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

• Published in: Atmospheric chemistry and physics 2021-09, Vol.21 (17), p.13455-13481
• Main Authors: Weigel, Ralf, Mahnke, Christoph, Baumgartner, Manuel, Krämer, Martina, Spichtinger, Peter, Spelten, Nicole, Afchine, Armin, Rolf, Christian, Viciani, Silvia, D'Amato, Francesco, Tost, Holger, Borrmann, Stephan
• Format: Article
• Language: English
• Subjects: Air; Altitude; Anticyclones; Asian monsoons; Atmospheric aerosols; Carbon monoxide; Clear sky; Clouds; Condensation; Condensation nuclei; Convection; Diameters; Ice; Ice clouds; Ice particles; Mixing ratio; Monsoons; Nucleation; Numerical simulations; Particle formation; Pollutant load; Pollutants; Pollution load; Precursors; Saturation; Stratosphere; Sulfuric acid; Sulphuric acid; Supersaturation; Temperature; Time measurement; Tropical climate; Tropical tropopause; Tropopause; Troposphere; Upper troposphere; Water vapor; Water vapour
• ISSN: 1680-7324; 1680-7316; 1680-7324
• DOI: 10.5194/acp-21-13455-2021

From 27 July to 10 August 2017, the airborne StratoClim mission took place in Kathmandu, Nepal, where eight mission flights were conducted with the M-55 Geophysica up to altitudes of 20 km. New particle formation (NPF) was identified by the abundant presence of nucleation-mode aerosols, with particle diameters dp smaller than 15 nm, which were in-situ-detected by means of condensation nuclei (CN) counter techniques. NPF fields in clear skies as well as in the presence of cloud ice particles (dp > 3 µm) were encountered at upper troposphere–lowermost stratosphere (UTLS) levels and within the Asian monsoon anticyclone (AMA). NPF-generated nucleation-mode particles in elevated concentrations (Nnm) were frequently found together with cloud ice (in number concentrations Nice of up to 3 cm−3) at heights between ∼ 11 and 16 km. From a total measurement time of ∼ 22.5 h above 10 km altitude, in-cloud NPF was in sum detected over ∼ 1.3 h (∼ 50 % of all NPF records throughout StratoClim). Maximum Nnm of up to ∼ 11 000 cm−3 was detected coincidently with intermediate ice particle concentrations Nice of 0.05–0.1 cm−3 at comparatively moderate carbon monoxide (CO) contents of ∼ 90–100 nmol mol−1. Neither under clear-sky nor during in-cloud NPF do the highest Nnm concentrations correlate with the highest CO mixing ratios, suggesting that an elevated pollutant load is not a prerequisite for NPF. Under clear-air conditions, NPF with elevated Nnm (> 8000 cm−3) occurred slightly less often than within clouds. In the presence of cloud ice, NPF with Nnm between 1500–4000 cm−3 was observed about twice as often as under clear-air conditions. NPF was not found when ice water contents exceeded 1000 µmol mol−1 in very cold air (