A new parameterization of uptake coefficient of SO2 and its impact on sulfate concentration

Abstract

Current chemical transport models (CTMs), such as the Community Multiscale Air Quality (CMAQ) model, often struggle to reproduce the peak levels of sulfate accurately due to missing sulfate production pathways. To address this issue, we propose an unaccounted pathway via the heterogeneous reaction of SO₂, implementing a new parameterization of the uptake coefficient of SO₂ (${\gamma }_{{\text{SO}}_2}$). This parameterization accounts for relative humidity and the mixing ratios of NO₂ and NH₃. When the parameterization was incorporated into the CMAQ model (i.e., EXP), sulfate concentrations increased from 3.85 $\mu$ g/m³ to 4.78 $\mu$ g/m³ during the Korea-United States Air Quality (KORUS-AQ) campaign. These enhancements improved model performance, as shown by a reduction in mean bias (MB) from −1.94 $\mu$ g/m³ to −1.00 $\mu$ g/m³ and an increase in the index of agreement (IOA) from 0.63 to 0.70. Additionally, we investigated sulfate concentrations in the EXP across four seasons to confirm that our approach is broadly applicable and not confined to specific temporal conditions. The results also demonstrated a significant improvement in sulfate prediction accuracy, with MB decreasing from −1.68 $\mu$ g/m³ to 0.18 $\mu$ g/m³ and IOA increasing from 0.65 to 0.69. Notably, the most substantial increases in sulfate concentrations were observed during spring and winter, with enhancements of 1.50 $\mu$ g/m³ (57.9 %) and 3.95 $\mu$ g/m³ (213.5 %), respectively. These findings emphasize the importance of utilizing an appropriate ${\gamma }_{{\text{SO}}_2}$ to improve the representation of sulfate chemistry in CTMs.