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Recently significant advances have been made in the collection, detection, and characterization of ice nucleating particles (INP). Ice nuclei are particles that facilitate the heterogeneous formation of ice within the atmospheric aerosol by lowering the free energy barrier to spontaneous nucleation and growth of ice from atmospheric water and/or vapor. The Frankfurt isostatic diiffusion chamber (FRIDGE) is an INP collection and offline detection system that has become widely deployed and shows additional potential for ambient measurements. Since its initial development FRIDGE has gone through several iterations and improvements. Here we describe improvements that have been made in the collection and analysis techniques. We detail the uncertainties inherent in the measurement method, and suggest a systematic method of error analysis for FRIDGE measurements. Thus what is presented herein should serve as a foundation for the dissemination of all current and future measurements using FRIDGE instrumentation.
Desert dust is one of the most abundant ice nucleating particle types in the atmosphere. Traditionally, clay minerals were assumed to determine the ice nucleation ability of desert dust and constituted the focus of ice nucleation studies over several decades. Recently some feldspar species were identified to be ice active at much higher temperatures than clay minerals, redirecting studies to investigate the contribution of feldspar to ice nucleation on desert dust. However, so far no study has shown the atmospheric relevance of this mineral phase.
For this study four dust samples were collected after airborne transport in the troposphere from the Sahara to different locations (Crete, the Peloponnese, Canary Islands, and the Sinai Peninsula). Additionally, 11 dust samples were collected from the surface from nine of the biggest deserts worldwide. The samples were used to study the ice nucleation behavior specific to different desert dusts. Furthermore, we investigated how representative surface-collected dust is for the atmosphere by comparing to the ice nucleation activity of the airborne samples. We used the IMCA-ZINC setup to form droplets on single aerosol particles which were subsequently exposed to temperatures between 233 and 250 K. Dust particles were collected in parallel on filters for offline cold-stage ice nucleation experiments at 253–263 K. To help the interpretation of the ice nucleation experiments the mineralogical composition of the dusts was investigated. We find that a higher ice nucleation activity in a given sample at 253 K can be attributed to the K-feldspar content present in this sample, whereas at temperatures between 238 and 245 K it is attributed to the sum of feldspar and quartz content present. A high clay content, in contrast, is associated with lower ice nucleation activity. This confirms the importance of feldspar above 250 K and the role of quartz and feldspars determining the ice nucleation activities at lower temperatures as found by earlier studies for monomineral dusts. The airborne samples show on average a lower ice nucleation activity than the surface-collected ones. Furthermore, we find that under certain conditions milling can lead to a decrease in the ice nucleation ability of polymineral samples due to the different hardness and cleavage of individual mineral phases causing an increase of minerals with low ice nucleation ability in the atmospherically relevant size fraction. Comparison of our data set to an existing desert dust parameterization confirms its applicability for climate models. Our results suggest that for an improved prediction of the ice nucleation ability of desert dust in the atmosphere, the modeling of emission and atmospheric transport of the feldspar and quartz mineral phases would be key, while other minerals are only of minor importance.
Desert dust is one of the most abundant ice nucleating particle types in the atmosphere. Tra ditionally, clay minerals were assumed to determine the ice nucleation ability of desert dust and constituted the focus of ice nucleation studies. Only recently some feldspar species were identified to be ice-active at much higher temperatures than clay minerals, redirecting studies to investigate the contribution of feldspar to ice nucleation on desert dust. However, so far no study has shown the atmospheric relevance of this mineral phase.
