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- ItemIntercomparison of 15 aerodynamic particle size spectrometers (APS 3321): Uncertainties in particle sizing and number size distribution(München : European Geopyhsical Union, 2016) Pfeifer, Sascha; Müller, Thomas; Weinhold, Kay; Zikova, Nadezda; dos Santos, Sebastiao Martins; Marinoni, Angela; Bischof, Oliver F.; Kykal, Carsten; Ries, Ludwig; Meinhardt, Frank; Aalto, Pasi; Mihalopoulos, Nikolaos; Wiedensohler, AlfredAerodynamic particle size spectrometers are a well-established method to measure number size distributions of coarse mode particles in the atmosphere. Quality assurance is essential for atmospheric observational aerosol networks to obtain comparable results with known uncertainties. In a laboratory study within the framework of ACTRIS (Aerosols, Clouds, and Trace gases Research Infrastructure Network), 15 aerodynamic particle size spectrometers (APS model 3321, TSI Inc., St. Paul, MN, USA) were compared with a focus on flow rates, particle sizing, and the unit-to-unit variability of the particle number size distribution. Flow rate deviations were relatively small (within a few percent), while the sizing accuracy was found to be within 10 % compared to polystyrene latex (PSL) reference particles. The unit-to-unit variability in terms of the particle number size distribution during this study was within 10 % to 20 % for particles in the range of 0.9 up to 3 µm, which is acceptable for atmospheric measurements. For particles smaller than that, the variability increased up to 60 %, probably caused by differences in the counting efficiencies of individual units. Number size distribution data for particles smaller than 0.9 µm in aerodynamic diameter should only be used with caution. For particles larger than 3 µm, the unit-to-unit variability increased as well. A possible reason is an insufficient sizing accuracy in combination with a steeply sloping particle number size distribution and the increasing uncertainty due to decreasing counting. Particularly this uncertainty of the particle number size distribution must be considered if higher moments of the size distribution such as the particle volume or mass are calculated, which require the conversion of the aerodynamic diameter measured to a volume equivalent diameter. In order to perform a quantitative quality assurance, a traceable reference method for the particle number concentration in the size range 0.5–3 µm is needed.
- ItemAn optical particle size spectrometer for aircraft-borne measurements in IAGOS-CARIBIC(München : European Geopyhsical Union, 2016) Hermann, Markus; Weigelt, Andreas; Assmann, Denise; Pfeifer, Sascha; Müller, Thomas; Conrath, Thomas; Voigtländer, Jens; Heintzenberg, Jost; Wiedensohler, Alfred; Martinsson, Bengt G.; Deshler, Terry; Brenninkmeijer, Carl A.M.; Zahn, AndreasThe particle number size distribution is an important parameter to characterize the atmospheric aerosol and its influence on the Earth's climate. Here we describe a new optical particle size spectrometer (OPSS) for measurements of the accumulation mode particle number size distribution in the tropopause region on board a passenger aircraft (IAGOS-CARIBIC observatory: In-service Aircraft for a Global Observing System – Civil Aircraft for Regular Investigation of the Atmosphere Based on an Instrument Container). A modified KS93 particle sensor from RION Co., Ltd., together with a new airflow system and a dedicated data acquisition system, is the key component of the CARIBIC OPSS. The instrument records individual particle pulse signal curves in the particle size range 130–1110 nm diameter (for a particle refractive index of 1.47-i0.006) together with a time stamp and thus allows the post-flight choice of the time resolution and the size distribution bin width. The CARIBIC OPSS has a 50 % particle detection diameter of 152 nm and a maximum asymptotic counting efficiency of 98 %. The instrument's measurement performance shows no pressure dependency and no particle coincidence for free tropospheric conditions. The size response function of the CARIBIC OPSS was obtained by a polystyrene latex calibration in combination with model calculations. Particle number size distributions measured with the new OPSS in the lowermost stratosphere agreed within a factor of 2 in concentration with balloon-borne measurements over western North America. Since June 2010 the CARIBIC OPSS is deployed once per month in the IAGOS-CARIBIC observatory.
- ItemProperties of cloud condensation nuclei (CCN) in the trade wind marine boundary layer of the western North Atlantic(München : European Geopyhsical Union, 2016) Kristensen, Thomas B.; Müller, Thomas; Kandler, Konrad; Benker, Nathalie; Hartmann, Markus; Prospero, Joseph M.; Wiedensohler, Alfred; Stratmann, FrankCloud optical properties in the trade winds over the eastern Caribbean Sea have been shown to be sensitive to cloud condensation nuclei (CCN) concentrations. The objective of the current study was to investigate the CCN properties in the marine boundary layer (MBL) in the tropical western North Atlantic, in order to assess the respective roles of inorganic sulfate, organic species, long-range transported mineral dust and sea-salt particles. Measurements were carried out in June–July 2013, on the east coast of Barbados, and included CCN number concentrations, particle number size distributions and offline analysis of sampled particulate matter (PM) and sampled accumulation mode particles for an investigation of composition and mixing state with transmission electron microscopy (TEM) in combination with energy-dispersive X-ray spectroscopy (EDX). During most of the campaign, significant mass concentrations of long-range transported mineral dust was present in the PM, and influence from local island sources can be ruled out. The CCN and particle number concentrations were similar to what can be expected in pristine marine environments. The hygroscopicity parameter κ was inferred, and values in the range 0.2–0.5 were found during most of the campaign, with similar values for the Aitken and the accumulation mode. The accumulation mode particles studied with TEM were dominated by non-refractory material, and concentrations of mineral dust, sea salt and soot were too small to influence the CCN properties. It is highly likely that the CCN were dominated by a mixture of sulfate species and organic compounds.
