The ATAL within the 2017 Asian Monsoon Anticyclone: Microphysical aerosol properties derived from aircraft-borne in situ measurements

The Asian summer monsoon is an effective pathway for aerosol particles and precursor substances from the planetary boundary layer over Central, South, and East Asia into the upper troposphere and lower stratosphere. An enhancement of aerosol particles within the Asian monsoon anticyclone (AMA) has been observed by satellites, called the Asian Tropopause Aerosol Layer (ATAL). In this paper we discuss airborne in situ and remote sensing observations of aerosol microphysical properties conducted during the 2017 StratoClim field campaign within the region of the Asian monsoon anticyclone. The aerosol particle measurements aboard the high-altitude research aircraft M55 Geophysica (reached a maximum altitude of about 20.5 km) were conducted by a modified Ultra High Sensitivity Aerosol Spectrometer Airborne (UHSAS-A; particle diameter detection range from 65 nm to 1 µm), the COndensation PArticle counting System (COPAS, for detecting total aerosol densities of submicrometer sized particles), and the Cloud and Aerosol Spectrometer with Detection of POLarization (NIXE-CAS-DPOL). In the COPAS and UHSAS-A vertical particle mixing ratio profiles, the ATAL is evident as a distinct layer between 15 km (≈ 370 K) and 18.5 km altitude (≈ 420 K potential temperature). Within the ATAL, the maximum detected particle mixing ratios (from the median profiles) were 700 mg−1 for diameters between 65 nm to 1 µm (UHSAS-A) and higher than 2500 mg−1 for diameters larger than 10 nm (COPAS). These values are up to two times higher than previously found at similar altitudes in other tropical locations. The difference between the particle mixing ratio profiles measured by the UHSAS-A and the COPAS indicate that the region below the ATAL at potential temperatures from 350 to 370 K is influenced by the fresh nucleation of aerosol particles (diameter < 65 nm). We provide detailed analyses of the vertical distribution of the aerosol particle size distributions and the particle mixing ratios and compare these with previous tropical and extratropical measurements. The aerosol scattering ratio was calculated based on the in situ measured aerosol particle size distributions. The resulting dataset was compared with the vertical profiles of the aerosol scattering ratios detected by the Multiwavelength Aerosol Scatterometer (MAS) and an airborne Miniature Aerosol Lidar (MAL) aboard the M55 Geophysica and by the satellite-borne Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP). The data of all four methods largely agree with each other, showing enhanced values of aerosol scattering ratio in the altitude range of the Asian Tropopause Aerosol Layer with a maximum at 17.5 km altitude. By means of the AMA-centered equivalent latitude calculated from meteorological reanalysis data it is shown that such enhanced values of the aerosol scattering ratio larger 1.08 could only be observed within the confinement of the Asian monsoon anticyclone