How Stratospheric Chemistry and Transport Drive Surface Variability of N2O

N₂O is a long-lived greenhouse gas which has steadily increased over the past 50 years. It directly affects climate and participates in ozone depletion, thus further altering climate and atmospheric composition. There are many natural and anthropogenic surface sources of N₂O, and significant efforts have gone into using surface variability to infer the magnitude and location of these sources (Tian et al., 2018). The stratospheric loss of N₂O is manifest at the surface as a negative perturbation in N₂O abundance, and this confounds the effort to derive surface emissions from the variability in surface abundance. In an effort to constrain the stratospheric photochemical sink, we calculate the N₂O loss (TgN/yr) and also estimate a semi-empirical N₂O lifetime from measurements of stratospheric N₂O, O₃, and temperature from the Microwave Limb Sounder (MLS) instrument on the NASA Aura satellite. This derived stratospheric sink of N₂O is consistent with most models for the same period and extends previous work (Prather et al., 2015) from 2004-2010 to 2004-2018.

Stratospheric loss of N₂O has a strong seasonal cycle and a persistent Quasi-Biennial Oscillation (QBO). The QBO can increase/decrease the N₂O sink depending on whether it enhances/suppresses tropical upwelling in the middle stratosphere. We use multiple models (GMI, LMDZ5, and UCI CTM) to investigate how variations in stratospheric N₂O loss interact with the circulation and stratosphere-troposphere exchange (STE) to propagate negative anomalies of N₂O into the troposphere and down to the surface. In our simulations we calculate the stratospheric sink in isolation with no surface sources and directly compare to observations (from HATS at NOAA and from the ACE-FTS instrument on the CSA SCISAT-1 satellite) monthly. We examine how the QBO affects STE and transport of negative-N₂O in each hemisphere and its impact on surface variability. The variations in surface N₂O on seasonal, hemispheric, or long-term interannual scales are mostly driven by surface sources, but are systematically offset by the stratospheric signal. The observed QBO signal, however, is uniquely stratospheric. 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.