscale Air Quality (CMAQ) model, cannot account for such effects due to inadequate canopy representation and lack of sub-canopy processes. To address this knowledge gap, we implemented detailed forest canopy processes—including in-canopy photolysis attenuation and turbulence—into the CMAQv5.3.1 model, driven by the Global Forecast System and enhanced with high-resolution vegetation datasets. Simulations were conducted for August 2019 over the contiguous U.S. The canopy-aware model shows substantial improvement, with mean O₃ bias reduced from +0.70 ppb (Base) to −0.10 ppb (Canopy), and fractional bias from +9.71% to +6.37%. Monthly mean O₃ in the lowest model layer (~0–40 m) decreased by up to 9 ppb in dense forests, especially in the East. Process analysis reveals a 75.2% drop in first-layer O₃, with daily surface production declining from 673 to 167 ppb d⁻¹, driven by suppressed photolysis and vertical mixing. This enhances NOₓ titration and reduces O₃ formation under darker, stable conditions. The results highlight the critical role of canopy processes in atmospheric chemistry and demonstrate the importance of incorporating realistic vegetation-atmosphere interactions in CTMs to improve air quality forecasts and health-relevant exposure assessments.

Aerosol particles contain complex mixtures of polar and non-polar species that can undergo organic-inorganic phase separations. 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 CMAQ version (version 5.3) 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 implemented in CMAQ, and the subsequent model predictions were used to compare 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, on average, decreased IEPOX reactive uptake by up to 99.99 % compared to Base CMAQ, resulting in mixed model performance. While 2-methyltetrol performance improved with phase separation and phase state updates, methyltetrol sulfates and total IEPOX-SOA concentrations further underpredicted field observations in comparison to Base CMAQ.

The presence of dense forest canopies significantly alters the near-field dynamical, physical, and chemical environment, with implications for atmospheric composition and air quality variables such as boundary layer ozone (O₃). Observations show profound vertical gradients in O₃ concentration beneath forest canopies; however, most chemical transport models (CTMs) used in the operational and research community, such as the Community Multiscale Air Quality (CMAQ) model, cannot account for such effects due to inadequate canopy representation and lack of sub-canopy processes. To address this knowledge gap, we implemented detailed forest canopy processes—including in-canopy photolysis attenuation and turbulence—into the CMAQv5.3.1 model, driven by the Global Forecast System and enhanced with high-resolution vegetation datasets. Simulations were conducted for August 2019 over the contiguous U.S. The canopy-aware model shows substantial improvement, with mean O₃ bias reduced from +0.70 ppb (Base) to −0.10 ppb (Canopy), and fractional bias from +9.71% to +6.37%. Monthly mean O₃ in the lowest model layer (~0–40 m) decreased by up to 9 ppb in dense forests, especially in the East. Process analysis reveals a 75.2% drop in first-layer O₃, with daily surface production declining from 673 to 167 ppb d⁻¹, driven by suppressed photolysis and vertical mixing. This enhances NOₓ titration and reduces O₃ formation under darker, stable conditions. The results highlight the critical role of canopy processes in atmospheric chemistry and demonstrate the importance of incorporating realistic vegetation-atmosphere interactions in CTMs to improve air quality forecasts and health-relevant exposure assessments.

The presence of dense forest canopies significantly alters the near-field dynamical, physical, and chemical environment, with implications for atmospheric composition and air quality variables such as boundary layer ozone (O₃). Observations show profound vertical gradients in O₃ concentration beneath forest canopies; however, most chemical transport models (CTMs) used in the operational and research community, such as the Community Multiscale Air Quality (CMAQ) model, cannot account for such effects due to inadequate canopy representation and lack of sub-canopy processes. To address this knowledge gap, we implemented detailed forest canopy processes—including in-canopy photolysis attenuation and turbulence—into the CMAQv5.3.1 model, driven by the Global Forecast System and enhanced with high-resolution vegetation datasets. Simulations were conducted for August 2019 over the contiguous U.S. The canopy-aware model shows substantial improvement, with mean O₃ bias reduced from +0.70 ppb (Base) to −0.10 ppb (Canopy), and fractional bias from +9.71% to +6.37%. Monthly mean O₃ in the lowest model layer (~0–40 m) decreased by up to 9 ppb in dense forests, especially in the East. Process analysis reveals a 75.2% drop in first-layer O₃, with daily surface production declining from 673 to 167 ppb d⁻¹, driven by suppressed photolysis and vertical mixing. This enhances NOₓ titration and reduces O₃ formation under darker, stable conditions. The results highlight the critical role of canopy processes in atmospheric chemistry and demonstrate the importance of incorporating realistic vegetation-atmosphere interactions in CTMs to improve air quality forecasts and health-relevant exposure assessments.