Coastal megacities represent dynamic physical environments where anthropogenic, biogenic, and marine influences exert a determining impact on atmospheric composition and subsequent air quality, and climate-relevant processes. This session invites contributions that examine the sources, transformations, and impacts of trace gases and aerosols in coastal urban environments. Topics of interest include, but are not limited to: VOC speciation, reactivity, and source attribution across anthropogenic and biogenic regimes; secondary organic aerosol (SOA) formation and evolution from mixed precursor sources; the role of multiphase and heterogeneous chemistry, including halogen-driven processes, in modifying atmospheric composition; diel variability in oxidation pathways and the influence of nighttime chemistry on subsequent daytime conditions; and the impacts of air mass history, boundary layer dynamics, and coastal meteorology on chemical transformations. We particularly encourage studies that integrate observations, laboratory experiments, and modeling approaches to better understand interactions across the atmosphere–biosphere–hydrosphere system.