Contaminated Tijuana River Contributes to Regional Particulate Matter (PM) Levels Highlighting Overlooked Water–Air Transfer of Gaseous PM Precursors

The San Diego–Tijuana border region has experienced rapid urbanization and industrial growth with unmitigated environmental consequences. For nearly a century, the Tijuana River (TR) has carried untreated sewage and industrial waste into the United States, contributing to a long-recognized and ongoing water pollution crisis. However, the impact of this pollution on air quality has been almost entirely overlooked until very recently. Analysis of low-cost air sensor data reveals that gases released from the polluted TR directly contribute to the formation of fine aerosols, increasing PM₁ (particulate matter <1 μm) concentrations, particularly when river flow is high and atmospheric dispersion is low. Analysis of PM₁ size distributions revealed the enhancement of smaller particle fractions, and persistently high PM₁-to-PM_2.5 ratios (≥0.56 ± 0.15) showed that submicrometer particles constitute the majority of fine PM mass. Combined with recent evidence of elevated gas-phase emissions from the polluted TR, these results point to secondary aerosol formation─driven by the chemical transformations of river-emitted gaseous precursors─as a major source of PM_1. Concentrations peaked near a turbulent riverine hotspot, particularly at night when intensified flow and stable conditions promoted secondary aerosol formation. These findings identify a previously unrecognized source of urban air pollution, showing that the river-to-air transfer of particulate precursors can perpetuate poor air quality and heighten environmental justice and public health concerns.