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 Ralf , Mahnke Christoph , Baumgartner Manuel , Krämer Martina , Spichtinger Peter , Spelten Nicole , Afchine Armin , Rolf Christian , Viciani Silvia , D'Amato Francesco , Tost Holger , Borrmann Stephan 
Published: September 2021

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 ∼ 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 (< 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 dmdt 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.