2018, Schmale, Julia, Henning, Silvia, Decesari, Stefano, Henzing, Bas, Keskinen, Helmi, Sellegri, Karine, Ovadnevaite, Jurgita, Pöhlker, Mira L., Brito, Joel, Bougiatioti, Aikaterini, Kristensson, Adam, Kalivitis, Nikos, Stavroulas, Iasonas, Carbone, Samara, Jefferson, Anne, Park, Minsu, Schlag, Patrick, Iwamoto, Yoko, Aalto, Pasi, Äijälä, Mikko, Bukowiecki, Nicolas, Ehn, Mikael, Frank, Göran, Fröhlich, Roman, Frumau, Arnoud, Herrmann, Erik, Herrmann, Hartmut, Holzinger, Rupert, Kos, Gerard, Kulmala, Markku, Mihalopoulos, Nikolaos, Nenes, Athanasios, O'Dowd, Colin, Petäjä, Tuukka, Picard, David, Pöhlker, Christopher, Pöschl, Ulrich, Poulain, Laurent, Prévôt, André Stephan Henry, Swietlicki, Erik, Andreae, Meinrat O., Artaxo, Paulo, Wiedensohler, Alfred, Ogren, John, Matsuki, Atsushi, Yum, Seong Soo, Stratmann, Frank, Baltensperger, Urs, Gysel, Martin

Aerosol-cloud interactions (ACI) constitute the single largest uncertainty in anthropogenic radiative forcing. To reduce the uncertainties and gain more confidence in the simulation of ACI, models need to be evaluated against observations, in particular against measurements of cloud condensation nuclei (CCN). Here we present a data set - ready to be used for model validation - of long-term observations of CCN number concentrations, particle number size distributions and chemical composition from 12 sites on 3 continents. Studied environments include coastal background, rural background, alpine sites, remote forests and an urban surrounding. Expectedly, CCN characteristics are highly variable across site categories. However, they also vary within them, most strongly in the coastal background group, where CCN number concentrations can vary by up to a factor of 30 within one season. In terms of particle activation behaviour, most continental stations exhibit very similar activation ratios (relative to particles 20nm) across the range of 0.1 to 1.0% supersaturation. At the coastal sites the transition from particles being CCN inactive to becoming CCN active occurs over a wider range of the supersaturation spectrum. Several stations show strong seasonal cycles of CCN number concentrations and particle number size distributions, e.g. at Barrow (Arctic haze in spring), at the alpine stations (stronger influence of polluted boundary layer air masses in summer), the rain forest (wet and dry season) or Finokalia (wildfire influence in autumn). The rural background and urban sites exhibit relatively little variability throughout the year, while short-term variability can be high especially at the urban site. The average hygroscopicity parameter, calculated from the chemical composition of submicron particles was highest at the coastal site of Mace Head (0.6) and lowest at the rain forest station ATTO (0.2-0.3). We performed closure studies based on -Köhler theory to predict CCN number concentrations. The ratio of predicted to measured CCN concentrations is between 0.87 and 1.4 for five different types of . The temporal variability is also well captured, with Pearson correlation coefficients exceeding 0.87. Information on CCN number concentrations at many locations is important to better characterise ACI and their radiative forcing. But long-term comprehensive aerosol particle characterisations are labour intensive and costly. Hence, we recommend operating migrating-CCNCs to conduct collocated CCN number concentration and particle number size distribution measurements at individual locations throughout one year at least to derive a seasonally resolved hygroscopicity parameter. This way, CCN number concentrations can only be calculated based on continued particle number size distribution information and greater spatial coverage of long-term measurements can be achieved.

