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. The Asian summer monsoon is an effective pathway for aerosol particles andprecursors from the planetary boundary layer over Central, South, and EastAsia into the upper troposphere and lower stratosphere. An enhancement ofaerosol particles within the Asian monsoon anticyclone (AMA), called the Asian tropopause aerosol layer (ATAL), has been observedby satellites. We discussairborne in situ and remote sensing observations of aerosol microphysicalproperties conducted during the 2017 StratoClim field campaign within the AMAregion. The aerosol particle measurements aboard the high-altitude researchaircraft M55 Geophysica (maximum altitude reached of ∼20.5 km)were conducted with a modified ultra-high-sensitivity aerosolspectrometer – airborne (UHSAS-A; particle diameter detection range of65 nm to 1 µm), the COndensation PArticle counting System(COPAS, detecting total concentrations of submicrometer-sized particles), andthe New Ice eXpEriment – Cloud and Aerosol Spectrometer with Detection ofPOLarization (NIXE-CAS-DPOL). In the COPAS and UHSAS-A vertical particlemixing ratio (PMR) profiles and the size distribution profiles (for number,surface area, and volume concentration), the ATAL is evident as a distinctlayer between ∼370 and 420 K potential temperature (Θ). Within the ATAL, the maximum detected PMRs (from the median profiles) were∼700 mg−1 for particle diameters between 65 nm and1 µm (UHSAS-A) and higher than 2500 mg−1 for diameterslarger than 10 nm (COPAS). These values are up to 2 times higherthan those previously found at similar altitudes in other tropical locations. Thedifference between the PMR profiles measured by the UHSAS-A and the COPASindicate that the region below the ATAL at Θ levels from 350 to370 K is influenced by the nucleation of aerosol particles (diameter&lt;65 nm). We provide detailed analyses of the vertical distributionof the aerosol particle size distributions and the PMR and compare these withprevious tropical and extratropical measurements. The backscatter ratio (BR)was calculated based on the aerosol particle sizedistributions measured in situ. The resulting data set was compared with the vertical profilesof the BR detected by the multiwavelength aerosol scatterometer (MAS) and anairborne miniature aerosol lidar (MAL) aboard the M55 Geophysica and by thesatellite-borne Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP). Thedata of all four methods largely agree with one another, showing enhanced BR values in the altitude range of the ATAL (between ∼15 and18.5 km) with a maximum at 17.5 km altitude. By means of theAMA-centered equivalent latitude calculated from meteorological reanalysisdata, it is shown that such enhanced values of the BR larger than 1.1 could only beobserved within the confinement of the AMA.

The Asian tropopause aerosol layer within the 2017 monsoon anticyclone: microphysical properties derived from aircraft-borne in situ measurements