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. Editor's summary: Nitrous oxide (N2O) is an important greenhouse gas with an atmospheric abundance that is increasing faster than predicted. It is the chemical species primarily responsible for stratospheric ozone depletion. Leon-Palmero et al. describe a pathway that is absent from current greenhouse gas budgets in which sunlight drives abiotic N2O formation in oxic fresh and marine surface waters. They found that production rates are proportional to radiation flux and estimate that this pathway creates more N2O than biological production in surface waters, thus constituting a major source of N2O emissions globally. —Jesse Smith [ABSTRACT FROM AUTHOR]