2016, Kürten, Andreas, Bianchi, Federico, Almeida, Joao, Kupiainen-Määttä, Oona, Dunne, Eimear M., Duplissy, Jonathan, Williamson, Christina, Barmet, Peter, Breitenlechner, Martin, Dommen, Josef, Donahue, Neil M., Flagan, Richard C., Franchin, Alessandro, Gordon, Hamish, Hakala, Jani, Hansel, Armin, Heinritzi, Martin, Ickes, Luisa, Jokinen, Tuija, Kangasluoma, Juha, Kim, Jaeseok, Kirkby, Jasper, Kupc, Agnieszka, Lehtipalo, Katrianne, Leiminger, Markus, Makhmutov, Vladimir, Onnela, Antti, Ortega, Ismael K., Petäjä, Tuukka, Praplan, Arnaud P., Riccobono, Francesco, Rissanen, Matti P., Rondo, Linda, Schnitzhofer, Ralf, Schobesberger, Siegfried, Smith, James N., Steiner, Gerhard, Stozhkov, Yuri, Tomé, António, Tröstl, Jasmin, Tsagkogeorgas, Georgios, Wagner, Paul E., Wimmer, Daniela, Ye, Penglin, Baltensperger, Urs, Carslaw, Ken, Kulmala, Markku, Curtius, Joachim

Binary nucleation of sulfuric acid and water as well as ternary nucleation involving ammonia are thought to be the dominant processes responsible for new particle formation (NPF) in the cold temperatures of the middle and upper troposphere. Ions are also thought to be important for particle nucleation in these regions. However, global models presently lack experimentally measured NPF rates under controlled laboratory conditions and so at present must rely on theoretical or empirical parameterizations. Here with data obtained in the European Organization for Nuclear Research CLOUD (Cosmics Leaving OUtdoor Droplets) chamber, we present the first experimental survey of NPF rates spanning free tropospheric conditions. The conditions during nucleation cover a temperature range from 208 to 298 K, sulfuric acid concentrations between 5 × 105 and 1 × 109 cm−3, and ammonia mixing ratios from zero added ammonia, i.e., nominally pure binary, to a maximum of ~1400 parts per trillion by volume (pptv). We performed nucleation studies under pure neutral conditions with zero ions being present in the chamber and at ionization rates of up to 75 ion pairs cm−3 s−1 to study neutral and ion-induced nucleation. We found that the contribution from ion-induced nucleation is small at temperatures between 208 and 248 K when ammonia is present at several pptv or higher. However, the presence of charges significantly enhances the nucleation rates, especially at 248 K with zero added ammonia, and for higher temperatures independent of NH3 levels. We compare these experimental data with calculated cluster formation rates from the Atmospheric Cluster Dynamics Code with cluster evaporation rates obtained from quantum chemistry.

2018, Pandolfi, Marco, Alados-Arboledas, Lucas, Alastuey, Andrés, Andrade, Marcos, Angelov, Christo, Artiñano, Begoña, Backman, John, Baltensperger, Urs, Bonasoni, Paolo, Bukowiecki, Nicolas, Collaud Coen, Martine, Conil, Sébastien, Coz, Esther, Crenn, Vincent, Dudoitis, Vadimas, Ealo, Marina, Eleftheriadis, Kostas, Favez, Olivier, Fetfatzis, Prodromos, Fiebig, Markus, Flentje, Harald, Ginot, Patrick, Gysel, Martin, Henzing, Bas, Hoffer, Andras, Holubova Smejkalova, Adela, Kalapov, Ivo, Kalivitis, Nikos, Kouvarakis, Giorgos, Kristensson, Adam, Kulmala, Markku, Lihavainen, Heikki, Lunder, Chris, Luoma, Krista, Lyamani, Hassan, Marinoni, Angela, Mihalopoulos, Nikos, Moerman, Marcel, Nicolas, José, O'Dowd, Colin, Petäjä, Tuukka, Petit, Jean-Eudes, Pichon, Jean Marc, Prokopciuk, Nina, Putaud, Jean-Philippe, Rodríguez, Sergio, Sciare, Jean, Sellegri, Karine, Swietlicki, Erik, Titos, Gloria, Tuch, Thomas, Tunved, Peter, Ulevicius, Vidmantas, Vaishya, Aditya, Vana, Milan, Virkkula, Aki, Vratolis, Stergios, Weingartner, Ernest, Wiedensohler, Alfred, Laj, Paolo