For this study four dust samples were collected after airborne transport in the troposphere from the Sahara to different locations (Crete, the Peloponnese, Canary Islands and the Sinai Peninsula). Additionally, eleven dust samples were collected from the surface from nine of the biggest deserts worldwide. The samples were used to study the ice nucleation behavior specific to different desert dusts. Furthermore we investigated how representative ice nucleation on surface-collected dust is for that in the atmosphere by comparing to the ice nucleation activity of the airborne samples. We used the IMCA-ZINC set-up to form droplets on single aerosol particles which were subsequently exposed to temperatures between 233 - 250 K. Dust particles were collected in parallel on filters for offline cold stage ice nucleation experiments at 253 - 263 K. To help the interpretation of the results from the ice nucleation experiments the mineralogical composition of the dusts was investigated.We found that a higher ice nucleation activity in a given sample can be attributed at 253 K to the K-feldspar content present in this sample whereas at temperatures between 238 - 245 K it is attributed to the sum of feldspar and quartz content present. A high clay content on the other hand is associated with a lower ice nucleation activity of a sample. This confirms the importance of feldspar at T > 250 K and the role of quartz and feldspars determining the ice nucleation activities at lower T as found by earlier studies for monomineral dust surrogates. Furthermore, we find that milling may lead to a decrease in the ice nucleation ability of polymineral samples due to a change in mineralogical composition in the atmospherically relevant size fraction arising from the different hardness and cleavage of individual mineral phases. Comparison of our comprehensive data set to an existing desert dust parameterization confirms its applicability for climate models. Our results suggest that for an improved prediction of the ice nucleation ability of desert dust in the atmosphere, the modelling of emission and atmospheric transport of the feldspar and quartz mineral phases would be key while other minerals are only of minor importance.
Recently significant advances have been made in the collection, detection and characterization of ice nucleating particles (INPs). Ice nuclei are particles that facilitate the heterogeneous formation of ice within the atmospheric aerosol by lowering the free energy barrier to spontaneous nucleation and growth of ice from atmospheric water and/or vapor. The Frankfurt isostatic diffusion chamber (FRankfurt Ice nucleation Deposition freezinG Experiment: FRIDGE) is an INP collection and offline detection system that has become widely deployed and shows additional potential for ambient measurements. Since its initial development FRIDGE has gone through several iterations and improvements. Here we describe improvements that have been made in the collection and analysis techniques. We detail the uncertainties inherent in the measurement method and suggest a systematic method of error analysis for FRIDGE measurements. Thus what is presented herein should serve as a foundation for the dissemination of all current and future measurements using FRIDGE instrumentation.
Immersion freezing is the most relevant heterogeneous ice nucleation mechanism through which ice crystals are formed in mixed-phase clouds. In recent years, an increasing number of laboratory experiments utilizing a variety of instruments have examined immersion freezing activity of atmospherically relevant ice nucleating particles (INPs). However, an inter-comparison of these laboratory results is a difficult task because investigators have used different ice nucleation (IN) measurement methods to produce these results. A remaining challenge is to explore the sensitivity and accuracy of these techniques and to understand how the IN results are potentially influenced or biased by experimental parameters associated with these techniques.
Within the framework of INUIT (Ice Nucleation research UnIT), we distributed an illite rich sample (illite NX) as a representative surrogate for atmospheric mineral dust particles to investigators to perform immersion freezing experiments using different IN measurement methods and to obtain IN data as a function of particle concentration, temperature (T), cooling rate and nucleation time. Seventeen measurement methods were involved in the data inter-comparison. Experiments with seven instruments started with the test sample pre-suspended in water before cooling, while ten other instruments employed water vapor condensation onto dry-dispersed particles followed by immersion freezing. The resulting comprehensive immersion freezing dataset was evaluated using the ice nucleation active surface-site density (ns) to develop a representative ns(T) spectrum that spans a wide temperature range (−37 °C < T < −11 °C) and covers nine orders of magnitude in ns.
Our inter-comparison results revealed a discrepancy between suspension and dry-dispersed particle measurements for this mineral dust. While the agreement was good below ~ −26 °C, the ice nucleation activity, expressed in ns, was smaller for the wet suspended samples and higher for the dry-dispersed aerosol samples between about −26 and −18 °C. Only instruments making measurement techniques with wet suspended samples were able to measure ice nucleation above −18 °C. A possible explanation for the deviation between −26 and −18 °C is discussed. In general, the seventeen immersion freezing measurement techniques deviate, within the range of about 7 °C in terms of temperature, by three orders of magnitude with respect to ns. In addition, we show evidence that the immersion freezing efficiency (i.e., ns) of illite NX particles is relatively independent on droplet size, particle mass in suspension, particle size and cooling rate during freezing. A strong temperature-dependence and weak time- and size-dependence of immersion freezing efficiency of illite-rich clay mineral particles enabled the ns parameterization solely as a function of temperature. We also characterized the ns (T) spectra, and identified a section with a steep slope between −20 and −27 °C, where a large fraction of active sites of our test dust may trigger immersion freezing. This slope was followed by a region with a gentler slope at temperatures below −27 °C. A multiple exponential distribution fit is expressed as ns(T) = exp(23.82 × exp(−exp(0.16 × (T + 17.49))) + 1.39) based on the specific surface area and ns(T) = exp(25.75 × exp(−exp(0.13 × (T + 17.17))) + 3.34) based on the geometric area (ns and T in m−2 and °C, respectively). These new fits, constrained by using an identical reference samples, will help to compare IN measurement methods that are not included in the present study and, thereby, IN data from future IN instruments.
