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
Authors: Weigel R., Mahnke C., Baumgartner M., Krämer M., Spichtinger P., Spelten N., Afchine A., Rolf C., Viciani S., D’Amato F., Tost H., Borrmann S.
Autors Affiliation: Institut für Physik der Atmosphäre, Johannes Gutenberg University Mainz, Mainz, Germany
Abteilung Partikelchemie, Max-Planck-Institut für Chemie, Mainz, Germany
Zentrum für Datenverarbeitung, Johannes Gutenberg University Mainz, Mainz, Germany
Institute of Energy and Climate Research (IEK-7), Forschungszentrum Jülich, Jülich, Germany
National Institute of Optics, National Research Council (CNR-INO), Florence, Italy
Abstract: 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 about 11 and 16 km. From a total measurement time of about 22.5 h above 10 km altitude, incloud NPF was in sum detected over about 1.3 h (about 50% of all NPF records throughout StratoClim). Maximum Nnm of up to about 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 about 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 (<195 K) at tropopause levels. This indicates a reduction in NPF once deep convection is prevalent together with the presence of mainly liquid-origin ice particles. Within in situ cirrus near the cold point tropopause, recent NPF or intense events with mixing ration nnm larger than 5000 mg-1 were observed only in about 6% of the in-cloud NPF data. In determining whether the cloud-internal NPF is attenuated or prevented by the microphysical properties of cloud elements, the integral radius (IR) of the ice cloud population turned out to be indicative. Neither the number of ice particles nor the free distance between the ice particles is clearly related to the NPF rate detected. While the increase in ice particles’ mass per time (dm)/(dt) is proportional to the IR and mainly due to the condensation of water vapour, additional condensation of NPF precursors proceeds at the expense of the NPF rate as the precursor’s saturation ratio declines. Numerical simulations show the impact of the IR on the supersaturation of a condensable vapour, such as sulfuric acid, and furthermore illustrate that the IR of the cloud ice determines the effective limitation of NPF rates. Journal/Review: ATMOSPHERIC CHEMISTRY AND PHYSICS (PRINT)
Volume: 21 Pages from: 13455 to: 13481
KeyWords: new particle formation, Asian monsoon anticyclone, STRATOCLIMDOI: 10.5194/acp-21-13455-2021