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.