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    In situ aerosol characterization at Cape Verde, Part 2: Parametrization of relative humidity- and wavelength-dependent aerosol optical properties
    (Milton Park : Taylor & Francis, 2017) Schladitz, Alexander; Müller, Thomas; Nordmann, Stephan; Tesche, Matthias; Silke Groß, Silke Groß; Freudenthaler, Volker; Gasteiger, Josef; Wiedensohler, Alfred
    An observation-based numerical study of humidity-dependent aerosol optical properties of mixed marine and Saharan mineral dust aerosol is presented. An aerosol model was developed based on measured optical and microphysical properties to describe the marine and Saharan dust aerosol at Cape Verde. A wavelength-dependent optical equivalent imaginary part of the refractive index and a scattering non-sphericity factor for Saharan dustwere derived. Simulations of humidity effects on optical properties by the aerosol model were validated with relative measurements of the extinction coefficient at ambient conditions. Parametrizations were derived to describe the humidity dependence of the extinction, scattering, and absorption coefficients as well as the asymmetry parameter and single scattering albedo. For wavelengths (300–950 nm) and dry dust volume fractions (0–1), aerosol optical properties as a function of relative humidity (RH = 0–90%) can be calculated from tabulated parameters. For instance, at a wavelength of 550 nm, a volume fraction of 0.5 of dust on the total particle volume (dry conditions) and a RH of 90%, the enhancements for the scattering, extinction and absorption coefficients are 2.55, 2.46 and 1.04, respectively, while the enhancements for the asymmetry parameter and single scattering albedo are 1.11 and 1.04.
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    Cloud condensation nuclei spectra derived from size distributions and hygroscopic properties of the aerosol in coastal south-west Portugal during ACE-2
    (Milton Park : Taylor & Francis, 2016) Dusek, Ulrike; Covert, David S.; Wiedensohler, Alfred; Neusüss, Christian; Weise, Diana; Cantrell, Will
    In this work we propose and test a method to calculate cloud condensation nuclei (CCN) spectra basedon aerosol number size distributions and hygroscopic growth factors. Sensitivity studies show thatthis method can be used in a wide variety of conditions except when the aerosol consist mainly oforganic compounds. One crucial step in the calculations, estimating soluble ions in an aerosol particlebased on hygroscopic growth factors, is tested in an internal hygroscopic consistency study. The resultsshow that during the second Aerosol Characterization Experiment (ACE-2) the number concentrationof inorganic ions analyzed in impactor samples could be reproduced from measured growth factorswithin the measurement uncertainties at the measurement site in Sagres, Portugal. CCN spectra were calculated based on data from the ACE-2 field experiment at the Sagres site.The calculations overestimate measured CCN spectra on average by approximately 30%, which iscomparable to the uncertainties in measurements and calculations at supersaturations below 0.5%. Thecalculated CCN spectra were averaged over time periods when Sagres received clean air masses and airmasses influenced by aged and recent pollution. Pollution outbreaks enhance the CCN concentrationsat supersaturations near 0.2% by a factor of 3 (aged pollution) to 5 (recent pollution) compared to theclean marine background concentrations. In polluted air masses, the shape of the CCN spectra changes.The clean spectra can be approximated by a power function, whereas the polluted spectra are betterapproximated by an error function.
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    Understanding aerosol microphysical properties from 10 years of data collected at Cabo Verde based on an unsupervised machine learning classification
    (Katlenburg-Lindau : EGU, 2022) Gong, Xianda; Wex, Heike; Müller, Thomas; Henning, Silvia; Voigtländer, Jens; Wiedensohler, Alfred; Stratmann, Frank
    The Cape Verde Atmospheric Observatory (CVAO), which is influenced by both marine and desert dust air masses, has been used for long-term measurements of different properties of the atmospheric aerosol from 2008 to 2017. These properties include particle number size distributions (PNSD), light-absorbing carbon (LAC) and concentrations of cloud condensation nuclei (CCN) together with their hygroscopicity. Here we summarize the results obtained for these properties and use an unsupervised machine learning algorithm for the classification of aerosol types. Five types of aerosols, i.e., marine, freshly formed, mixture, moderate dust and heavy dust, were classified. Air masses during marine periods are from the Atlantic Ocean and during dust periods are from the Sahara Desert. Heavy dust was more frequently present during wintertime, whereas the clean marine periods were more frequently present during springtime. It was observed that during the dust periods CCN number concentrations at a supersaturation of 0.30g% were roughly 2.5 times higher than during marine periods, but the hygroscopicity (κ) of particles in the size range from g1/4g30 to g1/4g175gnm during marine and dust periods were comparable. The long-term data presented here, together with the aerosol classification, can be used as a basis to improve our understanding of annual cycles of the atmospheric aerosol in the eastern tropical Atlantic Ocean and on aerosol-cloud interactions and it can be used as a basis for driving, evaluating and constraining atmospheric model simulations.