Immersion freezing is the most relevant heterogeneous ice nucleation mechanism through which ice crystals are formed in mixed-phase clouds. In recent years, an increasing number of laboratory experiments utilizing a variety of instruments have examined immersion freezing activity of atmospherically relevant ice-nucleating particles. However, an intercomparison of these laboratory results is a difficult task because investigators have used different ice nucleation (IN) measurement methods to produce these results. A remaining challenge is to explore the sensitivity and accuracy of these techniques and to understand how the IN results are potentially influenced or biased by experimental parameters associated with these techniques.
Within the framework of INUIT (Ice Nuclei Research Unit), we distributed an illite-rich sample (illite NX) as a representative surrogate for atmospheric mineral dust particles to investigators to perform immersion freezing experiments using different IN measurement methods and to obtain IN data as a function of particle concentration, temperature (T), cooling rate and nucleation time. A total of 17 measurement methods were involved in the data intercomparison. Experiments with seven instruments started with the test sample pre-suspended in water before cooling, while 10 other instruments employed water vapor condensation onto dry-dispersed particles followed by immersion freezing. The resulting comprehensive immersion freezing data set was evaluated using the ice nucleation active surface-site density, ns, to develop a representative ns(T) spectrum that spans a wide temperature range (−37 °C < T < −11 °C) and covers 9 orders of magnitude in ns.
In general, the 17 immersion freezing measurement techniques deviate, within a range of about 8 °C in terms of temperature, by 3 orders of magnitude with respect to ns. In addition, we show evidence that the immersion freezing efficiency expressed in ns of illite NX particles is relatively independent of droplet size, particle mass in suspension, particle size and cooling rate during freezing. A strong temperature dependence and weak time and size dependence of the immersion freezing efficiency of illite-rich clay mineral particles enabled the ns parameterization solely as a function of temperature. We also characterized the ns(T) spectra and identified a section with a steep slope between −20 and −27 °C, where a large fraction of active sites of our test dust may trigger immersion freezing. This slope was followed by a region with a gentler slope at temperatures below −27 °C. While the agreement between different instruments was reasonable below ~ −27 °C, there seemed to be a different trend in the temperature-dependent ice nucleation activity from the suspension and dry-dispersed particle measurements for this mineral dust, in particular at higher temperatures. For instance, the ice nucleation activity expressed in ns was smaller for the average of the wet suspended samples and higher for the average of the dry-dispersed aerosol samples between about −27 and −18 °C. Only instruments making measurements with wet suspended samples were able to measure ice nucleation above −18 °C. A possible explanation for the deviation between −27 and −18 °C is discussed. Multiple exponential distribution fits in both linear and log space for both specific surface area-based ns(T) and geometric surface area-based ns(T) are provided. These new fits, constrained by using identical reference samples, will help to compare IN measurement methods that are not included in the present study and IN data from future IN instruments.
Ziel dieser Arbeit, die im Rahmen der Ice Nuclei Research Unit (INUIT) Forschergruppe erstellt wurde, war ursprünglich die saisonale und geographische Variabilität von bodennahen Eiskeimen zu untersuchen. Die Konzentrationen, Quellen und Zusammensetzung der Eisnuklei (ice nuclei, IN) sollte als Basis für Parametrisierungen dienen. Das Verständnis von Eiskeimen und deren Einfluss auf Wetter und Klima sind nur zum Teil bekannt und bedürfen daher noch weitgehender Forschung. Auch die Änderung der Eiskeimkonzentration mit der Zeit kann von Bedeutung sein, diese sollte durch die Fortführung einer Langzeitmessreihe untersucht werden. Durch Hinzuziehen von lokalen Parametern und Trajektorien sollten Proxies für die IN Konzentration ermittelt werden.