This paper presents the light-scattering properties of atmospheric aerosol particles measured over the past decade at 28 ACTRIS observatories, which are located mainly in Europe. The data include particle light scattering (σsp) and hemispheric backscattering (σbsp) coefficients, scattering Ångström exponent (SAE), backscatter fraction (BF) and asymmetry parameter (g). An increasing gradient of σsp is observed when moving from remote environments (arctic/mountain) to regional and to urban environments. At a regional level in Europe, σsp also increases when moving from Nordic and Baltic countries and from western Europe to central/eastern Europe, whereas no clear spatial gradient is observed for other station environments. The SAE does not show a clear gradient as a function of the placement of the station. However, a west-to-east-increasing gradient is observed for both regional and mountain placements, suggesting a lower fraction of fine-mode particle in western/south-western Europe compared to central and eastern Europe, where the fine-mode particles dominate the scattering. The g does not show any clear gradient by station placement or geographical location reflecting the complex relationship of this parameter with the physical properties of the aerosol particles. Both the station placement and the geographical location are important factors affecting the intraannual variability. At mountain sites, higher σsp and SAE values are measured in the summer due to the enhanced boundary layer influence and/or new particle-formation episodes. Conversely, the lower horizontal and vertical dispersion during winter leads to higher σsp values at all low-altitude sites in central and eastern Europe compared to summer. These sites also show SAE maxima in the summer (with corresponding g minima). At all sites, both SAE and g show a strong variation with aerosol particle loading. The lowest values of g are always observed together with low σsp values, indicating a larger contribution from particles in the smaller accumulation mode. During periods of high σsp values, the variation of g is less pronounced, whereas the SAE increases or decreases, suggesting changes mostly in the coarse aerosol particle mode rather than in the fine mode. Statistically significant decreasing trends of σsp are observed at 5 out of the 13 stations included in the trend analyses. The total reductions of σsp are consistent with those reported for PM2.5 and PM10 mass concentrations over similar periods across Europe.

2020, Petäjä, Tuukka, Duplissy, Ella-Maria, Tabakova, Ksenia, Schmale, Julia, Altstädter, Barbara, Ancellet, Gerard, Arshinov, Mikhail, Balin, Yurii, Baltensperger, Urs, Bange, Jens, Beamish, Alison, Belan, Boris, Berchet, Antoine, Bossi, Rossana, Cairns, Warren R.L., Ebinghaus, Ralf, El Haddad, Imad, Ferreira-Araujo, Beatriz, Franck, Anna, Huang, Lin, Hyvärinen, Antti, Humbert, Angelika, Kalogridis, Athina-Cerise, Konstantinov, Pavel, Lampert, Astrid, MacLeod, Matthew, Magand, Olivier, Mahura, Alexander, Marelle, Louis, Masloboev, Vladimir, Moisseev, Dmitri, Moschos, Vaios, Neckel, Niklas, Onishi, Tatsuo, Osterwalder, Stefan, Ovaska, Aino, Paasonen, Pauli, Panchenko, Mikhail, Pankratov, Fidel, Pernov, Jakob B., Platis, Andreas, Popovicheva, Olga, Raut, Jean-Christophe, Riandet, Aurélie, Sachs, Torsten, Salvatori, Rosamaria, Salzano, Roberto, Schröder, Ludwig, Schön, Martin, Shevchenko, Vladimir, Skov, Henrik, Sonke, Jeroen E., Spolaor, Andrea, Stathopoulos, Vasileios K., Strahlendorff, Mikko, Thomas, Jennie L., Vitale, Vito, Vratolis, Sterios, Barbante, Carlo, Chabrillat, Sabine, Dommergue, Aurélien, Eleftheriadis, Konstantinos, Heilimo, Jyri, Law, Kathy S., Massling, Andreas, Noe, Steffen M., Paris, Jean-Daniel, Prévôt, André S.H., Riipinen, Ilona, Wehner, Birgit, Xie, Zhiyong, Lappalainen, Hanna K.