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    Aerosol physical properties and processes in the lower marine boundary layer: A comparison of shipboard sub-micron data from ACE-1 and ACE-2
    (Milton Park : Taylor & Francis, 2016) Bates, Timothy S.; Quinn, Patricia K.; Covert, David S.; Coffman, Derek J.; Johnson, James E.; Wiedensohler, Alfred
    The goals of the IGAC Aerosol Characterization Experiments (ACE) are to determine and understand the properties and controlling processes of the aerosol in a globally representative range of natural and anthropogenically perturbed environments. ACE-1 was conducted in the remote marine atmosphere south of Australia while ACE-2 was conducted in the anthropogenically modified atmosphere of the Eastern North Atlantic. In-situ shipboard measurements from the RV Discoverer(ACE-1) and the RV Professor Vodyanitskiy(ACE-2), combined with calculated back trajectories can be used to define the physical properties of the sub-micron aerosol in marine boundary layer (MBL) air masses from the remote Southern Ocean, Western Europe, the Iberian coast, the Mediterranean and the background Atlantic Ocean. The differences in these aerosol properties, combined with dimethylsulfide, sulfur dioxide and meteorological measurements provide a means to assess processes that affect the aerosol distribution. The background sub-micron aerosol measured over the Atlantic Ocean during ACE-2 was more abundant (number and volume) and appeared to be more aged than that measured over the Southern Ocean during ACE-1. Based on seawater DMS measurements and wind speed, the oceanic source of non-sea-salt sulfur and sea-salt to the background marine atmosphere during ACE-1 and ACE-2 was similar. However, the synoptic meteorological pattern was quite different during ACE-1 and ACE-2. The frequent frontal passages during ACE-1 resulted in the mixing of nucleation mode particles into the marine boundary layer from the free troposphere and relatively short aerosol residence times. In the more stable meteorological setting of ACE-2, a significant nucleation mode aerosol was observed in the MBL only for a half day period associated with a weak frontal system. As a result of the longer MBL aerosol residence times, the average background ACE-2 accumulation mode aerosol had a larger diameter and higher number concentration than during ACE-1. The sub-micron aerosol number size distributions in the air masses that passed over Western Europe, the Mediterranean, and coastal Portugal were distinctly different from each other and the background aerosol. The differences can be attributed to the age of the air mass and the degree of cloud processing.
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    A European aerosol phenomenology - 6: Scattering properties of atmospheric aerosol particles from 28 ACTRIS sites
    (Katlenburg-Lindau : EGU, 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.
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    Biomass burning and urban emission impacts in the Andes Cordillera region based on in situ measurements from the Chacaltaya observatory, Bolivia (5240a.s.l.)
    (Katlenburg-Lindau : EGU, 2019) Chauvigné, Aurélien; Aliaga, Diego; Sellegri, Karine; Montoux, Nadège; Krejci, Radovan; Močnik, Griša; Moreno, Isabel; Müller, Thomas; Pandolfi, Marco; Velarde, Fernando; Weinhold, Kay; Ginot, Patrick; Wiedensohler, Alfred; Andrade, Marcos; Laj, Paolo
    This study documents and analyses a 4-year continuous record of aerosol optical properties measured at the Global Atmosphere Watch (GAW) station of Chacaltaya (CHC; 5240a.s.l.), in Bolivia. Records of particle light scattering and particle light absorption coefficients are used to investigate how the high Andean Cordillera is affected by both long-range transport and by the fast-growing agglomeration of La Paz-El Alto, located approximately 20km away and 1.5km below the sampling site. The extended multi-year record allows us to study the properties of aerosol particles for different air mass types, during wet and dry seasons, also covering periods when the site was affected by biomass burning in the Bolivian lowlands and the Amazon Basin. The absorption, scattering, and extinction coefficients (median annual values of 0.74, 12.14, and 12.96Mm-1 respectively) show a clear seasonal variation with low values during the wet season (0.57, 7.94, and 8.68Mm-1 respectively) and higher values during the dry season (0.80, 11.23, and 14.51Mm-1 respectively). The record is driven by variability at both seasonal and diurnal scales. At a diurnal scale, all records of intensive and extensive aerosol properties show a pronounced variation (daytime maximum, night-time minimum), as a result of the dynamic and convective effects. The particle light absorption, scattering, and extinction coefficients are on average 1.94, 1.49, and 1.55 times higher respectively in the turbulent thermally driven conditions than the more stable conditions, due to more efficient transport from the boundary layer. Retrieved intensive optical properties are significantly different from one season to the other, reflecting the changing aerosol emission sources of aerosol at a larger scale. Using the wavelength dependence of aerosol particle optical properties, we discriminated between contributions from natural (mainly mineral dust) and anthropogenic (mainly biomass burning and urban transport or industries) emissions according to seasons and local circulation. The main sources influencing measurements at CHC are from the urban area of La Paz-El Alto in the Altiplano and from regional biomass burning in the Amazon Basin. Results show a 28% to 80% increase in the extinction coefficients during the biomass burning season with respect to the dry season, which is observed in both tropospheric dynamic conditions. From this analysis, long-term observations at CHC provide the first direct evidence of the impact of biomass burning emissions of the Amazon Basin and urban emissions from the La Paz area on atmospheric optical properties at a remote site all the way to the free troposphere. © Author(s) 2019.
