Sunlight drives the abiotic formation of nitrous oxide in fresh and marine waters

Nitrous oxide (N2O) is a potent greenhouse gas and the main stratospheric ozone-depleting agent, yet its sources are not well resolved. In this work, we experimentally show a N2O production pathway not previously considered in greenhouse gas budgets, which we name photochemodenitrification. Sunlight induces substantial and consistent N2O production under oxic abiotic conditions in fresh and marine waters. We measured photochemical N2O production rates using isotope tracers and determined that nitrite is the main substrate and that nitrate can also contribute after being photoreduced to nitrite. Additionally, this N2O production was strongly correlated to the radiation dose. Photochemodenitrification exceeded biological N2O production in surface waters. Although previously overlooked, this process may contribute considerably to global N2O emissions through its occurrence in fresh and marine surface waters.