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

For trace gases destroyed in the stratosphere, mass flux across the tropopause can substantially influence observed surface hemispheric differences (NH-SH). Here, we quantify associations between observed stratospheric and tropospheric NH-SH growth rate anomalies of CFC-11, CFC-12, and N₂O. We employ a chemistry climate model along with satellite and global surface station observations. Our model explains 60% of observed N₂O NH-SH growth rate variability from 2005 to 2019, compared to 30% for CFC-11% and 40% for CFC-12, supporting evidence that unexpected anthropogenic emissions caused sustained positive NH-SH anomalies in these CFCs from 2012 to 2017. Between 2012 and 2015, the observed CFC-11 NH-SH difference grew by 1.7 ppt; our model explains 0.5 ± 0.1 ppt of this growth, but not the duration. Our model suggests that in the absence of further emission anomalies, new NH-SH positive tracer anomalies should have occurred in 2020, and predicts small negative anomalies in 2021.