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    Characterization of the planetary boundary layer during SAMUM-2 by means of lidar measurements
    (Milton Park : Taylor & Francis, 2017) Groß, Silke; Gasteiger, Josef; Freudenthaler, Volker; Wiegner, Matthias; Geiß, Alexander; Schladitz, Alexander; Toledano, Carlos; Kandler, Konrad; Tesche, Matthias; Ansmann, Albert; Wiedensohler, Alfred
    Measurements with two Raman-depolarization lidars of the Meteorological Institute of the Ludwig-Maximilians- Universit¨at, M¨unchen, Germany, performed during SAMUM-2, were used to characterize the planetary boundary layer (PBL) over Praia, Cape Verde. A novel approach was used to determine the volume fraction of dust υd in the PBL. This approach primarily relies on accurate measurements of the linear depolarization ratio. Comparisons with independent in situ measurements showed the reliability of this approach. Based on our retrievals, two different phases could be distinguished within the measurement period of almost one month. The first (22–31 January 2008) was characterized by high aerosol optical depth (AOD) in the PBL and large υd > 95%. During the second phase, the AOD in the PBL was considerably lower and υd less than ∼40%. These findings were in very good agreement with ground based in situ measurements, when ambient volume fractions are considered that were calculated from the actual measurements of the dry volume fraction. Only in cases when dust was not the dominating aerosol component (second phase), effects due to hygroscopic growth became important.
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    Regional modelling of Saharan dust and biomass-burning smoke, Part I: Model description and evaluation
    (Milton Park : Taylor & Francis, 2017) Heinold, Bernd; Tegen, Ina; Schepanski, Kerstin; Tesche, Matthias; Esselborn, Michael; Freudenthaler, Volker; Gross, Silke; Kandler, Konrad; Knippertz, Peter; Müller, Detlef; Schladitz, Alexander; Toledano, Carlos; Weinzierl, Bernadett; Ansmann, Albert; Althausen, Dietrich; Müller, Thomas; Petzold, Andreas; Wiedensohler, Alfred
    The spatio-temporal evolution of the Saharan dust and biomass-burning plume during the SAMUM-2 field campaign in January and February 2008 is simulated at 28 km horizontal resolution with the regional model-system COSMOMUSCAT. The model performance is thoroughly tested using routine ground-based and space-borne remote sensing and local field measurements. Good agreement with the observations is found in many cases regarding transport patterns, aerosol optical thicknesses and the ratio of dust to smoke aerosol. The model also captures major features of the complex aerosol layering. Nevertheless, discrepancies in the modelled aerosol distribution occur, which are analysed in detail. The dry synoptic dynamics controlling dust uplift and transport during the dry season are well described by the model, but surface wind peaks associated with the breakdown of nocturnal low-level jets are not always reproduced. Thus, a strong dust outbreak is underestimated. While dust emission modelling is a priori more challenging, since strength and placement of dust sources depend on on-line computed winds, considerable inaccuracies also arise in observation-based estimates of biomass-burning emissions. They are caused by cloud and spatial errors of satellite fire products and uncertainties in fire emission parameters, and can lead to unrealistic model results of smoke transport.