- ItemIn situ aerosol characterization at Cape Verde, Part 1: Particle number size distributions, hygroscopic growth and state of mixing of the marine and Saharan dust aerosol(Milton Park : Taylor & Francis, 2017) Schladitz, Alexander; Müller, Thomas; Nowak, Andreas; Kandler, Konrad; Lieke, Kirsten; Massling, Andreas; Wiedensohler, AlfredParticle number size distributions and hygroscopic properties of marine and Saharan dust aerosol were investigated during the SAMUM-2 field study at Cape Verde in winter 2008. Aitken and accumulation mode particles were mainly assigned to the marine aerosol, whereas coarse mode particles were composed of sea-salt and a variable fraction of Saharan mineral dust. A new methodical approach was used to derive hygroscopic growth and state of mixing for a particle size range (volume equivalent) from dpve = 26 nm to 10 μm. For hygroscopic particles with dpve < 100 nm, the median hygroscopicity parameter κ is 0.35. From 100 nm < dpve < 350 nm, κ increases to 0.65. For larger particles, κ at dpve = 350 nm was used. For nearly hydrophobic particles, κ is between 0 and 0.1 for dpve < 250 nm and decreases to 0 for dpve > 250 nm. The mixing state of Saharan dust in terms of the number fraction of nearly hydrophobic particles showed the highest variation and ranges from 0.3 to almost 1. This study was used to perform a successful mass closure at ambient conditions and demonstrates the important role of hygroscopic growth of large sea-salt particles.
- ItemIn 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, AlfredAn 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.
- ItemInfluence of water uptake on the aerosol particle light scattering coefficients of the Central European aerosol(Milton Park : Taylor & Francis, 2014) Zieger, Paul; Fierz-Schmidhauser, Rahel; Poulain, Laurent; Müller, Thomas; Birmili, Wolfram; Spindler, Gerald; Wiedensohler, Alfred; Baltensperger, Urs; Weingartner, ErnestThe influence of aerosol water uptake on the aerosol particle light scattering was examined at the regional continental research site Melpitz, Germany. The scattering enhancement factor f(RH), defined as the aerosol particle scattering coefficient at a certain relative humidity (RH) divided by its dry value, was measured using a humidified nephelometer. The chemical composition and other microphysical properties were measured in parallel. f(RH) showed a strong variation, e.g. with values between 1.2 and 3.6 at RH85% and l550 nm. The chemical composition was found to be the main factor determining the magnitude of f(RH), since the magnitude of f(RH) clearly correlated with the inorganic mass fraction measured by an aerosol mass spectrometer (AMS). Hysteresis within the recorded humidograms was observed and explained by long-range transported sea salt. A closure study using Mie theory showed the consistency of the measured parameters.
- ItemRegional 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, AlfredThe 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.
- ItemRegional Saharan dust modelling during the SAMUM 2006 campaign(Milton Park : Taylor & Francis, 2017) Heinold, Bernd; Tegen, Ina; Esselborn, Michael; Kandler, Konrad; Knippertz, Peter; Müller, Detlef; Schladitz, Alexander; Tesche, Matthias; Weinzierl, Bernadett; Ansmann, Albert; Althausen, Dietrich; Laurent, Benoit; Massling, Andreas; Müller, Thomas; Petzold, Andreas; Schepanski, Kerstin; Wiedensohler, AlfredThe regional dust model system LM-MUSCAT-DES was developed in the framework of the SAMUM project. Using the unique comprehensive data set of near-source dust properties during the 2006SAMUMfield campaign, the performance of the model system is evaluated for two time periods in May and June 2006. Dust optical thicknesses, number size distributions and the position of the maximum dust extinction in the vertical profiles agree well with the observations. However, the spatio-temporal evolution of the dust plumes is not always reproduced due to inaccuracies in the dust source placement by the model. While simulated winds and dust distributions are well matched for dust events caused by dry synoptic-scale dynamics, they are often misrepresented when dust emissions are caused by moist convection or influenced by small-scale topography that is not resolved by the model. In contrast to long-range dust transport, in the vicinity of source regions the model performance strongly depends on the correct prediction of the exact location of sources. Insufficiently resolved vertical grid spacing causes the absence of inversions in the model vertical profiles and likely explains the absence of the observed sharply defined dust layers.