The role of polar regions is increasing in terms of megatrends such as globalization, new transport routes, demography, and the use of natural resources with consequent effects on regional and transported pollutant concentrations. We set up the ERA-PLANET Strand 4 project “iCUPE – integrative and Comprehensive Understanding on Polar Environments” to provide novel insights and observational data on global grand challenges with an Arctic focus. We utilize an integrated approach combining in situ observations, satellite remote sensing Earth observations (EOs), and multi-scale modeling to synthesize data from comprehensive long-term measurements, intensive campaigns, and satellites to deliver data products, metrics, and indicators to stakeholders concerning the environmental status, availability, and extraction of natural resources in the polar areas. The iCUPE work consists of thematic state-of-the-art research and the provision of novel data in atmospheric pollution, local sources and transboundary transport, the characterization of arctic surfaces and their changes, an assessment of the concentrations and impacts of heavy metals and persistent organic pollutants and their cycling, the quantification of emissions from natural resource extraction, and the validation and optimization of satellite Earth observation (EO) data streams. In this paper we introduce the iCUPE project and summarize initial results arising out of the integration of comprehensive in situ observations, satellite remote sensing, and multi-scale modeling in the Arctic context.

2018, Nieminen, Tuomo, Kerminen, Veli-Matti, Petäjä, Tuukka, Aalto, Pasi P., Arshinov, Mikhail, Asmi, Eija, Baltensperger, Urs, Beddows, David C. S., Beukes, Johan Paul, Collins, Don, Ding, Aijun, Harrison, Roy M., Henzing, Bas, Hooda, Rakesh, Hu, Min, Hõrrak, Urmas, Kivekäs, Niku, Komsaare, Kaupo, Krejci, Radovan, Kristensson, Adam, Laakso, Lauri, Laaksonen, Ari, Leaitch, W. Richard, Lihavainen, Heikki, Mihalopoulos, Nikolaos, Németh, Zoltán, Nie, Wei, O'Dowd, Colin, Salma, Imre, Sellegri, Karine, Svenningsson, Birgitta, Swietlicki, Erik, Tunved, Peter, Ulevicius, Vidmantas, Vakkari, Ville, Vana, Marko, Wiedensohler, Alfred, Wu, Zhijun, Virtanen, Annele, Kulmala, Markku

Atmospheric new particle formation (NPF) is an important phenomenon in terms of global particle number concentrations. Here we investigated the frequency of NPF, formation rates of 10 nm particles, and growth rates in the size range of 10–25 nm using at least 1 year of aerosol number size-distribution observations at 36 different locations around the world. The majority of these measurement sites are in the Northern Hemisphere. We found that the NPF frequency has a strong seasonal variability. At the measurement sites analyzed in this study, NPF occurs most frequently in March–May (on about 30 % of the days) and least frequently in December-February (about 10 % of the days). The median formation rate of 10 nm particles varies by about 3 orders of magnitude (0.01–10 cm−3 s−1) and the growth rate by about an order of magnitude (1–10 nm h−1). The smallest values of both formation and growth rates were observed at polar sites and the largest ones in urban environments or anthropogenically influenced rural sites. The correlation between the NPF event frequency and the particle formation and growth rate was at best moderate among the different measurement sites, as well as among the sites belonging to a certain environmental regime. For a better understanding of atmospheric NPF and its regional importance, we would need more observational data from different urban areas in practically all parts of the world, from additional remote and rural locations in North America, Asia, and most of the Southern Hemisphere (especially Australia), from polar areas, and from at least a few locations over the oceans.

