Trace gas observations can be used to define chemical tropopauses based on differences between stratospheric and tropospheric characteristics. Notable examples are O₃ and N₂O, which have a well-defined positive and negative gradient in the lower stratosphere, respectively. We propose a vertical coordinate relative to the chemical tropopause that can be derived directly from trace gas measurements. Taking advantage of the extensive global coverage of regular ozone soundings, we develop an ozone-based tropopause using an observation-based climatology expressed as mixing ratios relative to the tropopause, ∆&Theta;(O₃). We apply this new coordinate to two exemplary data sets in the extratropical upper troposphere and lower stratosphere. Compared to the dynamic tropopause, the short-term dynamical variability of 2D-binned N₂O is reduced by 18 % and that of SF₆ by up to 38 % in the two data sets. This reflects the implicit incorporation of transport and mixing processes in ∆&Theta;(O₃). In both data sets, ∆&Theta;(O₃) also compresses the vertical extent of observations, indicating that air masses at high ∆&Theta;(<em>dyn</em>) are chemically more related to lower-lying air. Overall, we show that ∆&Theta;(O₃) provides a chemically consistent vertical coordinate for large observational data sets, facilitating studies of smaller-scale processes without requiring model reanalysis data.