The modeled seasonal cycles of land biosphere and ocean N2O fluxes and
atmospheric N2O
Abstract
Nitrous oxide (N2O) is a greenhouse gas and an ozone-depleting agent
with large and growing anthropogenic emissions. Previous studies
identified the influx of N2O-depleted air from the stratosphere to
partly cause the seasonality in tropospheric N2O (aN2O), but other
contributions remain unclear. Here we combine surface fluxes from eight
land and four ocean models from phase 2 of the Nitrogen/N2O Model
Intercomparison Project with tropospheric transport modeling to simulate
aN2O at the air sampling sites: Alert, Barrow, Ragged Point, Samoa,
Ascension Island, and Cape Grim for the modern and preindustrial
periods. Models show general agreement on the seasonal phasing of
zonal-average N2O fluxes for most sites, but, seasonal peak-to-peak
amplitudes differ severalfold across models. After transport, the
seasonal amplitude of surface aN2O ranges from 0.25 to 0.80 ppb
(interquartile ranges 21-52% of median) for land, 0.14 to 0.25 ppb
(19-42%) for ocean, and 0.13 to 0.76 ppb (26-52%) for combined flux
contributions. The observed range is 0.53 to 1.08 ppb. The stratospheric
contributions to aN2O, inferred by the difference between
surface-troposphere model and observations, show 36-126% larger
amplitudes and minima delayed by ~1 month compared to
Northern Hemisphere site observations. Our results demonstrate an
increasing importance of land fluxes for aN2O seasonality, with land
fluxes and their seasonal amplitude increasing since the preindustrial
era and are projected to grow under anthropogenic activities. In situ
aN2O observations and atmospheric transport-chemistry models will
provide opportunities for constraining terrestrial and oceanic biosphere
models, critical for projecting surface N2O sources under ongoing global
warming.