288 results
Search Results
Now showing 1 - 10 of 288
- ItemSpectral surface albedo over Morocco and its impact on radiative forcing of Saharan dust(Abingdon : Taylor & Francis, 2009) Bierwirth, E.; Wendisch, M.; Ehrlich, A.; Heese, B.; Tesche, M.; Althausen, D.; Schladitz, A.; Müller, D.; Otto, S.; Trautmann, T.; Dinter, T.; Von Hoyningen-Huene, W.; Kahn, R.In May-June 2006, airborne and ground-based solar (0.3-2.2 μm) and thermal infrared (4-42 μm) radiation measurements have been performed in Morocco within the Saharan Mineral Dust Experiment (SAMUM). Upwelling and downwelling solar irradiances have been measured using the Spectral Modular Airborne Radiation Measurement System (SMART)-Albedometer. With these data, the areal spectral surface albedo for typical surface types in southeastern Morocco was derived from airborne measurements for the first time. The results are compared to the surface albedo retrieved from collocated satellite measurements, and partly considerable deviations are observed. Using measured surface and atmospheric properties, the spectral and broad-band dust radiative forcing at top-of-atmosphere (TOA) and at the surface has been estimated. The impact of the surface albedo on the solar radiative forcing of Saharan dust is quantified. In the SAMUM case of 19 May 2006, TOA solar radiative forcing varies by 12 W m-2 per 0.1 surface-albedo change. For the thermal infrared component, values of up to +22 W m-2 were derived. The net (solar plus thermal infrared) TOA radiative forcing varies between -19 and +24 W m-2 for a broad-band solar surface albedo of 0.0 and 0.32, respectively. Over the bright surface of southeastern Morocco, the Saharan dust always has a net warming effect. © 2008 The Author Journal compilation © 2008 Blackwell Munksgaard.
- ItemCharging of mesospheric aerosol particles: The role of photodetachment and photoionization from meteoric smoke and ice particles(Göttingen : Copernicus, 2009) Rapp, M.Time constants for photodetachment, photoemission, and electron capture are considered for two classes of mesospheric aerosol particles, i.e., meteor smoke particles (MSPs) and pure water ice particles. Assuming that MSPs consist of metal oxides like Fe2O3 or SiO, we find that during daytime conditions photodetachment by solar photons is up to 4 orders of magnitude faster than electron attachment such that MSPs cannot be negatively charged in the presence of sunlight. Rather, even photoemission can compete with electron capture unless the electron density becomes very large (≫1000 cm-3) such that MSPs should either be positively charged or neutral in the case of large electron densities. For pure water ice particles, however, both photodetachment and photoemission are negligible due to the wavelength characteristics of its absorption cross section and because the flux of solar photons has already dropped significantly at such short wavelengths. This means that water ice particles should normally be negatively charged. Hence, our results can readily explain the repeated observation of the coexistence of positive and negative aerosol particles in the polar summer mesopause, i.e., small MSPs should be positively charged and ice particles should be negatively charged. These results have further important implications for our understanding of the nucleation of mesospheric ice particles as well as for the interpretation of incoherent scatter radar observations of MSPs. © 2009 Author(s).
- ItemAerosol number-size distributions during clear and fog periods in the summer high Arctic: 1991, 1996 and 2001(Milton Park : Taylor & Francis, 2017) Heintzenberg, Jost; Leck, Caroline; Birmili, Wolfram; Wehner, Birgit; Tjernström, Michael; Wiedensohler, AlfredThe present study covers submicrometer aerosol size distribution data taken during three Arctic icebreaker expeditions in the summers of 1991, 1996 and 2001. The size distributions of all expeditions were compared in log-normally fitted form to the statistics of the marine number size distribution provided by Heintzenberg et al. (2004) yielding rather similar log-normal parameters of the modes. Statistics of the modal concentrations revealed strong concentration decreases of large accumulation mode particles with increasing length of time spent over the pack ice. The travel-time dependencies of both Aitken and ultrafine modes strongly indicate, as other studies did before, the occurrence of fine-particle sources in the inner Arctic. With two approaches evidence of fog-related aerosol source processeswas sought for in the data sets of 1996 and 2001 because they included fog drop size distributions. With increasing fog intensity modes in interstitial particle number concentrations appeared in particular in the size range around 80 nm that was nearly mode free in clear air. A second, dynamic approach revealed that Aitken mode concentrations increased strongly above their respective fog-period medians in both years before maximum drop numbers were reached in both years. We interpret the results of both approaches as strong indications of fog-related aerosol source processes as discussed in Leck and Bigg (1999) that need to be elucidated with further data from dedicated fog experiments in future Arctic expeditions in order to understand the life cycle of the aerosol over the high Arctic pack ice area.
