Eisnukleierende Aerosole in der Atmosphäre

Eiskeime (INP) sind Aerosolpartikel, die das Entstehen von Eiskristallen in der Atmosphäre zwischen 0 und -37°C ermöglichen, indem sie die zur Ausbildung der Eisphase nötige Energie gegenüber einem reinen Wassersystem stark herabsetzen. Dabei sind aktive Stellen auf der Oberfläche dieser Partikel für die erste Nukleation von Eis verantwortlich. In der Folge können die Eiskristalle zulasten von verdunstenden Wasserdampfmolekülen und Wassertröpfchen weiter anwachsen. Über Eismultiplikationsprozesse zersplittern und vervielfältigen sich die Eiskristalle und wachsen über Bereifung schließlich zu einer kritischen Größe heran, wodurch sie als Niederschlag zu Boden fallen können. Auch wenn der Anteil der zur heterogenen Eisnukleation fähigen Aerosole vergleichsweise gering ist, spielen INP eine entscheidende Rolle für die Entwicklung von Niederschlag und nehmen Einfluss auf Strahlungsprozesse, indem sie auf die Phase der Wolken und damit auf deren Strahlungseigenschaften einwirken. Viele Fragen im Forschungsgebiet der heterogenen Eisnukleation sind jedoch weiterhin nicht hinreichend genau geklärt. Ohne eine verbesserte Kenntnis von Konzentrationen, geographischer und vertikaler Verteilung, sowie zeitlicher Variation, Quellen und Natur von INP, sind noch vorhandene Wissenslücken im Strahlungsantrieb durch Wechselwirkungen von Aerosolen und Wolken nur zu einem gewissem Grad zu reduzieren. Dies ist nötig, um aktuelle Beobachtungsdaten der sich erwärmenden Atmosphäre besser verstehen und die zukünftigen Änderungen des Klimas sicherer vorhersagen zu können. In dieser Arbeit wird die Vakuumdiffusionskammer FRIDGE verwendet, um atmosphärische INP-Konzentrationen zu bestimmen. Aerosolpartikel werden dabei in einem ersten Schritt auf einem Silicium-Probenträger elektrostatisch niedergeschlagen. Die Effizienz des Sammelprozesses, also der Anteil der Partikel die tatsächlich auf dem Si-Substrat abgeschieden werden, wurde mittels zweier unabhängiger Methoden auf etwa 60% bestimmt. In einem zweiten Mess-Schritt werden die Proben in FRIDGE typischen Bedingungen von Mischphasenwolken ausgesetzt, wodurch Eiskristalle an den INP aktiviert werden und im Verlauf einer Messung anwachsen. Eine Kamera beobachtet die durch das Eiswachstum entstehenden Helligkeitsänderungen auf dem dunklen Probensubstrat. Die Kriterien, wann ein Objekt als Eiskristall identifiziert und gezählt wird, mussten im Rahmen dieser Arbeit neu entwickelt werden. In der zu Beginn der Arbeit vorgefundenen Einstellung hatte bereits eine sehr geringe Helligkeitsänderung, wie sie durch das hygroskopische Wachstum von Aerosolpartikeln hervorgerufen wird, zu Signalen geführt, die fälschlicherweise als Eiskristalle gezählt wurden. Das reevaluierte Messverfahren von FRIDGE wurde im Zuge der FIN-02 Kampagne in einem groß angelegten Laborexperiment an der AIDA Wolkenkammer mit zahlreichen anderen INP-Zählern aus der ganzen Welt verglichen. Für den Großteil der Messungen der untersuchten Modell-Aerosoltypen konnte eine zufriedenstellende Übereinstimmung mit den anderen Instrumenten erzielt werden. In einer einmonatigen Feldmesskampagne im östlichen Mittelmeerraum konnten die ersten INP-Messungen an Bord eines unbemannten Flugzeugs durchgeführt werden. Während der Kampagne auf Zypern wurden mehrere Fälle von transportiertem Saharastaub beprobt, in denen die INP-Konzentration maßgeblich erhöht war. Lidar-Beobachtungen und ein Staubtransportmodell zeigten, dass sich das Maximum der Staubschichten zumeist in etwa 2-4 Kilometern Höhe befand. In der Höhe wurden INP-Konzentrationen gefunden, die im Mittel um einen Faktor 10 größer waren als auf Bodenniveau. Es wird gefolgert, dass INP-Messungen am Boden möglicherweise nur begrenzte Aussagekraft über die Situation nahe der Wolkenbildung besitzen. Im Rahmen BACCHUS-Projekts wurden zwischen August 2014 und Januar 2017 (mit Unterbrechungen) alle 1-2 Tage Proben an drei Reinluftstationen gesammelt (insgesamt über 900). Das INP-Messnetz mit einer geographischen Ausdehnung von der Arktis zum Äquator bestand aus Stationen in Spitzbergen, Martinique und im Amazonas. Die Station im brasilianischen Regenwald ist durch wechselnde Bedingungen von sauberer Regen- und verunreinigter Trockenzeit charakterisiert. In der Trockenzeit steigen die Partikelkonzentrationen durch starke Belastung aus Biomassenverbrennung um eine Größenordnung an; eine gleichzeitige Zunahme der INP-Konzentrationen konnte nicht beobachtet werden. Daraus kann vermutet werden, dass Partikel aus Feueremissionen keine ausgezeichneten Fähigkeiten zur Eisnukleation aufweisen. Die INP-Konzentrationen in der Karibik konnten mit dem Jahresgang von transportieren Saharastaub in Verbindung gebracht werden. In der Arktis