Calibrating the Tropospheric Air and Ozone Mass

We divide the atmosphere into distinct spheres based on their physical, chemical, and dynamical traits. In deriving chemical budgets and climate trends, which differ across spheres, we need clearly defined boundaries. Our primary spheres are the troposphere and stratosphere (∼99.9% by mass), and the boundary between them is the tropopause. Every global climate-weather model has one or more methods to calculate the lapse rate tropopause, but these involve subjective choices and are known to fail near the sub-tropical jets and polar regions. Age-of-air tracers clock the effective time-distance from the tropopause, allowing unambiguous separation of stratosphere from troposphere in the chaotic jet regions. We apply a global model with synthetic tracer e90 (90-day e-folding), focusing on ozone and temperature structures about the tropopause using ozone sonde and satellite observations. We calibrate an observation-consistent tropopause for e90 using tropics-plus-midlatitudes and then apply it globally to calculate total tropospheric air-mass and tropopause ozone values. The tropopause mixing barrier for the current UCI CTM is identified by a transition in the vertical transport gradient to stratospheric values of 15 days km⁻¹, corresponding to an e90 tropopause at 81 ± 2 ppb with a global tropospheric air mass of 82.2 ± 0.3%. The best e90 tropopause based on sonde pressures is 70–80 ppb; but that for ozone is 80–90 ppb, implying that the CTM tropopause ozone values are too large. This approach of calibrating an age-of-air tropopause can be readily applied to other models and possibly used with observed age-of-air tracers like sulfur hexafluoride.