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

Nitrous oxide (N 2 O) 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 N 2 O production pathway not previously considered in greenhouse gas budgets, which we name photochemodenitrification. Sunlight induces substantial and consistent N 2 O production under oxic abiotic conditions in fresh and marine waters. We measured photochemical N 2 O 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 N 2 O production was strongly correlated to the radiation dose. Photochemodenitrification exceeded biological N 2 O production in surface waters. Although previously overlooked, this process may contribute considerably to global N 2 O emissions through its occurrence in fresh and marine surface waters.