For the primary formation of ice crystals in clouds in the temperature range between 0°C and roughly -38°C, catalysts are needed which can initiate the nucleation of ice in a liquid, supercooled drop. Atmospheric particles that can act as such catalysts are called ice nucleating particles (INP).

Atmospheric concentrations of INP have been determined by TROPOS in a series of measurement campaigns at various locations around the world. Atmospheric filter samples were collected and evaluated in the laboratory. Off-line instrumentations were used, which can provide data in the temperature range from 0°C to approx. -25°C (the statements below therefore generally refer to this temperature range). The locations at which examinations were done (in the sequence listed below) include the Arctic, Antarctica, gobal oceans, southern Chile, Cape Verde/Cabo Verde, Beijing, Maumee River und Lake Erik (USA), Cyprus.

The obtained data are among others important input parameters for climate models and can help to improve future model calculations.

 

The following investigations and observations, shown separately for different regions of the Earth, were made:

Arctic

Time series of INP concentrations from Greenlandic ice cores from ~ 5 centuries.

  • i) Cooperation with PSI glaciologists in Switzerland, to measure INP in samples of two Arctic ice cores; no significant increase in INP concentrations compared to today was found in the period studied (past 500 years up t0 1989); on short time scales (samples than were less than one year long), INP concentrations showed a significantly higher scatter compared to perennial samples; oberall, the observed scatter in the data remained within that observed in current data (Hartmann et al., 2019).

Time series of INP concentrations at four different Arctic stations.

  • ii) Cooperation with partners from four different Arctic measuring stations (Alert, Ny Alesund, Utqiagvig (formerly Barrow), Villum Research Station); yearly variations of INP concentrations were found, with high values in summer and lower values in winter, which was particularly pronounced for very ice-active INP. Possible sources for the highly ice-active INP (presumably biogenic in origin) were identified in the Arctic (both terrestrial and maritime sources) (Wex et al., 2019).

INP spectra of filters collected during PamArcMip.

  • iii) Sample collection at Villum Research Station, both ground-based and on board the Polar 5 aircraft of the Alfred-Wegener-Institute in the framework of the PamArcMip campaign in March/April 2018. For days when flights were made at low altitudes above the mostly (but not completely) frozen ocean, elevated INP concentrations were found, while all other samples collected at higher altitudes showed low INP concentrations. For elevated INP concentrations, a biogenic origin from open leads and polynyas is assumed (Hartmann et al., 2020).

Figure from Hartmann et al. (2021), showing INP concentrations as obtained during the Polarstern cruise.

  • iv) Sample collection during a voyage of the German research vessel Polarstern within the framework of  (AC)3 (project B-04) in May - July 2017 - the observed atmospheric INP concentrations vary over 2 to 3 orders of magnitude at any particular temperature, are for the most part lower than at mid-latitudes, and cannot be explained by marine sources without enrichment factors of at least 104. But source attribution was not clearly possible and local marine sources must be considered. Initial data were published as part of a campaign review ((Wendisch et al., 2019), while Hartmann et al. (2021) discusses the results in detail.

Antarctica

Overview over INP concentrations obtained during the Antactic circumnavigation.

  • i) Examinations were made for the Southern Ocean region, based on data collected during an Antarctic circumnavigation during the ACE-SPACE campaign from November 2016 to March 2017 (for campaign overview see Schmale et al., 2019).

    INP concentrations were generally low, however, close to land masses (islands or continents), higher values were observed (Tatzelt et al., 2021).

INP spectra from filters collected at the Antarctic Neumayer station.

  • ii) Some first data for samples from the long-term filter collection at AWI's Antarctic Neumayer Measuring Station show INP concentrations similar to those described for the Antarctic Ocean in literature. One of the samples also contained INP that can be attributed to biogenic proteins, as they could be destroyed by heat.

Global oceans

Bild aus Welti et al. (2020) mit atmosphärischen INP Konzentrationen während verschiedener Schiffsmesskampagnen.

