In situ observation of new particle formation (NPF) in the tropical tropopause layer of the 2017 Asian monsoon anticyclone - Part 1: Summary of StratoClim results

During the monsoon season of the year 2017 the airborne StratoClim mission took place in Kathmandu, Nepal, with eight mission flights of the M-55 Geophysica in the upper troposphere-lower stratosphere (UTLS) of the Asian monsoon anticyclone (AMA) over northern India, Nepal, and Bangladesh. More than 100 events of new particle formation (NPF) were observed. In total, more than 2g h of flight time was spent under NPF conditions as indicated by the abundant presence of nucleation-mode aerosols, i.e. with particle diameters dp smaller than 15g nm, which were detected in situ by means of condensation nuclei counting techniques. Mixing ratios of nucleation-mode particles (nnm) of up to g1/4g 50g 000g mg-1 were measured at heights of 15-16g km (?g ?g 370g K). NPF was most frequently observed at g1/4g 12-16g km altitude (?g ?g 355-380g K) and mainly below the tropopause. Resulting nnm remained elevated (g1/4g 300-2000g mg-1) up to altitudes of g1/4g 17.5g km (?g ?g 400g K), while under NPF conditions the fraction (f) of sub-micrometre-sized non-volatile residues (dpg >g 10g nm) remained below 50g %. At g1/4g 12-14g km (?g ?g 355-365g K) the minimum of f (