Im Rahmen dieser Arbeit taten sich jedoch Probleme am Messverfahren auf, weshalb die ursprünglichen Ziele in den Hintergrund gerieten und die Verbesserung und Neuaufnahme des Messverfahrens in den Vordergrund trat. Anhand von zielgerichteten Experimenten wurde ein Messfehler ermittelt, der durch die vorherige Fehlinterpretation von deliqueszierenden Partikeln und von Tröpfchen als vermeintliche Eiskristalle entstand. Dieser Fehler wurde charakterisiert und durch optische Analysen dessen Ursprung ermittelt. Datensätze, die durch diese hygroskopischen Partikel fehlerbehaftet waren, wurden korrigiert und reanalysiert. Ein in früheren Arbeiten am Taunus Observatorium/Kleiner Feldberg ermittelter Jahresgang in der Eiskeimkonzentration mit einem Maximum im Sommer und einem Minimum im Winter konnte bestätigt werden, die Absolutzahlen sind jedoch deutlich geringer als bisher angenommen. Lokale Parameter sowie Trajektorien wurden zur weiteren Analyse hinzugezogen.
Die Reevaluierung der Datensätze vom Taunus Observatorium führte zu keinem abschließenden Ergebnis. Ein allgemein gültiger Zusammenhang zwischen Eiskeimkonzentration und Parametern, welche das Staubvorkommen in der Atmosphäre quantifizieren (PM10 und Aerosol Optische Dicke), konnte nicht festgestellt werden. Da die Messungen bei relativ warmen Bedingungen (≥-18°C) durchgeführt wurden, Staub aber erst bei kälteren Temperaturen als effektiver Eiskeim gilt, ist dieses Ergebnis jedoch zu erwarten gewesen.
Auch die Luftmassenherkunft scheint keinen eindeutigen Einfluss zu haben. Betrachtungen der Bodenfeuchte lieferten signifikante Korrelationen, welche jedoch monatsabhängig positiv oder negativ ausfallen können. Im Frühling ist eine hohe Bodenfeuchte mit einer erhöhten Konzentration von IN in Verbindung zu bringen, im Sommer liegt bei niedriger Bodenfeuchte eine tendenziell höhere Eiskeimkonzentration vor. Die Windrichtung hat für die Eiskeimkonzentration einen Einfluss, wenn der Wind aus Südost zum Taunus Observatorium strömt. Anthropogenes Aerosol aus Frankfurt am Main hemmt hier vermutlich die Eisbildung, was zu einer signifikant niedrigeren mittleren Konzentration aus dieser Richtung führt.
Da das Messverfahren noch nicht in seinem vollen Potential genutzt wurde, wurde es um eine Analysemethode erweitert. Mittels Tröpfchengefrierexperimenten konnte ein weiterer Gefriermodus betrachtet werden. Nun deckt das hier genutzte Messverfahren drei der vier bekannten Gefriermoden ab. Anhand von Testsubstanzen wurde die Zuverlässigkeit der neu eingeführten Methode überprüft und nachgewiesen.
Erste Parallelproben der korrigierten Depositions- und Kondensationsgefriermessmethode und der neu eingeführten Immersionsgefriermessung wurden am Taunus Observatorium/Kleiner Feldberg genommen. Dabei wurde auch ein Staubereignis beprobt und detailliert ausgewertet. Zwischen lokalen Parametern und Eiskeimkonzentration fanden sich Zusammenhänge. Bei Messbedingungen <-20°C konnte ein signifikanter Zusammenhang zwischen PM10 und Eiskeimkonzentration im Immersions- und Kondensationsmodus gefunden werden. Der Depositionsgefriermodus blieb unauffällig. Zwischen Bodenfeuchte und IN-Konzentration konnten ebenfalls wie bei der Reevaluierung der alten Messdaten Signifikanzen festgestellt werden.
Die neu eingeführte Immersionsmessmethode und die korrigierte Methode zur Bestimmung von Depositions- und Kondensationsgefrierkernen liefern Messdaten, welche im Bereich anderer Eiskeimzähler liegen. Vergleiche mit Parametrisierungen zeigen, dass die Messwerte dem aktuellen Stand der Forschung entsprechen und davon ausgegangen werden kann, dass sie vertrauenswürdig und belastbar sind.
