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

Abstract

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.

Chemical analysis of the Asian Tropopause Aerosol Layer (ATAL) with emphasis on secondary aerosol particles using aircraft based in situ aerosol mass spectrometry

“…COPAS, UHSAS-A, and NIXE-CAS) covering 10 nm to ~3 µm aerosol back scattering ratios then could be derived and compared with the CALIOP satellite measurements of roughly the same time period during StratoClim. The results from both methods agree well and exhibit an aerosol layer between 14 km and 18 km altitude inside the AMA (Mahnke et al, 2021). Thus, with respect to number density, the ATAL in 2017 consisted mainly of particles with diameters below 3 µm with a broad peak in the size distribution around 100 nm with the highest number densities near 40 000 cm −3 (Mahnke et al, 2021;Weigel et al, 2021a).…”

“…The results from both methods agree well and exhibit an aerosol layer between 14 km and 18 km altitude inside the AMA (Mahnke et al, 2021). Thus, with respect to number density, the ATAL in 2017 consisted mainly of particles with diameters below 3 µm with a broad peak in the size distribution around 100 nm with the highest number densities near 40 000 cm −3 (Mahnke et al, 2021;Weigel et al, 2021a). Previous balloon borne observations also had indicated a maximum in the size distribution at diameters of around 100 nm (Yu et al, 2017;Vernier et al, 2018).…”

“…Aerosol size distribution and cloud particle measurements: A modified UHSAS-A optical particle counter covering a nominal particle diameter from 65 nm to 1 µm was deployed on the aircraft as an underwing probe for aerosol size distribution measurements. As described in more detail by Mahnke et al (2021), for the operation of the UHSAS-A at atmospheric pressures below 130 hPa several modifications, e.g. of the flow system, had to be implemented to ensure reliable performance under the ambient conditions encountered during StratoClim.…”

“…Aerosol size distribution measurements were concurrently performed on the M-55 Geophysica during StratoClim inside of the 2017 AMA by Mahnke et al (2021), using optical particle counters (UHSAS-A and NIXE-CAS) for the size range of 65 nm to 3 µm in diameter. This range extends towards smaller particle sizes beyond the previous measurements by Yu et al (2017), who adopted a balloon borne POPS optical particle counter with a lower size detection limit of 140 nm.…”

“…This range extends towards smaller particle sizes beyond the previous measurements by Yu et al (2017), who adopted a balloon borne POPS optical particle counter with a lower size detection limit of 140 nm. As detailed in Mahnke et al (2021), additional data from the COPAS high altitude condensation particle counter (Weigel et al, 2009(Weigel et al, , 2021a allowed adding one broad size bin extending from 10 nm to 65 nm to size distributions delivered by the UHSAS-A. By means of these composite size distributions from the three instruments (i.e.…”