Qianjie Chen

Snow with high albedo enhances atmospheric photochemical reactions, influencing key oxidative processes. Nitryl chloride (ClNO ₂ ), as a strong oxidizing species, is generated by the heterogeneous reaction between dinitrogen pentoxide (N ₂ O ₅ ) and chloride adsorbed on aerosol and the ground surfaces. After sunrise, the photolysis of ClNO ₂ rapidly releases highly reactive chlorine radicals (Cl⚫), which contributes to the formation of secondary pollutants. However, the pollution mechanisms in high-latitude, snow-covered regions associated with increased chlorine emissions remain unclear. In this study, we employed the WRF-CAMx model (Weather Research and Forecasting Model-Comprehensive Air Quality Model with extensions) with a modified chemical mechanism (CB6r2h_lts, Carbon Bond 6 revision 2 with heterogeneous chemistry for low-temperature and snow-covered conditions) that incorporated heterogeneous N ₂ O ₅ reactions and ClNO ₂ photolysis on ground surfaces to assess their impact on regional atmosphere under snow-covered conditions in Northeast China. Our findings reveal that under snow-covered conditions, the YU20 aerosol scheme (from study by Yu et al., 2020) outperforms the BT09 scheme (from study by Bertram and Thornton, 2009) in simulating N ₂ O ₅ and ClNO ₂ concentrations within the CAMx model. Incorporating anthropogenic chlorine emissions and ground surface chemistry significantly improved model performance for ClNO ₂ , reducing the mean bias (MB) from −105.78 to 2.66 pptv and increasing the index of agreement (IOA) from 0.39 to 0.86. These processes resulted in a maximum hourly increase of 3.65 µg m ⁻³ in PM _2.5 (relative contribution: 15.34 %) and 3.41 ppbv in MDA8 O ₃ (5.68 %). Notably, ground surface chemical processes were identified as the dominant source of nocturnal ClNO ₂ , contributing approximately 28 % to nighttime accumulation across Northeast China. These findings not only highlight the pivotal role of chlorine chemistry in atmospheric processes under snow-covered conditions, but also provide crucial support for the refinement of the mechanisms governing the flux exchange of chemical substances between the atmosphere and the cryosphere.

Snow with high albedo enhances atmospheric photochemical reactions, influencing key oxidative processes. Nitryl chloride (ClNO₂), as a strong oxidizing species, is generated by the heterogeneous reaction between dinitrogen pentoxide (N₂O₅) and chloride adsorbed on aerosol and the ground surfaces. After sunrise, the photolysis of ClNO₂ rapidly releases highly reactive chlorine radicals (Cl·), which contributes to the formation of secondary pollutants. However, the pollution mechanisms in high-latitude, snow-covered regions associated with increased chlorine emissions remain unclear. In this study, we employed the WRF-CAMx model (Weather Research and Forecasting Model-Comprehensive Air Quality Model with extensions) with a modified chemical mechanism (CB6r2h_lts, Carbon Bond 6 revision 2 with heterogeneous chemistry for low-temperature and snow-covered conditions) that incorporated heterogeneous N₂O₅ reactions and ClNO₂ photolysis on ground surfaces to assess their impact on regional atmosphere under snow-covered conditions in Northeast China. Our findings reveal that under snow-covered conditions, the YU20 aerosol scheme (from study by YU et al., 2020) outperforms the BT09 scheme (from study by Bertram et al., 2009) in simulating N₂O₅ and ClNO₂ concentrations within the CAMx model. Incorporating anthropogenic chlorine emissions and ground surface chemistry significantly improved model performance for ClNO₂, reducing the mean bias (MB) from -105.78 pptv to 2.66 pptv and increasing the index of agreement (IOA) from 0.39 to 0.86. These processes resulted in a maximum hourly increase of 3.65 µg/m³ in PM_2.5 (relative contribution: 15.34 %) and 3.41 ppbv in MDA8 O₃ (5.68 %). Notably, ground surface chemical processes were identified as the dominant source of nocturnal ClNO₂, contributing approximately 28.36 % to nighttime accumulation across Northeast China. These findings not only highlight the pivotal role of chlorine chemistry in atmospheric processes under snow-covered conditions, but also provide crucial support for the refinement of the mechanisms governing the flux exchange of chemical substances between the atmosphere and the cryosphere.

