Abstract Reactive intermediates in surface waters are widely attributed to dissolved organic matter (DOM). Natural, incidental, or engineered nanomaterials simultaneously occur at parts per billion and can function as photocatalysts, yet the photochemical role of nanomaterials at environmentally relevant concentrations remains overlooked in surface water photochemistry. Here, we quantify reactive intermediates from TiO2 nanomaterials (1–100 μg L−1) and DOM (1–5 mgC L−1) under 222, 265, and 365 nm.
