Stratospheric ozone is important for the energy budget of the planetary atmosphere and for protecting life on Earth from harmful solar UV radiation. An unexpected decline in ozone in the extratropical lower stratosphere over the past two decades is therefore worrying and contrary to the Montreal Protocol's goal of ozone recovery. The study by Dube et al. adds important information to the ongoing discussion of this trend, which is being investigated internationally in activities such as SPARC/LOTUS, SPARC/OCTAVE-UTLS or the WMO ozone assessments. By their approach to use N2O as a tracer of stratospheric transport, the authors are able to separate ozone decrease due to circulation changes from a decrease caused by other, not further detailed reasons. They find that the latitudinal and altitudinal region where the ozone decrease cannot be explained by circulation changes is restricted to altitudes below 30hPa in the Northern hemisphere. They suggest a possible cause for the remaining ozone loss, namely a change in the tropopause altitude, thus narrowing successfully the search for processes leading to the observed ozone loss.