a, b, Points represent the observation-based (blue symbols) or modelled (red and grey symbols) hemispheric difference as a function of the global emission rate derived for that year in the three-box model for HCFC-22 (a) and for HFC-134a (b; see Methods; lines connect sequential years and the legend applies to a and b). The sensitivity of the hemispheric mole fraction difference (N−S difference) to exchange timescale (τ exch, N−S ) was tested in the model by incorporating values of ± 0.1 yr around 1.1 yr. If this timescale did not vary interannually, we would expect the observation-based points (blue) to overlay those from the model (red). A change in the annual mean value of this exchange would increase the difference between the observed and modelled N−S difference. Specifically, an annual mean change of ± 0.1 year would be reflected in the observed N−S difference being two-thirds of the way closer to the grey point associated with the emission derived for that year. The consistency between the model (red) and observed (blue) hemispheric differences in most years suggests that interannual changes in the exchange timescale are 0.1 yr (around 10%) or less, typically. More importantly, the results show no systematic change in this relationship before and after 2012, suggesting that any change in the rate of hemispheric air exchange in the troposphere is less than 10% during this period. We estimate that to fit the observed increase in the N–S difference measured for CFC-11 after 2012 without increasing the net CFC-11 flux to the Northern Hemisphere, this exchange time constant would have had to increase from 1.1 to 1.7 yr, which is inconsistent with the results presented here. Although the distribution of emissions between and within the hemispheres can affect the N–S difference, any considerable change in this distribution over time would probably be a shift to lower latitudes (away from the US and Europe) and would lead to a decrease in the N−S difference over time, not an increase as is observed for CFC-11 after 2012. Consistent with this, the best fit to the observations was obtained when the emission distribution (North Hemisphere/global) in these analyses was linearly decreased over time (from 0.95 in 1995 to 0.85 in 2015 for HFC-134a, and from 0.86 to 0.82 for HCFC-22). Assuming a constant hemispheric emission distribution (Northern Hemisphere/global) over time does not change the conclusions from this analysis.