Seasonal Variations of SF₆, CO₂, CH₄, and N₂O in the UT/LS Region due to Emissions, Transport, and Chemistry

Distributions of trace gases in the atmosphere provide key information on the transport and chemical properties of the Earth's environment. In this study, we examine the observed seasonal variability of SF₆, CO₂, CH₄, and N₂O from commercial aircraft-based measurements in the upper troposphere and lower stratosphere (UT/LS) in combination with atmospheric chemistry-transport model (ACTM) simulations. We used a dynamical tropopause defined based on maximum PV gradient that better allows for model-observation comparisons in the UT and LS regions separately. The ACTM simulated the following observed features well: (1) smaller seasonality in SF₆ in the UT than in the LS due to seasonal change in the transport processes, (2) large seasonality in CO₂ in the UT, in phase with the source-sink balance on the Earth's surface, (3) pronounced seasonality in CH₄ in the UT with opposite phase in the subtropics and Eurasian high latitudes as contrasting phases of source and chemical loss seasonality develop, and (4) small N₂O variability in the UT but stratospheric loss produces distinct seasonality in the LS. Vertical profile comparisons of all the species show that ACTM better simulated the photo-chemically inert CO₂ vertical gradients in all seasons, suggesting a realistic representation of the Brewer-Dobson circulation in the model. The seasonal variabilities of the long-lived species in the LS can be explained based on age of air derived from CO₂ in the older air regime, except for the months of May-July, when the model mixes UT and LS air more vigorously than suggested by the observations.