A two-year record of INP concentration measurements at the Sonnblick Observatory (3106 m): Insight into seasonal variability

In mixed-phase clouds, primary ice formation occurs only in the presence of ice-nucleating particles (INPs) [Vali et al., 2015], which are a very rare subset of the aerosols in the atmosphere. INPs are an important part of the earth climate system, as they can initiate the formation of precipitation [Mülmenstädt et al., 2015] and have an influence on the cloud radiative properties [Murray et. al, 2021]. In the last decades, different INP measurements have been performed within the atmospheric boundary layer at mixed-phase clouds conditions [Kanji et. al, 2017]. INP measurements in the free troposphere are challenging, as they can only be conducted by aircraft-based measurements or at high altitude mountain stations. However, it is important to study long-term INP concentrations at different altitudes and geographical locations to get a better understanding of the presence of INPs in the atmosphere.

The Sonnblick Observatory (SBO) in the Austrian Alps is located at an altitude of 3106 m above sea level and is an ideal location to investigate the INP concentration in the free troposphere, as the measured INPs are directly relevant for ice formation. Since August 2019, we continuously measure the INP concentration at the SBO via filter collection and offline analysis with INSEKT (Ice Nucleation Spectrometer of the Karlsruhe Institute of Technology) [Schneider et. al, 2020]. The analysis of the sampled aerosols gives us the temperature dependent number concentration of INPs at temperatures above -25°C with a time resolution of one week. In order to receive a better insight into short-term fluctuations, we performed additional measurements with the online INP measurement PINE (Portable Ice Nucleation Experiment) [Möhler et al., 2021], since the end of July 2021. PINE has a time resolution of 5 – 6 min and usually measures at a constant temperature of ~ -23°C. In addition, INP activity screenings in the range from -15°C to -30°C are performed in regular intervals and during interesting meteorological periods such as Saharan dust events. The overlap in temperature ranges of INSEKT and PINE enables a comparison between the two measuring instruments.

Our results show that there are significant seasonal variations in the INP concentration. Especially during the summer time, strong diurnal variations in the INP concentration were observed, which could be explained by the influence of convectively lifted air from the boundary layer during the day. Correlations of the measured INP concentrations to meteorological parameters, aerosol properties and boundary layer stability will be discussed. In addition, we will present a case study of a dust event, which shows a sudden, strong increase in the INP concentration.