2016, Lehtipalo, Katrianne, Rondo, Linda, Kontkanen, Jenni, Schobesberger, Siegfried, Jokinen, Tuija, Sarnela, Nina, Kürten, Andreas, Ehrhart, Sebastian, Franchin, Alessandro, Nieminen, Tuomo, Kulmala, Markku, Riccobono, Francesco, Sipila, Mikko, Yli-Juuti, Taina, Duplissy, Jonathan, Adamov, Alexey, Ahlm, Lars, Almeida, Joa˜o, Amorim, Antonio, Bianchi, Federico, Breitenlechner, Martin, Dommen, Josef, Downard, Andrew J., Dunne, Eimear M., Flagan, Richard C., Guida, Roberto, Hakala, Jani, Hansel, Armin, Jud, Werner, Kangasluoma, Juha, Kerminen, Veli-Matti, Keskinen, Helmi, Kim, Jaeseok, Kirkby, Jasper, Kupc, Agnieszka, Kupiainen-Määttä, Oona, Laaksonen, Ari, Lawler, Michael J., Leiminger, Markus, Mathot, Serge, Olenius, Tinja, Ortega, Ismael K., Onnela, Antti, Petäjä, Tuukka, Praplan, Arnaud, Rissanen, Matti P., Ruuskanen, Taina, Santos, Filipe D., Schallhart, Simon, Schnitzhofer, Ralf, Simon, Mario, Smith, James N., Tröstl, Jasmin, Tsagkogeorgas, Georgios, Tomé, António, Vaattovaara, Petri, Vehkamäki, Hanna, Vrtala, Aron E., Wagner, Paul E., Williamson, Christina, Wimmer, Daniela, Winkler, Paul M., Virtanen, Annele, Donahue, Neil M., Carslaw, Kenneth S., Baltensperger, Urs, Riipinen, Ilona, Curtius, Joachim, Worsnop, Douglas R., Kulmala, Markku

The growth of freshly formed aerosol particles can be the bottleneck in their survival to cloud condensation nuclei. It is therefore crucial to understand how particles grow in the atmosphere. Insufficient experimental data has impeded a profound understanding of nano-particle growth under atmospheric conditions. Here we study nano-particle growth in the CLOUD (Cosmics Leaving OUtdoors Droplets) chamber, starting from the formation of molecular clusters. We present measured growth rates at sub-3 nm sizes with different atmospherically relevant concentrations of sulphuric acid, water, ammonia and dimethylamine. We find that atmospheric ions and small acid-base clusters, which are not generally accounted for in the measurement of sulphuric acid vapour, can participate in the growth process, leading to enhanced growth rates. The availability of compounds capable of stabilizing sulphuric acid clusters governs the magnitude of these effects and thus the exact growth mechanism. We bring these observations into a coherent framework and discuss their significance in the atmosphere.

2022, Shen, Jiali, Scholz, Wiebke, He, Xu-Cheng, Zhou, Putian, Marie, Guillaume, Wang, Mingyi, Marten, Ruby, Surdu, Mihnea, Rörup, Birte, Baalbaki, Rima, Amorim, Antonio, Ataei, Farnoush, Bell, David M., Bertozzi, Barbara, Brasseur, Zoé, Caudillo, Lucía, Chen, Dexian, Chu, Biwu, Dada, Lubna, Duplissy, Jonathan, Finkenzeller, Henning, Granzin, Manuel, Guida, Roberto, Heinritzi, Martin, Hofbauer, Victoria, Iyer, Siddharth, Kemppainen, Deniz, Kong, Weimeng, Krechmer, Jordan E., Kürten, Andreas, Lamkaddam, Houssni, Lee, Chuan Ping, Lopez, Brandon, Mahfouz, Naser G. A., Manninen, Hanna E., Massabò, Dario, Mauldin, Roy L., Mentler, Bernhard, Müller, Tatjana, Pfeifer, Joschka, Philippov, Maxim, Piedehierro, Ana A., Roldin, Pontus, Schobesberger, Siegfried, Simon, Mario, Stolzenburg, Dominik, Tham, Yee Jun, Tomé, António, Umo, Nsikanabasi Silas, Wang, Dongyu, Wang, Yonghong, Weber, Stefan K., Welti, André, Wollesen de Jonge, Robin, Wu, Yusheng, Zauner-Wieczorek, Marcel, Zust, Felix, Baltensperger, Urs, Curtius, Joachim, Flagan, Richard C., Hansel, Armin, Möhler, Ottmar, Petäjä, Tuukka, Volkamer, Rainer, Kulmala, Markku, Lehtipalo, Katrianne, Rissanen, Matti, Kirkby, Jasper, El-Haddad, Imad, Bianchi, Federico, Sipilä, Mikko, Donahue, Neil M., Worsnop, Douglas R.

