The Asian Tropopause Aerosol Layer Mystery: Chemical and Physical Properties inferred from Aircraft-borne In-situ Measurements

During the Asian and West African monsoons large meteorological structures develop which reach into the upper troposphere and lower stratosphere (UTLS) with impact on the aerosols and precursor gases entering the stratosphere. The Asian Monsoon Anticyclone (AMA) forms from mid-June until October in an altitude band from roughly 12 to 20 km extending from East Asia to the Middle East. Long range transport from as far as India and Eastern China provides materials which are carried aloft by the deep convective clouds in the Himalaya region. The anvil outflows of the high reaching deep convective clouds release the uplifted (and partly processed) source gases and aerosols into the UTLS. Here New Particle Formation events (NPF) occur in clear cloud free air, as well as within the margins/outflows of Cb clouds, generating new aerosols from the inorganic and organic precursors. CALIPSO measurements revealed a distinct aerosol layer (i.e. the Asian Tropopause Aerosol Layer; ATAL) between 15 and 16.5 km within the AMA, the physical and chemical characteristics of which still were unclear. Since these phenomena occur at the tropopause in areas with slow upwelling motion, they may contribute to the global stratospheric aerosol. During the 2017 StratoClim field campaign the Russian high altitude research aircraft M-55 “Geophysica” operated in the AMA and ATAL at altitudes up to 20 km. In-situ chemical composition measurements were performed on the submicron ambient aerosol adopting a newly developed aerosol mass spectrometer. This new instrument combines the two available techniques for particle mass spectrometry (i.e. (1.) laser ablation and (2.) flash vaporization/electron impact ionization) in one apparatus. These complementary methods provide both, qualitative and quantitative information on the chemical composition of the sampled aerosol in real time by direct reading. One main result is that the ATAL particles consist mostly of ammonium nitrate with traces of sulfate. It was found that the precursor for the particulate ammonium is gaseous ammonia (at high concentration levels) which has its origin in regional agricultural activities. Also significant amounts of organics could be detected in the ATAL. A four channel condensation particle counter provided additional measurements of number concentration and volatility of aerosol particles as small as 6 nm and a modified UHSAS optical particle counter was operated on “Geophysica” delivering aerosol size distributions from 60 nm to 1 micrometer particle diameter. In the poster some of the key results from the 2017 StratoClim campaign in Nepal are presented and juxtaposed to previous high altitude “Geophysica” measurements from the West African monsoon (here the AMMA project in Burkina Faso, 2006).