Snow with high albedo enhances atmospheric photochemical reactions, influencing key oxidative processes. Nitryl chloride (ClNO₂), as a strong oxidizing species, is generated by the heterogeneous reaction between dinitrogen pentoxide (N₂O₅) and chloride adsorbed on aerosol and the ground surfaces. After sunrise, the photolysis of ClNO₂ rapidly releases highly reactive chlorine radicals (Cl·), which contributes to the formation of secondary pollutants. However, the pollution mechanisms in high-latitude, snow-covered regions associated with increased chlorine emissions remain unclear. In this study, we employed the WRF-CAMx model (Weather Research and Forecasting Model-Comprehensive Air Quality Model with extensions) with a modified chemical mechanism (CB6r2h_lts, Carbon Bond 6 revision 2 with heterogeneous chemistry for low-temperature and snow-covered conditions) that incorporated heterogeneous N₂O₅ reactions and ClNO₂ photolysis on ground surfaces to assess their impact on regional atmosphere under snow-covered conditions in Northeast China. Our findings reveal that under snow-covered conditions, the YU20 aerosol scheme (from study by YU et al., 2020) outperforms the BT09 scheme (from study by Bertram et al., 2009) in simulating N₂O₅ and ClNO₂ concentrations within the CAMx model. Incorporating anthropogenic chlorine emissions and ground surface chemistry significantly improved model performance for ClNO₂, reducing the mean bias (MB) from -105.78 pptv to 2.66 pptv and increasing the index of agreement (IOA) from 0.39 to 0.86. These processes resulted in a maximum hourly increase of 3.65 µg/m³ in PM_2.5 (relative contribution: 15.34 %) and 3.41 ppbv in MDA8 O₃ (5.68 %). Notably, ground surface chemical processes were identified as the dominant source of nocturnal ClNO₂, contributing approximately 28.36 % to nighttime accumulation across Northeast China. These findings not only highlight the pivotal role of chlorine chemistry in atmospheric processes under snow-covered conditions, but also provide crucial support for the refinement of the mechanisms governing the flux exchange of chemical substances between the atmosphere and the cryosphere.

Snow with high albedo enhances atmospheric photochemical reactions, influencing key oxidative processes. Nitryl chloride (ClNO₂), as a strong oxidizing species, is generated by the heterogeneous reaction between dinitrogen pentoxide (N₂O₅) and chloride adsorbed on aerosol and the ground surfaces. After sunrise, the photolysis of ClNO₂ rapidly releases highly reactive chlorine radicals (Cl·), which contributes to the formation of secondary pollutants. However, the pollution mechanisms in high-latitude, snow-covered regions associated with increased chlorine emissions remain unclear. In this study, we employed the WRF-CAMx model (Weather Research and Forecasting Model-Comprehensive Air Quality Model with extensions) with a modified chemical mechanism (CB6r2h_lts, Carbon Bond 6 revision 2 with heterogeneous chemistry for low-temperature and snow-covered conditions) that incorporated heterogeneous N₂O₅ reactions and ClNO₂ photolysis on ground surfaces to assess their impact on regional atmosphere under snow-covered conditions in Northeast China. Our findings reveal that under snow-covered conditions, the YU20 aerosol scheme (from study by YU et al., 2020) outperforms the BT09 scheme (from study by Bertram et al., 2009) in simulating N₂O₅ and ClNO₂ concentrations within the CAMx model. Incorporating anthropogenic chlorine emissions and ground surface chemistry significantly improved model performance for ClNO₂, reducing the mean bias (MB) from -105.78 pptv to 2.66 pptv and increasing the index of agreement (IOA) from 0.39 to 0.86. These processes resulted in a maximum hourly increase of 3.65 µg/m³ in PM_2.5 (relative contribution: 15.34 %) and 3.41 ppbv in MDA8 O₃ (5.68 %). Notably, ground surface chemical processes were identified as the dominant source of nocturnal ClNO₂, contributing approximately 28.36 % to nighttime accumulation across Northeast China. These findings not only highlight the pivotal role of chlorine chemistry in atmospheric processes under snow-covered conditions, but also provide crucial support for the refinement of the mechanisms governing the flux exchange of chemical substances between the atmosphere and the cryosphere.