wurden die niedrigsten INP-Konzentrationen der drei Stationen beobachtet. Zum Zeitpunkt des Erstellens dieser Arbeit können die determinierenden Einflussfaktoren, sowie der anthropogene Einfluss zur Zeit des arktischen Dunstes noch nicht abschließend geklärt werden.Ice nucleating particles (INPs) are aerosol particles that enable the emergence of ice crystals in the atmosphere at temperatures between 0 °C and ‒37 °C by significantly lowering the energy barrier that exists for spontaneous nucleation of water molecules. Active sites on the surface of these particles are responsible for the first nucleation of ice. The now formed ice crystals start to grow at the expense of water molecules and droplets. The ice crystals splinter and multiply, then grow further by riming to a critical size at which they start get relevant for precipitation processes. Although the fraction of aerosol particles able to nucleate ice is relatively small, INPs play an important role for the development of precipitation and influence the radiation budget by affecting the phase of the cloud. However, many questions in the topic of ice nucleation are not yet solved to a satisfying degree. As long as the knowledge of concentrations, geographical and vertical distribution, temporal variation, sources and nature of INPs will not improve, it is unlikely that the existing gaps in understanding the radiative forcings of aerosol-cloud interactions will reduce significantly. In turn, this is needed to better understand observations of the warming atmosphere and predict future climate changes more accurately. The isostatic vacuum diffusion chamber FRIDGE (FRankfurt Ice nucleation Deposition freezinG Experiment) was used in this thesis to measure INP concentrations. In a first step aerosol particles are electrostatically precipitated onto a silicon disc. The efficiency of this sampling process (the fraction of particles that are actually sampled on the substrate) was determined by two independent methods to be about 60%. In a second measurement step the samples are exposed to typical conditions of mixed-phase clouds within FRIDGE. During a measurement ice crystals activate on the INPs and start to grow. Changes in brightness associated with the emerging ice crystals on the dark substrate are observed by a CCD camera. The criteria, which determine if an object is counted as an ice crystal, had to be changed during this thesis. According to the settings from the beginning of this work even little changes in brightness due to the hygroscopic growth of water droplets were enough to cause an erroneous count of a supposedly ice crystal. The re-evaluated FRIDGE instrument was compared to many other INP counters from all over the world in a large-scale laboratory experiment at the AIDA cloud chamber during the Fifth International Workshop on Ice Nucleation part 2 (FIN-02). A reasonable agreement with the other instruments could be achieved for most measurements of the various aerosol types. The first INP measurements on board of an unmanned aerial vehicle could be performed during a one-month field campaign in the Eastern Mediterranean. The transport of Saharan dust was observed on several occasions during the Cyprus campaign, influencing the INP concentrations substantially. Measurements by lidar and a dust transport model revealed that the densest layer of the dust was usually found at 2 ‒ 4 km altitude. INP concentrations in elevated plumes were found to be a factor of 10 higher in average than the ground based measurements. It is concluded that INP measurements at ground level may only be of limited significance for the situation at the level cloud formation. As part of the EU project BACCHUS more than 900 FRIDGE samples were collected routinely every one or two days at three remote stations between August 2014 and January 2017 (with interruptions). The stations of the INP network were located at Svalbard, Martinique and the Amazon, covering a global geographic scale. The station in the Brazilian rainforest is characterized by changing conditions of the clean wet season and the polluted dry season. In the dry season particle concentrations rise one order of magnitude due to a strong increase of biomass burning. However, a simultaneous increase of INP concentrations could not be observed, suggesting that particles of fire emissions are not particularly ice active. The INP concentration in the Caribbean could be associated with the seasonal pattern of transported Saharan dust. The lowest INP concentrations of the three stations were found in the Arctic. The factors controlling the INP concentration, as well as the possible influence of the anthropogenic arctic haze, could not be resolved conclusively at the time this thesis was written