- ItemRegional modelling of Saharan dust and biomass-burning smoke, Part 2: Direct radiative forcing and atmospheric dynamic response(Milton Park : Taylor & Francis, 2017) Heinold, Bernd; Tegen, Ina; Bauer, Stefan; Wendisch, ManfredThe direct radiative forcing and dynamic atmospheric response due to Saharan dust and biomass-burning aerosol particles are presented for a case study during the SAMUM-2 field campaign in January and February 2008. The regional model system COSMO-MUSCAT is used. It allows online interaction of the computed dust and smoke load with the solar and terrestrial radiation and with the model dynamics. Model results of upward solar irradiances are evaluated against airborne radiation measurements in the Cape Verde region. The comparison shows a good agreement for the case of dust and smoke mixture. Dust and smoke particles influence the atmospheric dynamics by changing the radiative heating rates. The related pressure perturbations modify local and synoptic scale air-flow patterns. In the radiative feedback simulations, the Hadley circulation is enhanced and convergence zones occur along the Guinea coast. Thus, the smoke particles spread more than 5◦ further north and the equatorward transport is reduced. Within the convergence zones, Saharan dust and biomass-burning material are more effectively advected towards the Cape Verdes. Given the model uncertainties, the agreement between the modelled and observed aerosol distribution is locally improved when aerosol–radiation interaction is considered.
- ItemArctic haze over Central Europe(Milton Park : Taylor & Francis, 2017) Heintzenberg, Jost; Tuch, Thomas; Wehner, Birgit; Wiedensohler, Alfred; Wex, Heike; Ansmann, Albert; Mattis, Ina; Müller, Detlef; Wendisch, Manfred; Eckhardt, Sabine; Stohl, AndreasAn extraordinary aerosol situation over Leipzig, Germany in April 2002 was investigated with a comprehensive set of ground-based volumetric and columnar aerosol data, combined with aerosol profiles from lidar, meteorological data from radiosondes and air mass trajectory calculations. Air masses were identified to stem from the Arctic, partly influenced by the greater Moscow region. An evaluation of ground-based measurements of aerosol size distributions during these periods showed that the number concentrations below about 70 nm in diameter were below respective long-term average data, while number, surface and volume concentrations of the particles larger than about 70 nm in diameter were higher than the long-term averages. The lidar aerosol profiles showed that the imported aerosol particles were present up to about 3 km altitude. The particle optical depth was up to 0.45 at 550 nm wavelength. With a one-dimensional spectral radiative transfer model top of the atmosphere (TOA) radiative forcing of the aerosol layer was estimated for a period with detailed vertical information. Solar aerosol radiative forcing values between −23 and −38 W m−2 were calculated, which are comparable to values that have been reported in heavily polluted continental plumes outside the respective source regions. The present report adds weight to previous findings of aerosol import to Europe, pointing to the need for attributing the three-dimensional aerosol burden to natural and anthropogenic sources as well as to aerosol imports from adjacent or distant source regions. In the present case, the transport situation is further complicated by forward trajectories, indicating that some of the observed Arctic haze may have originated in Central Europe. This aerosolwas transported to the European Arctic before being re-imported in the modified and augmented form to its initial source region.
- ItemHorizontal homogeneity and vertical extent of new particle formation events(Milton Park : Taylor & Francis, 2017) Wehner, Birgit; Siebert, Holger; Stratmann, Frank; Tuch, Thomas; Wiedensohler, Alfred; PetäJä, Tuukka; Dal Maso, Miikka; Kulmala, MarkkuDuring the SATURN campaign 2002, new particle formation, i.e. the occurrence of ultrafine particles was investigated simultaneously at four ground-based measurement sites. The maximum distance between the sites was 50 km. Additionally, vertical profiles of aerosol particles from 5–10 nm have been measured by a tethered-balloonborne system at one of the sites. In general, two different scenarios have been found: (i) new particle formation was measured at all sites nearly in parallel with subsequent particle growth (homogeneous case) and (ii) new particle formation was observed at one to three sites irregularly (inhomogeneous case) where subsequent particle growth was often interrupted. The homogeneous case was connected with stable synoptical conditions, i.e. the region was influenced by a high pressure system. Here, the horizontal extent of the phenomenon has been estimated to be 400 km at maximum. In the vertical dimension, the ultrafine particles are well mixed within the entire boundary layer. In the inhomogeneous case the new particle formation depends mainly on the incoming solar radiation and was often interrupted due the occurrence of clouds. Thus, single point measurements are not representative for a larger region in that case.
- ItemAerosol number to volume ratios in Southwest Portugal during ACE-2(Milton Park : Taylor & Francis, 2017) Dusek, Ulrike; Covert, David S.; Wiedensohler, Alfred; Neusúss, Christian; Weise, DianaPast studies have indicated that long-term averages of the aerosol number to volume ratios (defined as the number of particles larger than a certain diameter divided by the particle volume over some range less than 1 μm) show little variability over the Atlantic. This work presents number to volume ratios (R) measured during the ACE-2 experiment on the land-based Sagres field site located in Southwest Portugal. The values of R measured in Sagres compare reasonably well with previous measurements over the Atlantic. The main emphasis of this work is therefore to investigate more closely possible reasons for the observed stability of the number to volume ratio. Aerosol number size distributions measured in Sagres are parametrized by the sum of two log-normal distributions fitted to the accumulation and to the Aitken mode. The main factor that limits the variability of R is that the parameters of these log-normal distributions are not always independent but show some covariance. In polluted air mass types correlations between parameters of the Aitken and accumulation mode are mostly responsible for stabilizing R. In marine air mass types the variability of R is reduced by an inverse relationship between the accumulation-mode mean diameter and standard deviation, consistent with condensational processes and cloud processing working on the aerosol. However, despite this reduction, the variability of R in marine air mass types is still considerable and R is linearly dependent on the number concentration of particles larger than 90 nm. This partly due to a mil of Aitken-mode particles extending to sizes larger than 90 nm.