Molecular chlorine (Cl ₂ ) plays a significant role in shaping atmospheric oxidative capacity (AOC), yet the GEOS-Chem global model tends to underestimate Cl ₂ concentrations due to incomplete representations of its formation pathways. Here, we adapt an iron (Fe)-mediated Cl ₂ formation mechanism into the GEOS-Chem model, explicitly representing the dynamic solubility of iron and Cl ₂ production. This implementation enables the GEOS-Chem model to better reproduce observed Cl ₂ concentrations, increasing correlation coefficient from 0.55 to 0.88 relative to the Base simulation (without Fe–Cl mechanism). Global surface mean Cl ₂ concentration increases about fivefold (from 0.4 to 2.2 pptv) which strengthens radical propagation, causing approximately threefold and fourfold rise in global Cl and ClO radicals, respectively. These radical perturbations further result in pronounced spatial heterogeneity in AOC. While global mean OH decreases by 5.7 % due to Cl-driven O ₃ removal and conversion of HO _x to ClO _x , eastern China experiences concurrent increases in O ₃ and OH (up to 14 %), as enhanced RO ₂ formation from Cl-accelerated VOCs oxidation elevates both OH and O ₃ under high-NO _x conditions. Consequently, the strengthened AOC intensifies regional secondary aerosol formation with wintertime PM _2.5 in eastern China surges by up to 6 %, driven primarily by accelerated nitrate production. Conversely, a discernible decline in PM _2.5 occurs in the downwind regions with enhanced AOC and also remote marine regions. These findings underscore the importance to consider iron-chlorine coupling chemistry in the GEOS-Chem global model for accurately representing atmospheric oxidation processes and enhancing its reliability of air quality assessments.

Snow with high albedo enhances atmospheric photochemical reactions, influencing key oxidative processes. Nitryl chloride (ClNO₂), as a strong oxidizing species, is generated by the heterogeneous reaction between dinitrogen pentoxide (N₂O₅) and chloride adsorbed on aerosol and the ground surfaces. After sunrise, the photolysis of ClNO₂ rapidly releases highly reactive chlorine radicals (Cl·), which contributes to the formation of secondary pollutants. However, the pollution mechanisms in high-latitude, snow-covered regions associated with increased chlorine emissions remain unclear. In this study, we employed the WRF-CAMx model (Weather Research and Forecasting Model-Comprehensive Air Quality Model with extensions) with a modified chemical mechanism (CB6r2h_lts, Carbon Bond 6 revision 2 with heterogeneous chemistry for low-temperature and snow-covered conditions) that incorporated heterogeneous N₂O₅ reactions and ClNO₂ photolysis on ground surfaces to assess their impact on regional atmosphere under snow-covered conditions in Northeast China. Our findings reveal that under snow-covered conditions, the YU20 aerosol scheme (from study by YU et al., 2020) outperforms the BT09 scheme (from study by Bertram et al., 2009) in simulating N₂O₅ and ClNO₂ concentrations within the CAMx model. Incorporating anthropogenic chlorine emissions and ground surface chemistry significantly improved model performance for ClNO₂, reducing the mean bias (MB) from -105.78 pptv to 2.66 pptv and increasing the index of agreement (IOA) from 0.39 to 0.86. These processes resulted in a maximum hourly increase of 3.65 µg/m³ in PM_2.5 (relative contribution: 15.34 %) and 3.41 ppbv in MDA8 O₃ (5.68 %). Notably, ground surface chemical processes were identified as the dominant source of nocturnal ClNO₂, contributing approximately 28.36 % to nighttime accumulation across Northeast China. These findings not only highlight the pivotal role of chlorine chemistry in atmospheric processes under snow-covered conditions, but also provide crucial support for the refinement of the mechanisms governing the flux exchange of chemical substances between the atmosphere and the cryosphere.