Testing mobile air purifiers in a school classroom: reducing the airborne transmission risk for SARS-CoV-2

Airborne transmission of SARS-CoV-2 through virus-containing aerosol particles has been established as an important pathway for Covid-19 infection. Suitable measures to prevent such infections are imperative, especially in situations when a high number of persons convene in closed rooms. Here we tested the efficiency and practicability of operating four air purifiers equipped with HEPA filters in a high school classroom while regular classes were taking place. We monitored the aerosol number concentration for particles >3 nm at two locations in the room, the aerosol size distribution in the range from 10 nm to 10 µm, PM10 and CO2 concentration. For comparison, we performed similar measurements in a neighboring classroom without purifiers. In times when classes were conducted with windows and door closed, the aerosol concentration was reduced by more than 90% within less than 30 min when running the purifiers (air exchange rate 5.5 h−1). The reduction was homogeneous throughout the room and for all particle sizes. The measurements are supplemented by a calculation estimating the maximum concentration levels of virus-containing aerosol from a highly contagious person speaking in a closed room with and without air purifiers. Measurements and calculation demonstrate that air purifiers potentially represent a well-suited measure to reduce the risks of airborne transmission of SARS-CoV-2 substantially. Staying for 2 h in a closed room with a highly infective person, we estimate that the inhaled dose is reduced by a factor of six when using air purifiers with a total air exchange rate of 5.7 h−1

Testing mobile air purifiers in a school classroom: reducing the airborne transmission risk for SARS-CoV-2

Airborne transmission of SARS-CoV-2 through virus-containing aerosol particles has been established as an important pathway for Covid-19 infection. Suitable measures to prevent such infections are imperative, especially in situations when a high number of persons convene in closed rooms. Here we tested the efficiency and practicability of operating four air purifiers equipped with HEPA filters in a high school classroom while regular classes were taking place. We monitored the aerosol number concentration for particles > 3 nm at two locations in the room, the aerosol size distribution in the range from 10 nm to 10 µm, PM10 and CO2 concentration. For comparison, we performed similar measurements in a neighboring classroom without purifiers. In times when classes were conducted with windows and door closed, the aerosol concentration was reduced by more than 90 % within less than 30 minutes when running the purifiers (air exchange rate 5.5 h-1). The reduction was homogeneous throughout the room and for all particle sizes. The measurements are supplemented by a calculation estimating the maximum concentration levels of virus-containing aerosol from a highly contagious person speaking in a closed room with and without air purifiers. Measurements and calculation demonstrate that air purifiers potentially represent a well-suited measure to reduce the risks of airborne transmission of SARS-CoV-2 substantially. Staying for two hours in a closed room with a highly infective person, we estimate that the inhaled dose is reduced by a factor of six when using air purifiers with a total air exchange rate of 5.7 h-1

Long-term deposition and condensation ice-nucleating particle measurements from four stations across the globe

