Balloon-borne measurements of ozone in Lhasa (29.66° N, 91.14° E, 3650 m above sea level) in August 2013 are investigated using backward trajectory calculations performed with the Chemical Lagrangian Model of the Stratosphere (CLaMS). Measurements show three time periods characterized by high ozone mixing ratios in the troposphere on 8, 11, and 18–20 August 2013 during the Asian summer monsoon season. Here, we could verify two different sources for the enhanced ozone values in the troposphere. First, transport of polluted air from the boundary layer, and second transport from the stratosphere by stratospheric intrusions. Air pollution from South Asia through convective and long-range transport plays a key role in enhancing middle/upper tropospheric ozone mixing ratios up to 90 % on 8 August and up to 125 % on 11 August 2013. Stratospheric air intruded from the northern high-latitude to the southeastern flank of the Asian summer monsoon (ASM) anticyclone to the troposphere and is identified as source of enhanced ozone according to backward trajectory calculation and satellite measurements by the Ozone Monitoring Instrument (OMI) and the Atmospheric Infrared Sounder (AIRS). Air parcels with high ozone moved from the high latitude lower stratosphere to the middle/upper troposphere and are then transported to Lhasa over a long-distance and enhanced upper middle/tropospheric ozone in Lhasa during 18–20 August 2013. Our findings demonstrate that the strong variability of ozone within the ASM anticyclone in the free troposphere is caused by transport from different regions of the atmosphere.