Quantifying the Imprints of Stratospheric Contributions to Interhemispheric Differences in Tropospheric CFC‐11, CFC‐12, and N₂O Abundances

Abstract

Understanding global emissions of long-lived trace gases requires careful interpretation of in situ measurements. Emissions can be inferred from observed changes in atmospheric mole fractions along with an assumed atmospheric lifetime (WMO, 2003(WMO, , 2018. However, sparse networks of in situ trace gas measurements together with large uncertainties in atmospheric lifetimes (Ko et al., 2013) can lead to large uncertainties in inferred emissions (Lickley, 2021). Experts have long looked to hemispheric differences in mole fractions of these chemicals as evidence to support conclusions about changes in anthropogenic emissions (Lovelock et al., 1973). However, recent work illustrates the importance of stratosphere -troposphere fluxes in driving anomalies of in situ trace gas measurements (Laube et al., 2020;Nevison et al., 2011;Ray et al., 2020;Ruiz et al., 2021). Ray et al. (2020) use modeling experiments to show that observed anomalies in North-South hemisphere differences (NH-SH) in CFC-11, CFC-12, and N 2 O tropospheric mole fractions are associated with stratospheric anomalies driven by the Quasi Biennial Oscillation (QBO). This association has yet to be validated or quantified with stratospheric measurements, limiting our ability to interpret

Evaluation of the Stratospheric Contribution to the Inter‐Annual Variabilities of Tropospheric Methane Growth Rates

“…We find that it is at most weakly correlated with the Brewer‐Dobson Circulation and the QBO indices that characterize the variations in the STE. In contrast, the inter‐annual growth rate anomalies of N 2 O, which has shown in previous studies to be controlled by the STE (Lickley et al., 2021; Nevison et al., 2004, 2011; Ray et al., 2020), have significant correlations with these indices. This result is consistent with the fact that the tropospheric sources and sinks play an important role for CH 4 , compared to N 2 O, but does not indicate that the contribution of the stratosphere‐troposphere CH 4 flux is negligible in all cases.…”

“…Previous studies have demonstrated that the $${F}_{S\to T}$$ term has a considerable impact on the tropospheric concentrations of N 2 O, another well‐mixed greenhouse gas, at seasonal and inter‐annual scales (Lickley et al., 2021; Nevison et al., 2004, 2011; Ray et al., 2020; Ruiz et al., 2021; Ruiz & Prather, 2022; Thompson et al., 2013). Driven by the global‐scale Brewer‐Dobson circulation, N 2 O‐rich tropospheric air enters the stratosphere (where N 2 O is destroyed by photochemistry) in the tropics, and N 2 O‐depleted stratospheric air returns to the troposphere in the extra‐tropics, leading to a net troposphere‐to‐stratosphere N 2 O flux ($${F}_{S\to T}$$ < 0) (Ruiz et al., 2021).…”