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 (<g 15g %) was found, and underneath, the median f generally remains below 25g %. The persistence of particles at nucleation-mode sizes is limited to a few hours, mainly due to coagulation, as demonstrated by a numerical simulation. The frequency of NPF events observed during StratoClim 2017 underlines the importance of the AMA as a source region for UTLS aerosols and for the formation and maintenance of the Asian tropopause aerosol layer (ATAL). The observed abundance of NPF-produced nucleation-mode particles within the AMA is not unambiguously attributable to (a) specific source regions in the boundary layer (according to backward trajectory analyses), or (b) the direct supply with precursor material by convective updraught (from correlations of NPF with carbon monoxide), or (c) the recent release of NPF-capable material from the convective outflow (according to air mass transport times in the tropical tropopause layer, TTL). Temperature anomalies with "T of 2g K (peak-to-peak amplitude), as observed at a horizontal wavelength of g1/4g 70-100g km during a level flight of several hours, match with NPF detections and represent an additional mechanism for local increases in supersaturation of the NPF precursors. Effective precursor supply and widely distributed temperature anomalies within the AMA can explain the higher frequency of intense NPF observed during StratoClim 2017 than all previous NPF detections with COPAS (COndensation PArticle counting System) at TTL levels over Brazil, northern Australia, or West Africa.