Stratospheric influence on the seasonal cycle of nitrous oxide in the troposphere as deduced from aircraft observations and model simulations

[1] The atmospheric N2O variations between the Earth's surface and the lower stratosphere, simulated by an atmospheric general circulation model–based chemistry transport model (ACTM), are compared with aircraft and satellite observations. We validate the newly developed ACTM simulations of N2O for loss rate and transport in the stratosphere using satellite observations from the Aura Microwave Limb Sounder (Aura-MLS), with optimized surface fluxes for reproducing N2O trends observed at the surface stations. Observations in the upper troposphere/lower stratosphere (UT/LS) obtained by the Japan AirLines commercial flights commuting between Narita (36°N), Japan, and Sydney (34°S), Australia, have been used to study the role of stratosphere-troposphere exchange (STE) on N2O variability near the tropopause. Low N2O concentration events in the UT region are shown to be captured statistically significantly by the ACTM simulation. This is attributed to successful reproduction of stratospheric air intrusion events and N2O vertical/horizontal gradients in the lower stratosphere. The meteorological fields and N2O concentrations reproduced in the ACTM are used to illustrate the mechanisms of STE and subsequent downward propagation of N2O-depleted stratospheric air in the troposphere. Aircraft observations of N2O vertical profile over Surgut (West Siberia, Russia; 61°N), Sendai-Fukuoka (Japan; 34°N–38°N), and Cape Grim (Tasmania, Australia; 41°S) have been used to estimate the relative contribution of surface fluxes, transport seasonality in the troposphere, and STE to N2O seasonal cycles at different altitude levels. Stratospheric N2O tracers are incorporated in the ACTM for quantitative estimation of the stratospheric influence on tropospheric N2O. The results suggest strong latitude dependency of the stratospheric contribution to the tropospheric N2O seasonal cycle. The periods of seasonal minimum in the upper troposphere, which are spring over Japan and summer over Surgut, are in good agreement between the ACTM and observation and indicate a different propagation path of the stratospheric signal between the two sites in the Northern Hemisphere. The stratospheric tracer simulations, when utilized with the observed seasonal cycle, also provide qualitative information on the seasonal variation in surface fluxes of N2O.