Assessing the Accuracy of Global HFC-134a Satellite Observations from Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) via Aircraft, Balloon, and Model Comparisons

HFC-134a is the most abundant hydrofluorocarbon and is a potent greenhouse gas, with rapidly increasing concentrations from use primarily in refrigeration and air conditioning applications. HFC-134a measurements mainly rely on ground-based globally distributed networks with sparse spatial coverage. Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) observes HFC-134a above 5 km globally, thus bridging observational gaps in the upper atmosphere and facilitating radiative forcing assessments. However, validation of ACE-FTS HFC-134a retrievals remains limited. Here, we evaluate the latest ACE-FTS HFC-134a retrievals using surface, aircraft, and balloon observations, with the GEOS-Chem chemical transport model (CTM) as the intercomparison platform. ACE-FTS HFC-134a retrievals exhibit systematic low biases across all altitudes during 2016-2024, varying from 0.7-15.3% (10-20 km), 12.1-26.6% (below 10 km), to 7.2-22.0% (above 20 km), likely due to retrieval limitations and water vapor interference not fully addressed in the current retrieval algorithm. Interperiod comparison suggests a mean observation-model difference ranging from -12.6% to -10.9%, implying stable ACE-FTS HFC-134a retrievals over time without degradation. This study integrates available <i>in situ</i> measurements with a CTM to indirectly validate HFC-134a observations from space, demonstrating a valuable framework for assessing satellite retrievals of a range of other fluorinated greenhouse gases.