Snow with high albedo enhances atmospheric photochemical reactions, influencing key oxidative processes. Nitryl chloride (ClNO₂), as a strong oxidizing species, is generated by the heterogeneous reaction between dinitrogen pentoxide (N₂O₅) and chloride adsorbed on aerosol and the ground surfaces. After sunrise, the photolysis of ClNO₂ rapidly releases highly reactive chlorine radicals (Cl·), which contributes to the formation of secondary pollutants. However, the pollution mechanisms in high-latitude, snow-covered regions associated with increased chlorine emissions remain unclear. In this study, we employed the WRF-CAMx model (Weather Research and Forecasting Model-Comprehensive Air Quality Model with extensions) with a modified chemical mechanism (CB6r2h_lts, Carbon Bond 6 revision 2 with heterogeneous chemistry for low-temperature and snow-covered conditions) that incorporated heterogeneous N₂O₅ reactions and ClNO₂ photolysis on ground surfaces to assess their impact on regional atmosphere under snow-covered conditions in Northeast China. Our findings reveal that under snow-covered conditions, the YU20 aerosol scheme (from study by YU et al., 2020) outperforms the BT09 scheme (from study by Bertram et al., 2009) in simulating N₂O₅ and ClNO₂ concentrations within the CAMx model. Incorporating anthropogenic chlorine emissions and ground surface chemistry significantly improved model performance for ClNO₂, reducing the mean bias (MB) from -105.78 pptv to 2.66 pptv and increasing the index of agreement (IOA) from 0.39 to 0.86. These processes resulted in a maximum hourly increase of 3.65 µg/m³ in PM_2.5 (relative contribution: 15.34 %) and 3.41 ppbv in MDA8 O₃ (5.68 %). Notably, ground surface chemical processes were identified as the dominant source of nocturnal ClNO₂, contributing approximately 28.36 % to nighttime accumulation across Northeast China. These findings not only highlight the pivotal role of chlorine chemistry in atmospheric processes under snow-covered conditions, but also provide crucial support for the refinement of the mechanisms governing the flux exchange of chemical substances between the atmosphere and the cryosphere.

Molecular chlorine (Cl₂) plays a significant role in shaping atmospheric oxidative capacity (AOC), yet global models tend to underestimate Cl₂ concentrations due to incomplete representations of its formation pathways. Here, we implement an iron (Fe)-mediated Cl₂ formation mechanism into the GEOS-Chem model, explicitly representing the dynamic solubility of iron driven by acid processing, organic complexation, and mineralogical variability. The updated mechanism substantially improves the model performance for tropospheric Cl₂, increasing correlation coefficient with observations from 0.55 to 0.88 relative to the Base simulation (without Fe-Cl mechanism). Global surface mean Cl₂ concentration increases about fivefold (from 0.4 to 2.2 pptv) which strengthens radical propagation, leading to approximately threefold and fourfold rise in global Cl and ClO radicals, respectively. These radical perturbations further result in pronounced spatial heterogeneity in AOC. While global mean OH decreases by 5.7 % due to removal of O₃ by Cl and conversion of HOx to ClOx, eastern China experiences concurrent increases in O₃ and OH (up to 14 %), as enhanced RO₂ formation from Cl-accelerated VOCs oxidation elevates both OH and O₃ under high‑NOx conditions. The enhanced AOC also intensifies secondary aerosol formation in eastern China, yielding a maximum of 6 % increase in PM_2.5 concentrations during wintertime, driven primarily by accelerated nitrate production. These findings demonstrate that iron-mediated chlorine activation is an important but previously underrepresented driver of global halogen chemistry. Incorporating iron-mediated photochemistry into global models is therefore essential for accurately representing atmospheric oxidation processes and enhancing the reliability of air quality assessments.

Molecular chlorine (Cl₂) plays a significant role in shaping atmospheric oxidative capacity (AOC), yet global models tend to underestimate Cl₂ concentrations due to incomplete representations of its formation pathways. Here, we implement an iron (Fe)-mediated Cl₂ formation mechanism into the GEOS-Chem model, explicitly representing the dynamic solubility of iron driven by acid processing, organic complexation, and mineralogical variability. The updated mechanism substantially improves the model performance for tropospheric Cl₂, increasing correlation coefficient with observations from 0.55 to 0.88 relative to the Base simulation (without Fe-Cl mechanism). Global surface mean Cl₂ concentration increases about fivefold (from 0.4 to 2.2 pptv) which strengthens radical propagation, leading to approximately threefold and fourfold rise in global Cl and ClO radicals, respectively. These radical perturbations further result in pronounced spatial heterogeneity in AOC. While global mean OH decreases by 5.7 % due to removal of O₃ by Cl and conversion of HOx to ClOx, eastern China experiences concurrent increases in O₃ and OH (up to 14 %), as enhanced RO₂ formation from Cl-accelerated VOCs oxidation elevates both OH and O₃ under high‑NOx conditions. The enhanced AOC also intensifies secondary aerosol formation in eastern China, yielding a maximum of 6 % increase in PM_2.5 concentrations during wintertime, driven primarily by accelerated nitrate production. These findings demonstrate that iron-mediated chlorine activation is an important but previously underrepresented driver of global halogen chemistry. Incorporating iron-mediated photochemistry into global models is therefore essential for accurately representing atmospheric oxidation processes and enhancing the reliability of air quality assessments.