  • INP concentrations from ship measurement campaigns (both our own and from literature) are 1 to 2 orders of magnitude lower than those from measurements performed on continents. No contribution to INP from ship emissions in the temperature range above -36°C was observed (Welti et al., 2020).

Southern Chile

INP spectra for roughly one year for Cerro Mirador.

  • As part of the DACAPO-PESO project, long-term in-situ aerosol data and filters are being collected to determine INP concentrations in southern Chile (Cerro Mirador west of Punta Areanas at an altitude of about 600m). No annual variation is observed, but INP concentrations are similar throughout the year as e.g. on Cape Verde (Cabo Verde).

Cape Verde islands / Cabo Verde

Figure from Welti et al. (2018) showing INP concentrations as measured on the Cape Verde islands.

  • 4 years of continuous filter samples (2009-2013) showed increased INP concentrations in combination with Sahara dust for INP active at lower temperatures (< -16°C). A source assignment for INP active at higher temperatures was not possible. A log-normal distribution of the INP concentrations, as observed at different separate temperatures, suggests that the INP do not originate from local sources, but from a number of different sources whose contributions were randomly turbulently mixed during transport (Welti et al., 2018).

Figure from Gong et al. (2020) showing INP concentrations as measured on the Cape Verde islands.

  • Within the framework of MarParCloud, INP concentrations were determined for various samples: Ocean water, oceanic surface micro layer (SML), in cloud water and in the air at two measuring stations (sea level and at an altitude of 700m on a mountain). Atmospheric INP are mostly supermicron in size, highly ice-active INP are of biogenic origin, and the observed atmospheric INP concentrations cannot be explained by marine sources unless there would be enrichment factors of at least 104 (Gong et al., 2020).

Beijing

Figure from Chen et al. (2018) showing INP concentrations measured in Beijing in comparison to other parameters.

  • Investigation of INP in atmospheric aerosol that was, at times, strongly influenced by anthropogenic pollution; it was found that no additional INP were introduced into the atmosphere even during phases of strong air pollution, i.e. that anthropogenic particles in Beijing did not function as INP in the atmosphere in the investigated temperature range (Chen et al., 2018).

 

Maumee River and Lake Erie (US)

Conceptualized sketch from Knackstedt et al. (2018), showing the circulation of biogenic INP in nature.

  • The farmland surrounding Maumee River was identified as the source of biogenic INP in the river (possibly the soil fungus Mortierella alpina). In Lake Erie (into which the Maumee River flows) additional different biogenic INP were found. Atmospheric INP concentrations above a weir were elevated compared to the surrounding area, suggesting an input of INP from the river (Knackstedt et al, 2018).

Cyprus

Figure from Gong et al. (2019) showing INP concentrations as measured on Cyprus.

  • Filter samples were collected both in the framework of a joint INUIT/BACCHUS-campaign (April 2016) as well as during A-LIFE (April 2017). Measurements of different devices for determining the INP concentration were consistent in their results, and no correlation was found between concentrations of particles > 500 nm and of INP (Gong et al., 2019).

Literature:

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Chen et al., 2021: Chen, J., Wu, Z., Zhao, X., Wang, Y., Chen, J., Qiu, Y., Zong, T., Chen, H., Wang, B., Lin, P., Liu, W., Guo, S., Yao, M., Zeng, L., Wex, H., Liu, X., Hu, M., and Li, S.-M. (2021), Atmospheric humic-like substances (HULIS) act as ice-nucleating particles, Geophys. Res. Lett., 48(e2021GL092443), doi:10.1029/2021GL092443.

Gong et al., 2019: Gong, X., H. Wex, T. Müller, A. Wiedensohler, K. Höhler, K. Kandler, N. Ma, B. Dietel, T. Schiebel, O. Möhler, and F. Stratmann (2019), Characterization of aerosol properties at Cyprus, focusing on cloud condensation nuclei and ice nucleating particles, Atmos. Chem. Phys., 19, 10883-10900, doi:10.5194/acp-19-10883-2019.

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