River and Coastal Water VOC Emissions Drive Spatial and Diurnal Air Quality Variability

As urbanization accelerates, accurately assessing the atmospheric consequences of wastewater pollution has become increasingly important. Polluted rivers, overlooked in air quality inventories, can rival or exceed other typically considered sources of airborne volatile organic compounds (VOCs). In this study, we deployed continuous sorbent tube air sampling with Thermal Desorption Gas Chromatography Mass Spectrometry (TD-GCMS) analysis, alongside purge and trap (P&T) dissolved gas measurements over a 25-day period in the Tijuana River Valley in San Diego, California, USA. Gas-phase VOC concentrations were consistently higher near the Tijuana River compared to the coast, with air masses often enriched in sulfur-containing compounds, ethers, and aromatic species. Peak VOC abundance near the Tijuana River aligned with river flow and turbulence at a previously identified Tijuana River hotspot, suggesting bubble-mediated transfer represents a key emission pathway. More than 30% of the elevated VOC species in the air were also present in Tijuana River water, providing chemical evidence of river-to-air gas transfer. These results extend the long-recognized water pollution crisis in the Tijuana River Valley, demonstrating that polluted water bodies can serve as major and compositionally distinct VOC sources in urban airsheds. By linking riverine VOC emissions to atmospheric composition, this study provides a scalable method for diagnosing wastewater impacts and informing mitigation strategies globally.