In-Situ observation of New Particle Formation in the upper troposphere/lower stratosphere of the Asian Monsoon Anticyclone

Abstract. 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 (UT/LS) of the Asian Monsoon Anticyclone (AMA) over northern India, Nepal and Bangladesh. More than hundred events of New Particle Formation (NPF) were observed. In total, more than two hours of flight time were spent under NPF conditions as indicated by the abundant presence of ultrafine aerosols, i.e. with particle diameters d_p smaller than 15 nm, which were in-situ detected by means of condensation nuclei counting techniques. Mixing ratios of ultrafine particles (n_uf) of up to ~ 50000 mg⁻¹ were measured at heights of 15–16 km (θ ≈ 370 K). NPF was most frequently observed at ~ 12–16 km altitude (θ ≈ 355–380 K) and mainly below the tropopause , but n_uf remained elevated (~ 300–2000 mg⁻¹) up to altitudes of ~ 17.5 km (θ ≈ 400 K) while under NPF conditions the fraction (f) of submicrometre-sized non-volatile particle residues (d_p > 10 nm) remained below 50 %. At ~ 12–14 km (θ ≈ 355–365 K) the minimum of f (< 15 %) was found, and underneath the median f generally remains below 25 %. The persistence of particles at ultrafine sizes is limited to a few hours, mainly due to coagulation, as demonstrated by a numerical simulation. Thus, NPF is detectable only for a limited period of time and the frequency of NPF events observed during StratoClim 2017 underlines the importance of the UT/LS within the AMA as a source region for aerosols. The effective in-situ production of aerosol in the tropopause region and subsequent coagulation and/or condensation likely contribute to the formation and maintenance of the Asian Tropopause Aerosol Layer (ATAL). The observed abundance of NPF-produced ultrafine 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 TTL). Temperature anomalies of more than one Kelvin, as observed with a wavelength of ~ 90 km during a level flight over several hours, could be associated with the NPF process as a possible cause for the increasing supersaturation of the NPF precursor system. The frequency of NPF observed during StratoClim 2017 exceeds all previous NPF detections with COPAS at TTL levels over Brazil, Northern Australia, or West Africa. The observed NPF abundance and productivity of fresh aerosols during StratoClim 2017 indicates that NPF is capable of directly affecting the extent and persistence of the ATAL.