Modelling the effects of mixing processes on the composition of the free troposphere using a three-dimensional chemical transport model

We have used a three-dimensional off-line chemistry transport model to identify the role of t mixing processes in the planetary boundary layer (PBL) and of convection on the global distributions of O3, and O3 precursors, processes whose effects are yet to be fully quantified. These effects are investigated in the model performing a sensitivity test, which takes the difference between a base run and a run where either convection or the PBL scheme have been switched off. With both PBL processes and convection, chemical species are redistributed in the troposphere, so that their mixing ratio profile becomes more uniform with height. In areas of strong convection, O3 is brought rapidly from the upper troposphere downwards to regions where its lifetime is shorter due to higher photochemical activity and the higher water vapour mixing ratio (the major sink for O3) than in the upper troposphere. This indicates that a direct effect of convection is to reduce the lifetime of O3 and thus lower the amount of tropospheric O3. More specifically, convection lowers O3 values by up to 5 ppbv in the upper troposphere, since it transports O3-poor surface air upwards. In regions where surface emissions are important, nitrogen species in the upper troposphere, most notably HNO3, show an increase of about 50 pptv. Conversely, in areas with strong lightning activity and low surface emissions, HNO3 decreases by about 10 pptv since convection dilutes locally produced nitrogen by lightning. The PBL acts primarily as a cleansing mechanism of the surface, transporting surface pollutants upwards and hence affecting the upper troposphere chemical concentrations as well. For instance, surface CO values decrease within areas of strong surface emissions by more than 100 pptv but in the lower free troposphere, CO values increase by more than 40 pptv through the injection of surface air rich in CO. Furthermore, the sensitivity of local, time varying concentrations to the processes in the PBL and convection is considered using a time series analysis, which reveals whether chemistry or transport dominates on particular days.