International audienceAbstract. Ice particle activation and evolution have important atmospheric implications for cloud formation, initiation of precipitation and radiative interactions. The initial formation of atmospheric ice by heterogeneous ice nucleation requires the presence of a nucleating seed, an ice-nucleating particle (INP), to facilitate its first emergence. Unfortunately, only a few long-term measurements of INPs exist, and as a result, knowledge about geographic and seasonal variations of INP concentrations is sparse. Here we present data from nearly 2 years of INP measurements from four stations in different regions of the world: the Amazon (Brazil), the Caribbean (Martinique), central Europe (Germany) and the Arctic (Svalbard). The sites feature diverse geographical climates and ecosystems that are associated with dissimilar transport patterns, aerosol characteristics and levels of anthropogenic impact (ranging from near pristine to mostly rural). Interestingly, observed INP concentrations, which represent measurements in the deposition and condensation freezing modes, do not differ greatly from site to site but usually fall well within the same order of magnitude. Moreover, short-term variability overwhelms all long-term trends and/or seasonality in the INP concentration at all locations. An analysis of the frequency distributions of INP concentrations suggests that INPs tend to be well mixed and reflective of large-scale air mass movements. No universal physical or chemical parameter could be identified to be a causal link driving INP climatology, highlighting the complex nature of the ice nucleation process. Amazonian INP concentrations were mostly unaffected by the biomass burning season, even though aerosol concentrations increase by a factor of 10 from the wet to dry season. Caribbean INPs were positively correlated to parameters related to transported mineral dust, which is known to increase during the Northern Hemisphere summer. A wind sector analysis revealed the absence of an anthropogenic impact on average INP concentrations at the site in central Europe. Likewise, no Arctic haze influence was observed on INPs at the Arctic site, where low concentrations were generally measured. We consider the collected data to be a unique resource for the community that illustrates some of the challenges and knowledge gaps of the field in general, while specifically highlighting the need for more long-term observations of INPs worldwide

Long-term ice nucleating particle concentrations by offline vacuum diffusion chamber measurements from the Amazon, the Caribbean, Central Europe, and the Norwegian Arctic, data for figures 1-10

The data set reports on long-term measurements of ice nucleating particles (INPs) from four observational sites: 1. Amazon Tall Tower Observatory (ATTO, 2.144° S, 59.000° W, 130 m AMSL, Brazil), 2. Volcanological and Seismological Observatory of Martinique (OVSM, 14.735° N, 61.147° W, 487 m AMSL, Martinique, Caribbean), 3. Taunus Observatory (TO, 50.221° N, 8.446° E, 825 m AMSL, Germany) and 4. Zeppelin Observatory (ZO, 78.908° N, 11.881° E, 474 m AMSL, Ny-Alesund, Svalbard, Arctic). The presented data covers data from May 2015 to January 2017. Samples were usually collected every day or every two days at local noon for 50 minutes at 2 L min-1, resulting in 100 L of air volume. After transport samples were analyzed in our laboratory in Frankfurt using the vacuum diffusion chamber FRIDGE. Each sample was analyzed at multiple combinations of temperature ( 20 °C, -25 °C, and -30 °C) and relative humidity (95%, 97%, 99%, 101% w.r.t. water). INP concentrations per liter are reported for each condition (the paper focuses on the highest humidity). This data supplement is related to a scientific publication (doi:10.5194/acp-2020-667)

Long-term ice nucleating particle concentrations by offline vacuum diffusion chamber measurements from the Amazon, the Caribbean, Central Europe, and the Norwegian Arctic

The data set reports on long-term measurements of ice nucleating particles (INPs) from four observational sites: 1. Amazon Tall Tower Observatory (ATTO, 2.144° S, 59.000° W, 130 m AMSL, Brazil), 2. Volcanological and Seismological Observatory of Martinique (OVSM, 14.735° N, 61.147° W, 487 m AMSL, Martinique, Caribbean), 3. Taunus Observatory (TO, 50.221° N, 8.446° E, 825 m AMSL, Germany) and 4. Zeppelin Observatory (ZO, 78.908° N, 11.881° E, 474 m AMSL, Ny-Alesund, Svalbard, Arctic). The presented data covers data from May 2015 to January 2017. Samples were usually collected every day or every two days at local noon for 50 minutes at 2 L min-1, resulting in 100 L of air volume. After transport samples were analyzed in our laboratory in Frankfurt using the vacuum diffusion chamber FRIDGE. Each sample was analyzed at multiple combinations of temperature ( 20 °C, -25 °C, and -30 °C) and relative humidity (95%, 97%, 99%, 101% w.r.t. water). INP concentrations per liter are reported for each condition (the paper focuses on the highest humidity). This data supplement is related to a scientific publication (doi:10.5194/acp-2020-667)