Dimethyl sulfide (DMS) influences climate via cloud condensation nuclei (CCN) formation resulting from its oxidation products (mainly methanesulfonic acid, MSA, and sulfuric acid, H2SO4). Despite their importance, accurate prediction of MSA and H2SO4from DMS oxidation remains challenging. With comprehensive experiments carried out in the Cosmics Leaving Outdoor Droplets (CLOUD) chamber at CERN, we show that decreasing the temperature from +25 to -10 °C enhances the gas-phase MSA production by an order of magnitude from OH-initiated DMS oxidation, while H2SO4production is modestly affected. This leads to a gas-phase H2SO4-to-MSA ratio (H2SO4/MSA) smaller than one at low temperatures, consistent with field observations in polar regions. With an updated DMS oxidation mechanism, we find that methanesulfinic acid, CH3S(O)OH, MSIA, forms large amounts of MSA. Overall, our results reveal that MSA yields are a factor of 2-10 higher than those predicted by the widely used Master Chemical Mechanism (MCMv3.3.1), and the NOxeffect is less significant than that of temperature. Our updated mechanism explains the high MSA production rates observed in field observations, especially at low temperatures, thus, substantiating the greater importance of MSA in the natural sulfur cycle and natural CCN formation. Our mechanism will improve the interpretation of present-day and historical gas-phase H2SO4/MSA measurements.

2017, Tsagkogeorgas, Georgios, Roldin, Pontus, Duplissy, Jonathan, Rondo, Linda, Tröstl, Jasmin, Slowik, Jay G., Ehrhart, Sebastian, Franchin, Alessandro, Kürten, Andreas, Amorim, Antonio, Bianchi, Federico, Kirkby, Jasper, Petäjä, Tuukka, Baltensperger, Urs, Boy, Michael, Curtius, Joachim, Flagan, Richard C., Kulmala, Markku, Donahue, Neil M., Stratmann, Frank

Evaporation of sulfuric acid from particles can be important in the atmospheres of Earth and Venus. However, the equilibrium constant for the dissociation of H2SO4 to bisulfate ions, which is the one of the fundamental parameters controlling the evaporation of sulfur particles, is not well constrained. In this study we explore the volatility of sulfate particles at very low relative humidity. We measured the evaporation of sulfur particles versus temperature and relative humidity in the CLOUD chamber at CERN. We modelled the observed sulfur particle shrinkage with the ADCHAM model. Based on our model results, we conclude that the sulfur particle shrinkage is mainly governed by H2SO4 and potentially to some extent by SO3 evaporation. We found that the equilibrium constants for the dissociation of H2SO4 to HSO4-(KH2SO4) and the dehydration of H2SO4 to SO3 (KSO3) are KH2SO4 Combining double low line 2-4 × 109 kg-1 and KSO3 ≥ 1.4 × g 1010 at 288.8± 5K.

2016, Kirkby, Jasper, Duplissy, Jonathan, Sengupta, Kamalika, Gordon, Hamish, Williamson, Christina, Heinritzi, Martin, Simon, Mario, Yan, Chao, Almeida, João, Tröstl, Jasmin, Nieminen, Tuomo, Ortega, Ismael K., Wagner, Robert, Adamov, Alexey, Amorim, Antonio, Bernhammer, Anne-Kathrin, Bianchi, Federico, Breitenlechner, Martin, Brilke, Sophia, Chen, Xuemeng, Craven, Jill, Dias, Antonio, Ehrhart, Sebastian, Flagan, Richard C., Franchin, Alessandro, Fuchs, Claudia, Guida, Roberto, Hakala, Jani, Hoyle, Christopher R., Jokinen, Tuija, Junninen, Heikki, Kangasluoma, Juha, Kim, Jaeseok, Krapf, Manuel, Kürten, Andreas, Laaksonen, Ari, Lehtipalo, Katrianne, Makhmutov, Vladimir, Mathot, Serge, Molteni, Ugo, Onnela, Antti, Peräkylä, Otso, Piel, Felix, Petäjä, Tuukka, Praplan, Arnaud P., Pringle, Kirsty, Rap, Alexandru, Richards, Nigel A.D., Riipinen, Ilona, Rissanen, Matti P., Rondo, Linda, Sarnela, Nina, Schobesberger, Siegfried, Scott, Catherine E., Seinfeld, John H., Sipilä, Mikko, Steiner, Gerhard, Stozhkov, Yuri, Stratmann, Frank, Tomé, Antonio, Virtanen, Annele, Vogel, Alexander L., Wagner, Andrea C., Wagner, Paul E., Weingartner, Ernest, Wimmer, Daniela, Winkler, Paul M., Ye, Penglin, Zhang, Xuan, Hansel, Armin, Dommen, Josef, Donahue, Neil M., Worsnop, Douglas R., Baltensperger, Urs, Kulmala, Markku, Carslaw, Kenneth S., Curtius, Joachim

