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. The South Asian summer monsoon is associated with a large-scale anticyclonic circulation in the upper troposphere and lower stratosphere (UTLS), which confines the air mass inside. During borealsummer, the confinement of this air mass leads to an accumulation ofaerosol between about 13 and 18 km (360and 440 K potential temperature); this accumulation of aerosolconstitutes the Asian Tropopause Aerosol Layer (ATAL). We presentballoon-borne aerosol backscatter measurements of the ATAL performedby the Compact Optical Backscatter Aerosol Detector (COBALD)instrument in Nainital in northern India in August 2016, and comparethese with COBALD measurements in the post-monsoon time in November2016. The measurements demonstrate a strong variability of the ATAL'saltitude, vertical extent, aerosol backscatter intensity and cirruscloud occurrence frequency. Such a variability cannot be deduced fromclimatological means of the ATAL as they are derived from satellitemeasurements. To explain this observed variability we performed aLagrangian back-trajectory analysis using the Chemical LagrangianModel of the Stratosphere (CLaMS). We identify the transport pathwaysas well as the source regions of air parcels contributing to the ATALover Nainital in August 2016. Our analysis reveals a variety offactors contributing to the observed day-to-day variability of theATAL: continental convection, tropical cyclones (maritime convection),dynamics of the anticyclone and stratospheric intrusions. Thus, the airin the ATAL is a mixture of air masses coming from different atmosphericaltitude layers. In addition, contributions from the model boundarylayer originate in different geographic source regions. The locationof the strongest updraft along the backward trajectories reveals a clusterof strong upward transport at the southern edge of the Himalayanfoothills. From the top of the convective outflow level (about13 km; 360 K) the air parcels ascend slowly to ATALaltitudes within a large-scale upward spiral driven by the diabaticheating in the anticyclonic flow of the South Asian summer monsoon atUTLS altitudes. Cases with a strong ATAL typically show boundary layercontributions from the Tibetan Plateau, the foothills of the Himalayasand other continental regions below the Asian monsoon. Weaker ATALcases show higher contributions from the maritime boundary layer,often related to tropical cyclones, indicating a mixing of cleanmaritime and polluted continental air. On the one hand increasinganthropogenic emissions in the future are expected due to the stronggrowth of Asian economies; on the other hand the implementation of newemission control measures (in particular in China) has reducedthe anthropogenic emissions of some pollutantscontributing to the ATAL substantially. It needs to be monitored in the future whether the thickness and intensity of the ATAL will furtherincrease, which will likely impact the surface climate.

Strong day-to-day variability of the Asian Tropopause Aerosol Layer (ATAL) in August 2016 at the Himalayan foothills