Chul Han Song

Halogen radicals (Cl, Br, and I) significantly influence atmospheric oxidation capacity, affecting both O₃ formation and destruction. However, understanding of halogen chemistry remains limited. To better investigate comprehensive atmospheric halogen chemistry, we incorporated halogen processes into the Community Multi-scale Air Quality (CMAQ) model: (i) emissions of Cl₂, HCl, Br₂, and HBr from anthropogenic sources and Br₂, I₂, HOI, and halocarbons from natural sources and (ii) 177 multi-phase halogen reactions. Model performance was evaluated against observed ClNO₂ levels and by comparison with reported ranges of BrO and IO levels. The updated model showed significant improvements in simulating ClNO₂ mixing ratios, with the index of agreement (IOA) increasing from 0.41 to 0.66 and mean bias (MB) decreasing from −159.36 to −25.07 ppt at supersites. Furthermore, simulated BrO and IO levels fell within the ranges reported in previous studies. We found that these improvements were driven by four key reactions: (i) ClO self-reaction, (ii) heterogeneous HOBr chemistry, (iii) NO₂ uptake, and (iv) revised N₂O₅ parameterization. Based on our modeling system, we found that the presence of halogen radicals led to changes in the net O_x production rate (P(O_x)), which increased from 3.08 to 3.33 ppb h⁻¹ on land and decreased from 0.21 to 0.07 ppb h⁻¹ over ocean. It was noted that levels of OH, HCHO, and NO_x also increased by ∼0.007 ppt (5.5 %), ∼0.03 ppb (1.6 %), and ∼0.29 ppb (2.9 %), respectively, while levels of HO₂ and volatile organic compounds (VOCs) decreased by ∼0.45 ppt (5.3 %) and ∼0.71 ppb (5.9 %). These results highlight the importance of accurately representing halogen processes in regional air quality models.

Recent studies have revealed that halogen radicals (Cl, Br, and I) significantly influence atmospheric oxidation capacity, affecting O₃ formation or destruction. However, understanding of halogen chemistries remains limited. To better investigate atmospheric halogen chemistries, we incorporated halogen processes into the Community Multi-scale Air Quality (CMAQ) model: (i) emissions of Cl₂, HCl, Br₂, and HBr from anthropogenic sources, and Br₂, I₂, HOI, and halocarbons from natural sources; and (ii) 177 multi-phase halogen reactions. After developing the model, we examined its performance against observed data. The results demonstrated significant improvements in simulating observed nitryl chloride (ClNO₂) mixing ratios at supersites. The index of agreement (IOA) improved from 0.41 to 0.66, and the mean bias (MB) decreased from -159.36 ppt to -25.07 ppt. These improvements were driven by four atmospheric key reactions: (i) ClO + ClO → Cl₂; (ii) HOBr + Cl^- → BrCl; (iii) different parameterization of γ_N2O5; and (iv) 2NO₂ + Cl^- → ClNO + NO₃^-. We then examined the net O_x production rate (P(O_x)), which increased from 3.08 ppb/h to 3.33 ppb/h on land and decreased from 0.21 ppb/h to 0.07 ppb/h over ocean in the presence of halogen radicals. Further analysis of the impacts of halogen processes on key atmospheric species revealed that levels of OH, HCHO, and NO_x increased by ~0.007 ppt (5.5 %), ~0.03 ppb (1.6 %), and ~0.29 ppb (2.9 %), respectively, while levels of HO₂ and VOCs decreased by ~0.45 ppt (5.3 %) and ~0.71 ppb (5.9 %), respectively.

Recent studies have revealed that halogen radicals (Cl, Br, and I) significantly influence atmospheric oxidation capacity, affecting O₃ formation or destruction. However, understanding of halogen chemistries remains limited. To better investigate atmospheric halogen chemistries, we incorporated halogen processes into the Community Multi-scale Air Quality (CMAQ) model: (i) emissions of Cl₂, HCl, Br₂, and HBr from anthropogenic sources, and Br₂, I₂, HOI, and halocarbons from natural sources; and (ii) 177 multi-phase halogen reactions. After developing the model, we examined its performance against observed data. The results demonstrated significant improvements in simulating observed nitryl chloride (ClNO₂) mixing ratios at supersites. The index of agreement (IOA) improved from 0.41 to 0.66, and the mean bias (MB) decreased from -159.36 ppt to -25.07 ppt. These improvements were driven by four atmospheric key reactions: (i) ClO + ClO → Cl₂; (ii) HOBr + Cl^- → BrCl; (iii) different parameterization of γ_N2O5; and (iv) 2NO₂ + Cl^- → ClNO + NO₃^-. We then examined the net O_x production rate (P(O_x)), which increased from 3.08 ppb/h to 3.33 ppb/h on land and decreased from 0.21 ppb/h to 0.07 ppb/h over ocean in the presence of halogen radicals. Further analysis of the impacts of halogen processes on key atmospheric species revealed that levels of OH, HCHO, and NO_x increased by ~0.007 ppt (5.5 %), ~0.03 ppb (1.6 %), and ~0.29 ppb (2.9 %), respectively, while levels of HO₂ and VOCs decreased by ~0.45 ppt (5.3 %) and ~0.71 ppb (5.9 %), respectively.