Atmospheric aerosols and their effect on clouds are thought to be important for anthropogenic radiative forcing of the climate, yet remain poorly understood1. Globally, around half of cloud condensation nuclei originate from nucleation of atmospheric vapours2. It is thought that sulfuric acid is essential to initiate most particle formation in the atmosphere3,4, and that ions have a relatively minor role5. Some laboratory studies, however, have reported organic particle formation without the intentional addition of sulfuric acid, although contamination could not be excluded6,7. Here we present evidence for the formation of aerosol particles from highly oxidized biogenic vapours in the absence of sulfuric acid in a large chamber under atmospheric conditions. The highly oxygenated molecules (HOMs) are produced by ozonolysis of α-pinene. We find that ions from Galactic cosmic rays increase the nucleation rate by one to two orders of magnitude compared with neutral nucleation. Our experimental findings are supported by quantum chemical calculations of the cluster binding energies of representative HOMs. Ion-induced nucleation of pure organic particles constitutes a potentially widespread source of aerosol particles in terrestrial environments with low sulfuric acid pollution.

2016, Järvinen, Emma, Ignatius, Karoliina, Nichman, Leonid, Kristensen, Thomas B., Fuchs, Claudia, Hoyle, Christopher R., Höppel, Niko, Corbin, Joel C., Craven, Jill, Duplissy, Jonathan, Ehrhart, Sebastian, El Haddad, Imad, Frege, Carla, Gordon, Hamish, Jokinen, Tuija, Kallinger, Peter, Kirkby, Jasper, Kiselev, Alexei, Naumann, Karl-Heinz, Petäjä, Tuukka, Pinterich, Tamara, Prevot, Andre S.H., Saathoff, Harald, Schiebel, Thea, Sengupta, Kamalika, Simon, Mario, Slowik, Jay G., Tröstl, Jasmin, Virtanen, Annele, Vochezer, Paul, Vogt, Steffen, Wagner, Andrea C., Wagner, Robert, Williamson, Christina, Winkler, Paul M., Yan, Chao, Baltensperger, Urs, Donahue, Neil M., Flagan, Rick C., Gallagher, Martin, Hansel, Armin, Kulmala, Markku, Stratmann, Frank, Worsnop, Douglas R., Möhler, Ottmar, Leisner, Thomas, Schnaiter, Martin

Under certain conditions, secondary organic aerosol (SOA) particles can exist in the atmosphere in an amorphous solid or semi-solid state. To determine their relevance to processes such as ice nucleation or chemistry occurring within particles requires knowledge of the temperature and relative humidity (RH) range for SOA to exist in these states. In the Cosmics Leaving Outdoor Droplets (CLOUD) experiment at The European Organisation for Nuclear Research (CERN), we deployed a new in situ optical method to detect the viscous state of α-pinene SOA particles and measured their transition from the amorphous highly viscous state to states of lower viscosity. The method is based on the depolarising properties of laboratory-produced non-spherical SOA particles and their transformation to non-depolarising spherical particles at relative humidities near the deliquescence point. We found that particles formed and grown in the chamber developed an asymmetric shape through coagulation. A transition to a spherical shape was observed as the RH was increased to between 35 % at −10 °C and 80 % at −38 °C, confirming previous calculations of the viscosity-transition conditions. Consequently, α-pinene SOA particles exist in a viscous state over a wide range of ambient conditions, including the cirrus region of the free troposphere. This has implications for the physical, chemical, and ice-nucleation properties of SOA and SOA-coated particles in the atmosphere.