Impacts of the QBO and Stratospheric Loss on N2O

Abstract:

N₂O is a long-lived greenhouse gas that affects atmospheric composition and climate. It has a wide range of natural and anthropogenic surface sources, with a sink in the middle stratosphere. Stratospheric loss results in air becoming depleted in N₂O, which is then transported down to the lower stratosphere, through the tropopause, and into the troposphere, manifesting as a negative perturbation in N₂O abundance at the surface. Using N₂O surface variations to determine its surface sources requires accurate knowledge of the variability caused by the sink. Using multiple chemistry transport models (CTMs; GMI, LMDz5, and UCI), satellite measurements (NASA’s Aura MLS and OMI, and CSA’s SCISAT-1 ACE-FTS), and surface observations (NOAA ESRL), we follow N₂O from where it is destroyed in the tropical middle stratosphere, down to the surface. To verify the stratospheric sink, we compare our model simulations to the N₂O loss (Tg/yr) calculated from MLS measurements. Stratospheric loss of N₂O has a strong seasonal cycle and is further modulated by the Quasi-Biennial Oscillation (QBO) in both observations and the models. The QBO influences tropical upwelling in the middle stratosphere, changing the N₂O sink, and thus its lifetime. For the model simulations to specifically predict stratospheric influence, we define the N₂O tracer N2OX that has no sources, decays quasi-exponentially, and is easily rescaled to current N₂O abundances. Next, we use a synthetic complementary N₂O tracer, CN2O, to calculate and follow the stratosphere-to-troposphere exchange (STE) flux of N₂O-depleted air. CN2O is created when N2OX is destroyed and has a rapid surface sink and thus a sharp gradient across the tropopause like O₃, enabling easy diagnostic of the flux. We also use stratospheric O₃ column measurements from OMI+MLS, stratospheric O₃ from our CTMs, and tracer-tracer correlations from ACE-FTS to track changes in STE flux, seasonally and over QBO cycles. Tracer correlations and STE fluxes of N₂O and O₃ show more negative-N₂O being propagated down to the troposphere in the Southern hemisphere than in the Northern Hemisphere, opposite to that of O₃. Constraining our modeled N₂O with these observations, we can derive a best estimate for the stratospherically driven surface signal in N₂O abundance on seasonal, hemispheric, and interannual scales.