Recent studies have revealed that halogen radicals (Cl, Br, and I) significantly influence atmospheric oxidation capacity, affecting O₃ formation or destruction. However, understanding of halogen chemistries remains limited. To better investigate atmospheric halogen chemistries, we incorporated halogen processes into the Community Multi-scale Air Quality (CMAQ) model: (i) emissions of Cl₂, HCl, Br₂, and HBr from anthropogenic sources, and Br₂, I₂, HOI, and halocarbons from natural sources; and (ii) 177 multi-phase halogen reactions. After developing the model, we examined its performance against observed data. The results demonstrated significant improvements in simulating observed nitryl chloride (ClNO₂) mixing ratios at supersites. The index of agreement (IOA) improved from 0.41 to 0.66, and the mean bias (MB) decreased from -159.36 ppt to -25.07 ppt. These improvements were driven by four atmospheric key reactions: (i) ClO + ClO → Cl₂; (ii) HOBr + Cl^- → BrCl; (iii) different parameterization of γ_N2O5; and (iv) 2NO₂ + Cl^- → ClNO + NO₃^-. We then examined the net O_x production rate (P(O_x)), which increased from 3.08 ppb/h to 3.33 ppb/h on land and decreased from 0.21 ppb/h to 0.07 ppb/h over ocean in the presence of halogen radicals. Further analysis of the impacts of halogen processes on key atmospheric species revealed that levels of OH, HCHO, and NO_x increased by ~0.007 ppt (5.5 %), ~0.03 ppb (1.6 %), and ~0.29 ppb (2.9 %), respectively, while levels of HO₂ and VOCs decreased by ~0.45 ppt (5.3 %) and ~0.71 ppb (5.9 %), respectively.

Recent studies have revealed that halogen radicals (Cl, Br, and I) significantly influence atmospheric oxidation capacity, affecting O₃ formation or destruction. However, understanding of halogen chemistries remains limited. To better investigate atmospheric halogen chemistries, we incorporated halogen processes into the Community Multi-scale Air Quality (CMAQ) model: (i) emissions of Cl₂, HCl, Br₂, and HBr from anthropogenic sources, and Br₂, I₂, HOI, and halocarbons from natural sources; and (ii) 177 multi-phase halogen reactions. After developing the model, we examined its performance against observed data. The results demonstrated significant improvements in simulating observed nitryl chloride (ClNO₂) mixing ratios at supersites. The index of agreement (IOA) improved from 0.41 to 0.66, and the mean bias (MB) decreased from -159.36 ppt to -25.07 ppt. These improvements were driven by four atmospheric key reactions: (i) ClO + ClO → Cl₂; (ii) HOBr + Cl^- → BrCl; (iii) different parameterization of γ_N2O5; and (iv) 2NO₂ + Cl^- → ClNO + NO₃^-. We then examined the net O_x production rate (P(O_x)), which increased from 3.08 ppb/h to 3.33 ppb/h on land and decreased from 0.21 ppb/h to 0.07 ppb/h over ocean in the presence of halogen radicals. Further analysis of the impacts of halogen processes on key atmospheric species revealed that levels of OH, HCHO, and NO_x increased by ~0.007 ppt (5.5 %), ~0.03 ppb (1.6 %), and ~0.29 ppb (2.9 %), respectively, while levels of HO₂ and VOCs decreased by ~0.45 ppt (5.3 %) and ~0.71 ppb (5.9 %), respectively.

