Continental precursor transport enhances ozone formation over the atmosphere of tropical coastal waters

Abstract. Ozone (O₃) formation can regulate the atmospheric chemistry and climate, but O₃ formation over the coastal marine atmosphere remains poorly understood, particularly in the tropical regions. To address this gap, our study combines shipborne measurements, coastal observations, and WRF-Chem simulations along the East Coast of Peninsular Malaysia (ECPM) to examine the spatial distribution and controlling processes of tropospheric O₃. Shipborne observations showed a clear north-to-south offshore ozone gradient, with a maximum hourly concentration of 97.7 ppbv in the northern sector. Offshore O₃ was generally higher than at nearby coastal land stations, indicating active ozone production and/or accumulation over the coastal marine atmosphere. Model results suggest that this enhancement was mainly driven by eastward transport of anthropogenic and biogenic precursors under the south-westerly monsoon. During transport, photochemical ageing promoted O₃ formation, while lower nitrogen oxides over the marine boundary layer reduced O₃ loss through nitric oxide titration. Strong solar radiation, warm temperatures, shallow boundary layers, and weak winds further favoured ozone production and accumulation. Sensitivity simulations showed that removing biogenic volatile organic compound emissions reduced surface ozone by 6–15 ppbv over the ECPM and adjacent South China Sea, highlighting the important role of tropical vegetation emissions in O₃ formation. Our study shows that under the influence of continental precursors outflow, the atmosphere above tropical coastal waters can act as active ozone accumulation zones, with important implications for future coastal air quality management and regional ozone chemistry studies in Southeast Asia.