A 3-D global modeling budget estimate of atmospheric N2O and its isotopologues for 1980-2019

Abstract:

Nitrous oxide (N₂O) is the third most important anthropogenic greenhouse gas (GHG), after CO₂ and CH₄. It is also a major ozone-depleting substance (ODS) in the stratosphere. Its main sources include anthropogenic activities (mostly agriculture), and natural emissions from ocean and soils. However, emission magnitudes from individual sources are highly uncertain and subject to uncertainties in its lifetime estimate and partitioning among various sources.

In this work, we use the NASA Goddard GEOS-5 chemistry climate model (CCM) to quantify the atmospheric budget of N₂O between 1980-2020. We derive annual global N₂O emissions for 1980-2019 using the NOAA GMD surface N₂O observations and the N₂O lifetime calculated in the NASA GEOSCCM. The inferred global mean N₂O emissions remain relatively unchanged between 1980 and mid-1990s, ~15.5 TgN/yr. Since 1995, N₂O emissions have been growing slowly at a rate of +0.1 TgN/yr and reaching an average emissions rate of 18.1 TgN/yr in the 2010s. With a best estimate pre-industrial N₂O lifetime of ~123 yr and the corresponding natural emissions of 9.1 TgN/yr from oceans and soil, this implies that anthropogenic N₂O emissions have grown rapidly from ~6.6 TgN/yr in the 1980s to about ~9.0 TgN/yr in the 2010s, a 36% increase.

N₂O isotopologues have been used increasingly to investigate the magnitudes of N₂O sources and sinks. We implemented a set of specially designed N₂O isotopic tracers in GEOSCCM to quantify the atmospheric isotopic budget of d¹⁵N and d¹⁸O for N₂O and the contribution of stratospheric enrichment to the N₂O isotopic budget. We estimate that, on global average, stratospheric enrichment contributes about +8.05‰/yr, +7.92‰/yr, and +8.32‰/yr to tropospheric d¹⁵N^α, d¹⁵N^b, and d¹⁸O budgets. The stratospheric isotopic influx contribution is slightly larger, by ~0.3 to ~0.4‰, in the Northern Hemisphere than in the Southern Hemisphere, due to stratosphere-to-troposphere transport with stronger wave activity driving mixing rates and downward Brewer-Dobson stratospheric circulation. Using the combination of a simple one-box model and a 3-D representation of N₂O isotopologues in GEOSCCM, assuming oceanic sources have approximately the same isotopic signature as the tropospheric background, we estimate that the global mean isotopic signature for pre-industrial terrestrial sources were d¹⁵N-a ~ 4.9‰, d¹⁵N-b ~ -12.5‰, d¹⁸O ~ 33.4‰ and the global mean anthropogenic isotopic signatures in recent decades are d¹⁵N-a ~ -20‰, d¹⁵N-b~ -22‰, d¹⁸O ~ 21‰. The 3-d model based anthropogenic isotopic estimates are significantly lighter than most of the previous one-box atmospheric model estimates.