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

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 20g km. New particle formation (NPF) was identified by the abundant presence of nucleation-mode aerosols, with particle diameters dp smaller than 15g 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 (dpg >g 3g 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 3g cm-3) at heights between g 1/4g 11 and 16g km. From a total measurement time of g 1/4g 22.5g h above 10g km altitude, in-cloud NPF was in sum detected over g 1/4g 1.3g h (g 1/4g 50g % of all NPF records throughout StratoClim). Maximum Nnm of up to g 1/4g 11g 000g cm-3 was detected coincidently with intermediate ice particle concentrations Nice of 0.05-0.1g cm-3 at comparatively moderate carbon monoxide (CO) contents of g 1/4g 90-100g nmolg 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 (>g 8000g cm-3) occurred slightly less often than within clouds. In the presence of cloud ice, NPF with Nnm between 1500-4000g cm-3 was observed about twice as often as under clear-air conditions. NPF was not found when ice water contents exceeded 1000g μmolg mol-1 in very cold air (