Evaluation of Age of Air derived from simulated infra-red satellite observations of trace gases

Following the increase of greenhouse gas emissions, atmospheric models predict a strengthening of the middle atmospheric Brewer-Dobson circulation (BDC). Changes in the BDC, inferred from age of air (AoA) trends, can influence UTLS exchange processes, including stratosphere–troposphere transport of ozone. While models predict an acceleration of the BDC (i.e. a decrease of AoA), in-situ balloon observations suggest the opposite, although not significantly, given the limited number of observations and the substantial uncertainties (Garny et al., 2024a, doi:10.1029/2023RG000832). Additionally, meteorological reanalyses disagree on the sign and magnitude of AoA trends, despite providing an optimal estimate of the state of the atmosphere. The Changing Atmosphere Infrared Tomography explorer (CAIRT) was a candidate satellite concept for ESA’s Earth Explorer 11. CAIRT is an infrared limb imager with unprecedented spatial coverage and was designed to provide level-2b AoA profiles with a precision of less than 0.5 years.
This contribution aims to evaluate the capability of CAIRT to achieve this precision. Synthetic
CAIRT profiles of six long-lived species (SF6, CH4, N2O, CFC-11, CFC-12 and HCFC-22)
are simulated using the atmospheric state from the Belgian Assimilation System for Chemical
ObsErvations (BASCOE) chemistry transport model, accounting for CAIRT’s expected
measurement errors and spatial resolution. CAIRT AoA observations, derived from the six
long-lived species using the method of Voet et al. (2025, doi:10.5194/acp-25-3541-2025), are
compared to clock tracer AoA, simulated by the BASCOE model, to evaluate the agreement.
The analysis is repeated three times by driving the model with the meteorological reanalyses
MERRA2, ERA5, and JRA-3Q, respectively. The sensitivity to the choice of driving reanalysis
and the correction for the mesospheric sink of SF6 is assessed. Emphasis is placed on
understanding the limitations of deriving AoA from observational tracers.