Impact of Vertical Mixing on CO2 Simulations during the ASIA-AQ campaign

Uncertainty in vertical mixing is a major source of error in simulations of long-lived trace gases such as CO₂ in atmospheric chemical transport models. We perform a set of sensitivity experiments with the GEOS-Chem model by applying different scaling factors to the vertical eddy diffusivity (K_z), thereby varying the strength of vertical mixing. Model results are evaluated using aircraft observations from the ASIA-AQ campaign conducted over Asia, a major anthropogenic CO₂ source region where simulations are particularly sensitive to the representation of vertical mixing. The observations cover a wide range of boundary-layer and free-tropospheric conditions. Model–observation agreement is quantified using a suite of statistical metrics. Simulations with weaker vertical mixing consistently show better agreement with aircraft observations across regions than the default model configuration. The improved agreement reflects a better representation of the observed vertical and temporal variability. This study suggests that vertical mixing in GEOS-Chem may be overestimated over Asia and provides a basis for improving the model representation of vertical transport.