A Newly Discovered Ozone Formation Mechanism Observed in a Coastal Island of East China

As an important secondary pollutant, ozone (O₃) presents a variety of adverse impacts on human health and air quality. Recently, some coastal background sites in East China suffered from severe O₃ pollution, while the reasons for the high O₃ concentrations were not fully clarified. In our study, a field experiment was performed on Chongming Island to explore the driving factors for locally worsening O₃ pollution. Analysis of the observation data of volatile organic compounds (VOCs) revealed that oxygenated VOCs (OVOCs) were present in higher concentrations compared with other VOC groups. OVOC photolysis was the largest primary source of RO_x radicals (48.2%), leading to the amplification of radical cycling. It is notable that both ozone formation potential (OFP) analysis and detailed chemical box modeling results demonstrated that CH₃CHO exhibited the highest OFP (34.4 ± 9.6 ppbv) and acted as a photochemical amplifier providing an important positive feedback mechanism to accelerate O₃ production. Moreover, sensitivity simulations suggested that aggressive CH₃CHO abatement could alleviate O₃ pollution effectively. Focusing on anthropogenic emission sources with larger contributions to CH₃CHO levels, the results from GEOS-Chem simulations and positive matrix factorization (PMF) model calculation revealed that the simulated CH₃CHO experienced significant contributions from shipping emissions, which could contribute substantially to the major VOC precursors of CH₃CHO. Considering the great contribution of the newly discovered formation mechanism to O₃ production, more stringent control measures for shipping emissions are needed to ensure the sustained improvement of air quality. The result might be also applicable to other similar areas around the world.