<p>We introduce a transformed isentropic coordinate Mθ<sub>e</sub>, defined as the dry air mass under a given equivalent potential temperature surface (θ<sub>e</sub>) within a hemisphere. Like θ<sub>e</sub>, the coordinate Mθ<sub>e</sub> follows the synoptic distortions of the atmosphere, but unlike θ<sub>e</sub>, has a nearly fixed relationship with latitude and altitude over the seasonal cycle. Calculation of Mθ<sub>e</sub> is straightforward from meteorological fields. Using observations from the recent HIPPO and Atom airborne campaigns, we map the CO<sub>2</sub> seasonal cycle as a function of pressure and Mθ<sub>e</sub>, where Mθ<sub>e</sub> is thereby effectively used as an alternative to latitude. We show that the CO<sub>2</sub> cycles are more constant as a function of pressure using Mθ<sub>e</sub> as the horizontal coordinate compared to latitude. Furthermore, short-term variability of CO<sub>2</sub> relative to the mean seasonal cycle is also smaller when the data are organized by Mθ<sub>e</sub> and pressure than when organized by latitude and pressure. We also present a method using Mθ<sub>e</sub> to compute mass-weighted averages of CO<sub>2</sub> on a hemispheric scale. Using this method with the same airborne data and applying corrections for limited coverage, we resolve the average CO<sub>2</sub> seasonal cycle in the Northern Hemisphere (mass weighted tropospheric climatological average for 2009–2018), yielding an amplitude of 7.8 ± 0.14 ppm and a downward zero-crossing at Julian day 173 ± 6.1 (i.e., late June). Mθ<sub>e</sub> may be similarly useful for mapping the distribution and computing inventories of any long-lived chemical tracer.</p>