Nitrous acid (HONO) is a main precursor of hydroxyl radicals (OH), which contribute to the formation of numerous secondary air pollutants in the troposphere. Despite its importance in atmospheric chemistry, HONO chemistry has not been fully incorporated into many chemical transport models (CTMs). Due to the lack of atmospheric HONO processes, CTM simulations often tend to underestimate atmospheric mixing ratios of HONO. This study was undertaken because simulations with the current Community Multiscale Air Quality (CMAQ) model have a strong tendency to underestimate the HONO mixing ratio. In search of missing sources of atmospheric HONO, we attempted to sequentially incorporate the following potential HONO sources and processes into the CMAQ modeling framework: (i) gas-phase HONO reactions, (ii) traffic HONO emissions, (iii) soil HONO emissions, (iv) heterogeneous HONO production on the surfaces of aerosols, (v) heterogeneous HONO formation on tree leaf and building surfaces, and (vi) photolysis reactions of particulates and deposited HNO₃/nitrates called “renoxification”. The simulation performances of the modified CMAQ models were then evaluated by comparing the modeled HONO mixing ratios with the HONO mixing ratios observed at the Olympic Park station in Seoul, South Korea. When HONO processes were fully added to the CMAQ model, average daily HONO mixing ratios increased from 0.06 to 1.18 ppb. The daytime HONO mixing ratios produced from the CMAQ model run with a full account of atmospheric HONO processes were found to be in better agreement with observations than those from the original CMAQ model (CMAQv5.2.1) runs with improved statistical metrics (e.g., the index of agreement (IOA) increased from 0.59 to 0.68, while the mean bias (MB) decreased dramatically from −0.57 to −0.34 ppb). In addition, we investigated the contributions of individual atmospheric HONO processes to HONO mixing ratios, as well as the impacts of HONO atmospheric processes on the concentrations of other atmospheric species in South Korea. All these issues are also discussed in this paper.

Nitrous acid (HONO) is a main precursor of hydroxyl radicals (OH), which contribute to the formation of numerous secondary air pollutants in the troposphere. Despite its importance in atmospheric chemistry, HONO chemistry has not been fully incorporated into many chemical transport models (CTMs). Due to the lack of atmospheric HONO processes, CTM simulations often tend to underestimate atmospheric mixing ratios of HONO. This study was undertaken because simulations with current Community Multiscale Air Quality (CMAQ) model have a strong tendency to underestimate the HONO mixing ratio. In search of missing sources of atmospheric HONO, we attempted to sequentially incorporate the following potential HONO sources and processes into the CMAQ modeling framework: (i) gas-phase HONO reactions; (ii) traffic HONO emissions; (iii) soil HONO emissions; (iv) heterogeneous HONO production on the surfaces of aerosols; (v) heterogeneous HONO formation on tree leaf and building surfaces; (vi) photolysis reactions of particulates and deposited HNO₃/nitrates called ‘renoxification’. The simulation performances of the modified CMAQ models were then evaluated by comparing the modeled HONO mixing ratios with the HONO mixing ratios observed at the Olympic Park station in Seoul, South Korea. When HONO processes were fully added to the CMAQ model, average daily HONO mixing ratios increased from 0.06 ppb to 1.18 ppb. The daytime HONO mixing ratios produced from the CMAQ model run with a full account of atmospheric HONO processes were found to be in better agreement with observations than those from the original CMAQ model (CMAQv5.2.1) runs with improved statistical metrics (e.g., IOA increased from 0.59 to 0.68, while MB decreased dramatically from -0.57 ppb to -0.34 ppb). In addition, we investigated the contributions of individual atmospheric HONO processes to HONO mixing ratios, as well as the impacts of HONO atmospheric processes on the concentrations of other atmospheric species in South Korea. All these issues are also discussed in this manuscript.

Nitrous acid (HONO) is a main precursor of hydroxyl radicals (OH), which contribute to the formation of numerous secondary air pollutants in the troposphere. Despite its importance in atmospheric chemistry, HONO chemistry has not been fully incorporated into many chemical transport models (CTMs). Due to the lack of atmospheric HONO processes, CTM simulations often tend to underestimate atmospheric mixing ratios of HONO. This study was undertaken because simulations with current Community Multiscale Air Quality (CMAQ) model have a strong tendency to underestimate the HONO mixing ratio. In search of missing sources of atmospheric HONO, we attempted to sequentially incorporate the following potential HONO sources and processes into the CMAQ modeling framework: (i) gas-phase HONO reactions; (ii) traffic HONO emissions; (iii) soil HONO emissions; (iv) heterogeneous HONO production on the surfaces of aerosols; (v) heterogeneous HONO formation on tree leaf and building surfaces; (vi) photolysis reactions of particulates and deposited HNO₃/nitrates called ‘renoxification’. The simulation performances of the modified CMAQ models were then evaluated by comparing the modeled HONO mixing ratios with the HONO mixing ratios observed at the Olympic Park station in Seoul, South Korea. When HONO processes were fully added to the CMAQ model, average daily HONO mixing ratios increased from 0.06 ppb to 1.18 ppb. The daytime HONO mixing ratios produced from the CMAQ model run with a full account of atmospheric HONO processes were found to be in better agreement with observations than those from the original CMAQ model (CMAQv5.2.1) runs with improved statistical metrics (e.g., IOA increased from 0.59 to 0.68, while MB decreased dramatically from -0.57 ppb to -0.34 ppb). In addition, we investigated the contributions of individual atmospheric HONO processes to HONO mixing ratios, as well as the impacts of HONO atmospheric processes on the concentrations of other atmospheric species in South Korea. All these issues are also discussed in this manuscript.

