Ice-nucleating particles in the free troposphere: long-term observation and first measurements at cirrus formation temperatures using the novel Portable Ice Nucleation Experiment PINEair

Ice formation induced by ice-nucleating particles (INPs) greatly influences the formation, life cycles, and climatic impact of tropospheric clouds, as well as their ability to form precipitation. However, knowledge about the abundance of INPs especially in the free troposphere (FT) is still missing. Therefore, this thesis aimed at observing INPs at the Sonnblick Observatory (SBO, 3106 m a.s.l.) which is frequently located in the FT. The comprehensive measurements were conducted over long term to investigate the seasonal variation, at high time resolution to obtain information on the diurnal and shorter-term variability, and in a wide temperature range to cover mixed-phase cloud (MPC) and cirrus cloud conditions. INPs that impact the ice formation in cirrus clouds were measured with the novel instrument PINEair (Portable Ice Nucleation Experiment airborne), which was developed as part of this PhD thesis.

The new aircraft-based INP instrument PINEair was developed especially for use on the HALO (High Altitude and Long Range) research aircraft. It is the first aircraft-based instrument that can perform automated in-situ measurements of INPs to temperatures of −65 °C. ... mehrPINEair simulates cloud-like conditions by expansion cooling. It consists of three expansion chambers operated in a cycling mode to achieve measurements with higher time and spatial resolution in a fast-flying jet aircraft. Laboratory measurements with a prototype of PINEair have successfully shown that the newly developed instrument can measure the INP concentration in a wide temperature range relevant for MPC and cirrus clouds. The distinction between homogeneous and heterogeneous freezing at cirrus conditions was demonstrated for ground-based conditions with ambient air sampling and for lower pressure (p = 250 mbar) conditions during aircraft flights which were simulated in a series of laboratory experiments. For this purpose, a new expansion procedure was developed where the pressure in the chambers can step-wise be reduced at the beginning of the expansion. This allows the calculation of the peak ice saturation ratio (Sice,p) in the chamber assuming ice-saturated conditions before expansion start and an adiabatic temperature decrease.

As part of this PhD thesis, a mobile prototype of PINEair was engineered, built, and tested during a field campaign at the SBO in the Austrian Alps. Measurements were performed quasi continuously during the time period from May 8 - 22, 2023, at both MPC (T = −22.7 °C and T = −27.5 °C) and cirrus (T = −47.8 °C) temperatures. At a temperature of −47.8 °C and Sice,p = 1.49−1.52, a median INP concentration of 1.1 std L^−1 was measured, and a maximum concentration of about 90 std L^−1. Higher Sice,p conditions in the chamber resulted in higher INP concentrations, which is consistent with the literature. During a case study with increased aerosol concentrations and particle mass concentrations, also increased INP concentrations up to 84 std L^−1 were measured at −47.8 °C and Sice, p = 1.52 − 1.55.

Another major part of this PhD thesis were long-term INP measurements in the MPC temperature range at the SBO. Due to its location, the site receives both air masses from the FT and the boundary layer (BL). The measurements were performed with the freezing experiment INSEKT (Ice Nucleation Spectrometer of the Karlsruhe Institute of Technology) from August 2019 to August 2022 with a time resolution of one week. To date, this is the longest, continuous INP measurement series. In addition, a 14-month INP time series with a high time resolution of 6 min was performed with PINE, starting in August 2021. Both data sets show a recurrent seasonal sinusoidal trend with the highest INP concentrations in spring/summer and the lowest in winter. From April to September, a daily cycle in the INP concentration is observed with a maximum around noon and a minimum at midnight. In contrast, from October to February, no daily cycle was observed and the INP concentrations were consistently low. The seasonal and diurnal variations of the INP concentration are likely caused by the influence of air masses from the BL, as the concentrations of the tracers (aerosol particles with a diameter larger than 90 nm and 214Polonium concentration) show the same trends. A heat analysis of the filters prior to INP analysis with INSEKT showed reduced INP activity, especially at higher temperatures above −13 °C for all seasons, indicating that biogenic compounds contributed to the INP abundance. Strong sudden increases in the INP concentration are caused by episodically occurring dust events, which were observed by enhanced and correlated concentrations of both the INP number concentration and the aerosol mass concentration. A comparison of the measured INP concentration during a dust event in March 2022 to the parameterization of DeMott et al. (2015) showed a good agreement, the temperature dependence and 98.5 % of the data within a factor of 10 were predicted correctly. The median INP concentration of the whole measurement period was slightly higher during clear-sky periods than during cloudy periods, which could be caused by processes such as wet-removal or pre-activation of INPs. Moreover, the INP concentration was found to be correlated with the aerosol concentration, size, and mass, especially at lower nucleation temperatures. However, the observed variation of the INP concentration was not correlated with meteorological parameters. From this study, it can be concluded that the INP concentration at SBO is mainly influenced by different sources including free tropospheric aerosols, long-range transported dust, and local or regional aerosol sources transported from the BL to the station.