- ItemSaharan Mineral Dust Experiments SAMUM-1 and SAMUM-2: What have we learned?(Milton Park : Taylor & Francis, 2011) Ansmann, Albert; Petzold, Andreas; Kandler, Konrad; Tegen, Ina; Wendisch, Manfred; Müller, Detlef; Weinzierl, Bernadett; Müller, Thomas; Heintzenberg, JostTwo comprehensive field campaigns were conducted in 2006 and 2008 in the framework of the Saharan Mineral Dust Experiment (SAMUM) project. The relationship between chemical composition, shape morphology, size distribution and optical effects of the dust particles was investigated. The impact of Saharan dust on radiative transfer and the feedback of radiative effects upon dust emission and aerosol transport were studied. Field observations (ground-based, airborne and remote sensing) and modelling results were compared within a variety of dust closure experiments with a strong focus on vertical profiling. For the first time, multiwavelength Raman/polarization lidars and an airborne high spectral resolution lidar were involved in major dust field campaigns and provided profiles of the volume extinction coefficient of the particles at ambient conditions (for the full dust size distribution), of particle-shape-sensitive optical properties at several wavelengths, and a clear separation of dust and smoke profiles allowing for an estimation of the single-scattering albedo of the biomass-burning aerosol. SAMUM–1 took place in southern Morocco close to the Saharan desert in the summer of 2006, whereas SAMUM–2 was conducted in Cape Verde in the outflow region of desert dust and biomass-burning smoke from western Africa in the winter of 2008. This paper gives an overview of the SAMUM concept, strategy and goals, provides snapshots (highlights) of SAMUM–2 observations and modelling efforts, summarizes main findings of SAMUM–1 and SAMUM–2 and finally presents a list of remaining problems and unsolved questions.
- 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.
- ItemAn overview of the ACE-2 clear sky column closure experiment (CLEARCOLUMN)(Milton Park : Taylor & Francis, 2016) Russell, Philip B.; Heintzenberg, JostAs 1 of 6 focused ACE-2 activities, a clear sky column closure experiment (CLEARCOLUMN) took place in June/July 1997 at the southwest corner of Portugal, in the Canary Islands, and over the eastern Atlantic Ocean surrounding and linking those sites. Overdetermined sets of volumetric, vertical profile and columnar aerosol data were taken from the sea surface to~5 km asl by samplers and sensors at land sites (20–3570 m asl), on a ship, and on 4 aircraft. In addition, 5 satellites measured upwelling radiances used to derive properties of the aerosol column. Measurements were made in a wide range of conditions and locations (e.g., the marine boundary layer with and without continental pollution, the free troposphere with and without African dust). Numerous tests of local and column closure, using unidisciplinary and multidisciplinary approaches, were conducted. This paper summarizes the methodological approach, the experiment sites and platforms, the types of measurements made on each, the types of analyses conducted, and selected key results, as a guide to the more complete results presented in other papers in this special issue and elsewhere. Example results include determinations of aerosol single scattering albedo by several techniques, measurements of hygroscopic effects on particle light scattering and size, and a wide range in the degree of agreement found in closure tests. In general, the smallest discrepancies were found in comparisons among (1) different techniques to measure an optical property of the ambient, unperturbed aerosol (e.g., optical depth, extinction, or backscatter by sunphotometer, lidar, and/or satellite) or (2) different techniques to measure an aerosol that had passed through a common sampling process (e.g., nephelometer and size spectrometer measurements with the same or similar inlets, humidities and temperatures). Typically, larger discrepancies were found between techniques that measure the ambient, unperturbed aerosol and those that must reconstruct the ambient aerosol by accounting for (a) processes that occur during sampling (e.g., aerodynamic selection, evaporation of water and other volatile material) or ( b) calibrations that depend on aerosol characteristics (e.g., sizedependent density or refractive index). A primary reason for the discrepancies in such cases is the lack of validated hygroscopic growth models covering the necessary range of particle sizes and compositions. Other common reasons include (1) using analysis or retrieval techniques that assume aerosol properties (e.g., density, single scattering albedo, shape) that do not apply in all cases and (2) using surface measurements to estimate column properties. Taken together, the ACE-2 CLEARCOLUMN data set provides a large collection of new information on the properties of the aerosol over the northeast Atlantic Ocean. CLEARCOLUMN studies have also pointed to improved techniques for analyzing current and future data sets (including satellite data sets) which will provide a more accurate and comprehensive description of the Atlantic–European–African aerosol. Thus they set the stage for an improved regional quantification of radiative forcing by anthropogenic aerosols.