Nitrous acid (HONO) is a main precursor of hydroxyl radicals (OH), which contribute to the formation of numerous secondary air pollutants in the troposphere. Despite its importance in atmospheric chemistry, HONO chemistry has not been fully incorporated into many chemical transport models (CTMs). Due to the lack of atmospheric HONO processes, CTM simulations often tend to underestimate atmospheric mixing ratios of HONO. This study was undertaken because simulations with current Community Multiscale Air Quality (CMAQ) model have a strong tendency to underestimate the HONO mixing ratio. In search of missing sources of atmospheric HONO, we attempted to sequentially incorporate the following potential HONO sources and processes into the CMAQ modeling framework: (i) gas-phase HONO reactions; (ii) traffic HONO emissions; (iii) soil HONO emissions; (iv) heterogeneous HONO production on the surfaces of aerosols; (v) heterogeneous HONO formation on tree leaf and building surfaces; (vi) photolysis reactions of particulates and deposited HNO₃/nitrates called ‘renoxification’. The simulation performances of the modified CMAQ models were then evaluated by comparing the modeled HONO mixing ratios with the HONO mixing ratios observed at the Olympic Park station in Seoul, South Korea. When HONO processes were fully added to the CMAQ model, average daily HONO mixing ratios increased from 0.06 ppb to 1.18 ppb. The daytime HONO mixing ratios produced from the CMAQ model run with a full account of atmospheric HONO processes were found to be in better agreement with observations than those from the original CMAQ model (CMAQv5.2.1) runs with improved statistical metrics (e.g., IOA increased from 0.59 to 0.68, while MB decreased dramatically from -0.57 ppb to -0.34 ppb). In addition, we investigated the contributions of individual atmospheric HONO processes to HONO mixing ratios, as well as the impacts of HONO atmospheric processes on the concentrations of other atmospheric species in South Korea. All these issues are also discussed in this manuscript.

Nitrous acid (HONO) is a main precursor of hydroxyl radicals (OH), which contribute to the formation of numerous secondary air pollutants in the troposphere. Despite its importance in atmospheric chemistry, HONO chemistry has not been fully incorporated into many chemical transport models (CTMs). Due to the lack of atmospheric HONO processes, CTM simulations often tend to underestimate atmospheric mixing ratios of HONO. This study was undertaken because simulations with current Community Multiscale Air Quality (CMAQ) model have a strong tendency to underestimate the HONO mixing ratio. In search of missing sources of atmospheric HONO, we attempted to sequentially incorporate the following potential HONO sources and processes into the CMAQ modeling framework: (i) gas-phase HONO reactions; (ii) traffic HONO emissions; (iii) soil HONO emissions; (iv) heterogeneous HONO production on the surfaces of aerosols; (v) heterogeneous HONO formation on tree leaf and building surfaces; (vi) photolysis reactions of particulates and deposited HNO₃/nitrates called ‘renoxification’. The simulation performances of the modified CMAQ models were then evaluated by comparing the modeled HONO mixing ratios with the HONO mixing ratios observed at the Olympic Park station in Seoul, South Korea. When HONO processes were fully added to the CMAQ model, average daily HONO mixing ratios increased from 0.06 ppb to 1.18 ppb. The daytime HONO mixing ratios produced from the CMAQ model run with a full account of atmospheric HONO processes were found to be in better agreement with observations than those from the original CMAQ model (CMAQv5.2.1) runs with improved statistical metrics (e.g., IOA increased from 0.59 to 0.68, while MB decreased dramatically from -0.57 ppb to -0.34 ppb). In addition, we investigated the contributions of individual atmospheric HONO processes to HONO mixing ratios, as well as the impacts of HONO atmospheric processes on the concentrations of other atmospheric species in South Korea. All these issues are also discussed in this manuscript.