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. Balloon-borne measurements of ozone in Lhasa (29.66 ∘  N,91.14 ∘  E; 3650 m above sea level) in August 2013 are investigatedusing backward trajectory calculations performed with the Chemical LagrangianModel of the Stratosphere (CLaMS). Measurements show three time periodscharacterized by high ozone mixing ratios (OMRs) in the troposphere on 8, 11,and 18–20 August 2013 during the Asian summer monsoon (ASM) season. Here, weverified two different sources for the enhanced ozone values in thetroposphere. First, transport of polluted air from the boundary layer, andsecond downward transport from the stratosphere by stratospheric intrusions.Air pollution from South Asia through convective and long-range transportplays a key role in enhancing middle tropospheric OMRs up to 90 % on8 August and up to 125 % on 11 August 2013 compared to monthly mean ozoneof August 2013. Stratospheric air intruded from the northern high-latitudesto the southeastern flank of the ASM anticyclone to the troposphere and isidentified as the source of enhanced ozone according to backward trajectorycalculation and satellite measurements by the Ozone Monitoring Instrument(OMI) and the Atmospheric Infrared Sounder (AIRS). Air parcels with highozone moved from the high-latitude lower stratosphere to the middle and uppertroposphere. These air parcels are then transported to Lhasa over longdistances and enhanced upper and middle tropospheric ozone over Lhasa during18–20 August 2013. Our findings demonstrate that the strong variability ofozone within the ASM anticyclone in the free troposphere is caused bytransport from very different regions of the atmosphere.

High tropospheric ozone in Lhasa within the Asian summer monsoon anticyclone in 2013: influence of convective transport and stratospheric intrusions