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

Abstract

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 ∼ 50 000 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. Resulting nnm remained elevated (∼ 300–2000 mg−1) up to altitudes of ∼ 17.5 km (θ ≈ 400 K), while under NPF conditions the fraction (f) of sub-micrometre-sized non-volatile residues (dp > 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 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 ΔT of 2 K (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.

The Asian tropopause aerosol layer within the 2017 monsoon anticyclone: microphysical properties derived from aircraft-borne in situ measurements

“…The difference in the particle mixing ratios between the median values of the COPAS and the UHSAS-A measurements of more than about 5500 mg −1 shows that very small aerosol particles between 10 and 65 nm in diameter dominate the aerosol total particle mixing ratios. This indicates that especially the range between 350 and 370 K is influenced by NPF, which has also been shown by Weigel et al (2021a) and Weigel et al (2021b).…”

In situ observation of new particle formation (NPF) in the tropical tropopause layer of the 2017 Asian monsoon anticyclone – Part 2: NPF inside ice clouds

“…From measurements in the upper troposphere it is commonly assumed that the occurrence of NPF is directly connected to deep-convective cloud systems (e.g. de Reus et al, 2001;Clarke and Kapustin, 2002;Weigelt et al, 2009;Andreae et al, 2018). The relationship between NPF and ice clouds is discussed in this study, whilst the immediate connection of NPF and deep-convective events is addressed in Weigel et al (2021).…”

Chemical analysis of the Asian Tropopause Aerosol Layer (ATAL) with emphasis on secondary aerosol particles using aircraft based in situ aerosol mass spectrometry

“…Such precursor gases reach the upper troposphere (e.g. Höpfner et al, 2016Höpfner et al, , 2019Fairlie et al, 2020), where homogeneous nucleation and subsequent particle growth processes form secondary particles in clear air as well as inside the outflows of convective cloud systems (Weigel et al, 2011(Weigel et al, , 2021aWilliamson et al, 2019). The exact details of the precursor gas mixture and the involved processes for inorganic homogeneous nucleation (including H 2 SO 4 , NH 3 , HNO 3 among others), as well as secondary organic aerosol (SOA) formation still are unknown.…”

“…This range extends towards smaller particle sizes beyond the previous measurements by Yu et al (2017), who adopted a balloon borne POPS optical particle counter with a lower size detection limit of 140 nm. As detailed in Mahnke et al (2021), additional data from the COPAS high altitude condensation particle counter (Weigel et al, 2009(Weigel et al, , 2021a allowed adding one broad size bin extending from 10 nm to 65 nm to size distributions delivered by the UHSAS-A. By means of these composite size distributions from the three instruments (i.e.…”