Scientific Results

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

Year: 2021

Authors: Weigel R., Mahnke C., Baumgartner M., Dragoneas A., Vogel B., Ploeger F., Viciani S., D’Amato F., Bucci S., Legras B., Luo B., Borrmann S.

Autors Affiliation: Johannes Gutenberg University Mainz
Max-Planck-Institut für Chemie
Forschungszentrum Jülich
École Polytechnique, Paris
ETH Zurich

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 (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 2 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 15 nm, which
were detected in situ by means of condensation nuclei counting techniques. Mixing ratios of nucleation-mode particles (nnm) of up to about50 000 mg-1 were measured at heights of 15–16 km (theta about 370 K). NPF was most frequently observed at about 12–16 km altitude (theta about 355–380 K) and mainly below the tropopause. Resulting nnm remained elevated (about 300–2000 mg-1) up to altitudes of about 17.5 km (theta about 400 K), while under NPF conditions the fraction (f ) of sub-micrometresized non-volatile residues (dp >10 nm) remained below 50 %. At about 12–14 km (theta about 355–365 K) the minimum of f(<15 %) was found, and underneath, the median f generally remains below 25 %. 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 1T of 2K (peak-to-peak amplitude), as observed at a horizontal wavelength of 70–100 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. Journal/Review: ATMOSPHERIC CHEMISTRY AND PHYSICS (PRINT)

Volume: 21      Pages from: 11689  to: 11722

KeyWords: Asian Monsoon Anticyclon
New particle formation
DOI: 10.5194/acp-21-11689-2021

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