Exploring the amplied role of HCHO during the wintertime ozone and PM2.5 pollution in a coastal city of southeast China.

To develop the effective strategies for controlling both PM_2.5 and O₃ levels, it is crucial to understand their synergistic mechanisms, key precursors, and atmospheric physiochemical processes involved. In this study, a wintertime co-occurring O₃ and PM_2.5 pollution event in a coastal city in southeast China was investigated based on high-time resolution measurements of criteria air pollutants, chemical compositions of PM_2.5, and O₃ precursors, such as NOx, HCHO, and VOCs. The results of this study revealed a positive correlation between PM_2.5 and MDA8 O₃ concentrations during the whole periods, suggesting an increase in atmospheric oxidation capacity (AOC) during the cold seasons. Strong correlations (R² = 0.415–0.477) were observed between HCHO, Fe, Mn, and sulfate concentrations, suggesting the influence of catalyzed oxidation processes in the coastal city. Through an observation-based model (OBM) analysis coupled with the regional atmospheric chemistry mechanism version 2 (RACM2) and the chemical aqueous-phase radical mechanism version 3.0 (CAPRAM 3.0), we found that high concentrations of precursors (SO₂ and HCHO), high relative humidity, and moderately acidic pH conditions enhanced the heterogeneous formation of hydroxymethanesulfonate (HMS) in PM_2.5. Furthermore, by employing the Master Chemical Mechanism (OBM-MCM), we verified that disabling the HCHO mechanism could decrease daytime net O₃ production rates by reducing the production rates of HO₂+NO. These results were consistent with the daily values of AOC, OH, HO₂, and RO₂ concentrations. This study contributes to a better understanding of the significance of HCHO in photochemical reactions and the formation of secondary aerosols in a coastal city. [ABSTRACT FROM AUTHOR]