The Asian tropopause aerosol layer within the 2017 monsoon anticyclone: microphysical properties derived from aircraft-borne in situ measurements
Authors: Mahnke Christoph , Weigel Ralf , Cairo Francesco , Vernier Jean-Paul , Afchine Armin , Krämer Martina , Mitev Valentin , Matthey Renaud , Viciani Silvia , D'Amato Francesco , Ploeger Felix , Deshler Terry , Borrmann Stephan 
Published: October 2021

Abstract. The Asian summer monsoon is an effective pathway for aerosol particles and
precursors 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), called the Asian tropopause aerosol layer (ATAL), has been observed
by satellites. We discuss
airborne in situ and remote sensing observations of aerosol microphysical
properties conducted during the 2017 StratoClim field campaign within the AMA
region. The aerosol particle measurements aboard the high-altitude research
aircraft M55 Geophysica (maximum altitude reached of ∼20.5 km)
were conducted with a modified ultra-high-sensitivity aerosol
spectrometer – airborne (UHSAS-A; particle diameter detection range of
65 nm to 1 µm), the COndensation PArticle counting System
(COPAS, detecting total concentrations of submicrometer-sized particles), and
the New Ice eXpEriment – Cloud and Aerosol Spectrometer with Detection of
POLarization (NIXE-CAS-DPOL). In the COPAS and UHSAS-A vertical particle
mixing ratio (PMR) profiles and the size distribution profiles (for number,
surface area, and volume concentration), the ATAL is evident as a distinct
layer 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 and
1 µm (UHSAS-A) and higher than 2500 mg−1 for diameters
larger than 10 nm (COPAS). These values are up to 2 times higher
than those previously found at similar altitudes in other tropical locations. The
difference between the PMR profiles measured by the UHSAS-A and the COPAS
indicate that the region below the ATAL at Θ levels from 350 to
370 K is influenced by the nucleation of aerosol particles (diameter
<65 nm). We provide detailed analyses of the vertical distribution
of the aerosol particle size distributions and the PMR and compare these with
previous tropical and extratropical measurements. The backscatter ratio (BR)
was calculated based on the aerosol particle size
distributions measured in situ. The resulting data set was compared with the vertical profiles
of the BR 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 one another, showing enhanced BR values in the altitude range of the ATAL (between ∼15 and
18.5 km) 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 BR larger than 1.1 could only be
observed within the confinement of the AMA.