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    Hygroscopic properties of aerosol particles in the northeastern Atlantic during ACE-2
    (Milton Park : Taylor & Francis, 2016) Swietlicki, Erik; Zhou, Jingchuan; Covert, David S.; Hämeri, Kaarle; Busch, Bernhard; Väkeva, Minna; Dusek, Ulrike; Berg, Olle H.; Wiedensohler, Alfred; Aalto, Pasi; Mäkelä, Jyrki; Martinsson, Bengt G.; Papaspiropoulos, Giorgos; Mentes, Besim; Frank, Göran; Stratmann, Frank
    Measurements of the hygroscopic properties of sub-micrometer atmospheric aerosol particles were performed with hygroscopic tandem differential mobility analysers (H-TDMA) at 5 sites in the subtropical north-eastern Atlantic during the second Aerosol Characterization Experiment (ACE-2) from 16 June to 25 July 1997. Four of the sites were in the marine boundary layer and one was, at least occasionally, in the lower free troposphere. The hygroscopic diameter growth factors of individual aerosol particles in the dry particle diameter range 10−440 nm were generally measured for changes in relative humidity (RH) from <10% to 90%. In the marine boundary layer, growth factors at 90% RH were dependent on location, air mass type and particle size. The data was dominated by a unimodal growth distribution of more-hygroscopic particles, although a bimodal growth distribution including less-hygroscopic particles was observed at times, most often in the more polluted air masses. In clean marine air masses the more-hygroscopic growth factors ranged from about 1.6 to 1.8 with a consistent increase in growth factor with increasing particle size. There was also a tendency toward higher growth factors as sodium to sulphate molar ratio increased with increasing sea-salt contribution at higher wind speeds. During outbreaks of European pollution in the ACE-2 region, the growth factors of the largest particles were reduced, but only slightly. Growth factors at all sizes in both clean and polluted air masses were markedly lower at the Sagres, Portugal site due to more proximate continental influences. The frequency of occurrence of less-hygroscopic particles with a growth factor of ca. 1.15 was greatest during polluted conditions at Sagres. The free tropospheric 50 nm particles were predominately less-hygroscopic, with an intermediate growth factor of 1.4, but more-hygroscopic particles with growth factors of about 1.6 were also frequent. While these particles probably originate from within the marine boundary layer, the less-hygroscopic particles are probably more characteristic of lower free tropospheric air masses. For those occasions when measurements were made at 90% and an intermediate 60% or 70% RH, the growth factor G(RH) of the more-hygroscopic particles could be modelled empirically by a power law expression. For the ubiquitous more-hygroscopic particles, the expressions G(RH)=(1-RH/100)-0.210 for 50 nm Aitken mode particles and G(RH)=(1-RH/100)-0.233 for 166 nm accumulation mode particles are recommended for clean marine air masses in the north-eastern Atlantic within the range 0
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    Vertical profiling of aerosol hygroscopic properties in the planetary boundary layer during the PEGASOS campaigns
    (München : European Geopyhsical Union, 2016) Rosati, Bernadette; Gysel, Martin; Rubach, Florian; Mentel, Thomas F.; Goger, Brigitta; Poulain, Laurent; Schlag, Patrick; Miettinen, Pasi; Pajunoja, Aki; Virtanen, Annele; Baltink, Henk Klein; Henzing, J.S. Bas; Größ, Johannes; Gobbi, Gian Paolo; Wiedensohler, Alfred; Kiendler-Scharr, Astrid; Decesari, Stefano; Facchini, Maria Cristina; Weingartner, Ernest; Baltensperger, Urs
    Vertical profiles of the aerosol particles hygroscopic properties, their mixing state as well as chemical composition were measured above northern Italy and the Netherlands. An aerosol mass spectrometer (AMS; for chemical composition) and a white-light humidified optical particle spectrometer (WHOPS; for hygroscopic growth) were deployed on a Zeppelin NT airship within the PEGASOS project. This allowed one to investigate the development of the different layers within the planetary boundary layer (PBL), providing a unique in situ data set for airborne aerosol particles properties in the first kilometre of the atmosphere. Profiles measured during the morning hours on 20 June 2012 in the Po Valley, Italy, showed an increased nitrate fraction at  ∼  100 m above ground level (a.g.l.) coupled with enhanced hygroscopic growth compared to  ∼  700 m a. g. l. This result was derived from both measurements of the aerosol composition and direct measurements of the hygroscopicity, yielding hygroscopicity parameters (κ) of 0.34  ±  0.12 and 0.19  ±  0.07 for 500 nm particles, at  ∼  100 and  ∼  700 m a. g. l., respectively. The difference is attributed to the structure of the PBL at this time of day which featured several independent sub-layers with different types of aerosols. Later in the day the vertical structures disappeared due to the mixing of the layers and similar aerosol particle properties were found at all probed altitudes (mean κ ≈ 0.18  ±  0.07). The aerosol properties observed at the lowest flight level (100 m a. g. l.) were consistent with parallel measurements at a ground site, both in the morning and afternoon. Overall, the aerosol particles were found to be externally mixed, with a prevailing hygroscopic fraction. The flights near Cabauw in the Netherlands in the fully mixed PBL did not feature altitude-dependent characteristics. Particles were also externally mixed and had an even larger hygroscopic fraction compared to the results in Italy. The mean κ from direct measurements was 0.28 ±  0.10, thus considerably higher than κ values measured in Italy in the fully mixed PBL.