Exploring the amplified role of HCHO in the formation of HMS and O.sub.3 during the co-occurring PM.sub.2.5 and O.sub.3 pollution in a coastal city of southeast China

To develop effective strategies for controlling both PM.sub.2.5 and O.sub.3 levels, it is crucial to understand their synergistic mechanisms and key precursors and the atmospheric physiochemical processes involved. In this study, a wintertime co-occurring O.sub.3 and PM.sub.2.5 pollution event in a coastal city in southeast China was investigated based on high-time-resolution measurements of criteria air pollutants and chemical compositions of PM.sub.2.5, and O.sub.3 precursors, such as NO.sub.x, HCHO, and volatile organic compounds (VOCs). The results of this study revealed the characteristics of positively correlated PM.sub.2.5 and MDA8 O.sub.3 concentrations, and an increase in atmospheric oxidation capacity (AOC) during the cold seasons. Strong correlations (R.sup.2 = 0.415-0.477) between HCHO, Fe, Mn, and sulfate concentrations were observed, 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 (SO.sub.2 and HCHO), high relative humidity, and moderately acidic pH conditions enhanced the heterogeneous formation of hydroxymethanesulfonate (HMS) in PM.sub.2.5 . Furthermore, by employing an OBM coupled to the Master Chemical Mechanism (OBM-MCM), we verified that disabling the HCHO mechanism could decrease daytime net O.sub.3 production rates by reducing the production rates of HO.sub.2 + NO. These results were consistent with the daily values of AOC, OH, HO.sub.2, and RO.sub.2 concentrations. This study contributes to a better understanding of the significance of HCHO in photochemical reactions and the formation of HMS in a coastal city.