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Now showing 1 - 10 of 408
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    Modifications in aerosol physical, optical and radiative properties during heavy aerosol events over Dushanbe, Central Asia
    (Amsterdam [u.a.] : Elsevier, 2021) Rupakheti, Dipesh; Rupakheti, Maheswar; Yin, Xiufeng; Hofer, Julian; Rai, Mukesh; Hu, Yuling; Abdullaev, Sabur F.; Kang, Shichang
    The location of Central Asia, almost at the center of the global dust belt region, makes it susceptible for dust events. The studies on atmospheric impact of dust over the region are very limited despite the large area occupied by the region and its proximity to the mountain regions (Tianshan, Hindu Kush-Karakoram-Himalayas, and Tibetan Plateau). In this study, we analyse and explain the modification in aerosols’ physical, optical and radiative properties during various levels of aerosol loading observed over Central Asia utilizing the data collected during 2010–2018 at the AERONET station in Dushanbe, Tajikistan. Aerosol episodes were classified as strong anthropogenic, strong dust and extreme dust. The mean aerosol optical depth (AOD) during these three types of events was observed a factor of ~3, 3.5 and 6.6, respectively, higher than the mean AOD for the period 2010–2018. The corresponding mean fine-mode fraction was 0.94, 0.20 and 0.16, respectively, clearly indicating the dominance of fine-mode anthropogenic aerosol during the first type of events, whereas coarse-mode dust aerosol dominated during the other two types of events. This was corroborated by the relationships among various aerosol parameters (AOD vs. AE, and EAE vs. AAE, SSA and RRI). The mean aerosol radiative forcing (ARF) at the top of the atmosphere (ARFTOA), the bottom of the atmosphere (ARFBOA), and in the atmosphere (ARFATM) were −35 ± 7, −73 ± 16, and 38 ± 17 Wm−2 during strong anthropogenic events, −48 ± 12, −85 ± 24, and 37 ± 15 Wm−2 during strong dust event, and −68 ± 19, −117 ± 38, and 49 ± 21 Wm−2 during extreme dust events. Increase in aerosol loading enhanced the aerosol-induced atmospheric heating rate to 0.5–1.6 K day−1 (strong anthropogenic events), 0.4–1.9 K day−1 (strong dust events) and 0.8–2.7 K day−1 (extreme dust events). The source regions of air masses to Dushanbe during the onset of such events are also identified. Our study contributes to the understanding of dust and anthropogenic aerosols, in particular the extreme events and their disproportionally high radiative impacts over Central Asia.
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    Strong impact of wildfires on the abundance and aging of black carbon in the lowermost stratosphere
    (Washington, DC : NAS, 2018) Ditas, Jeannine; Ma, Nan; Zhang, Yuxuan; Assmann, Denise; Neumaier, Marco; Riede, Hella; Karu, Einar; Williams, Jonathan; Scharffe, Dieter; Wang, Qiaoqiao; Saturno, Jorge; Schwarz, Joshua P.; Katich, Joseph M.; McMeeking, Gavin R.; Zahn, Andreas; Hermann, Markus; Brenninkmeijer, Carl A. M.; Andreae, Meinrat O.; Pöschl, Ulrich; Su, Hang; Cheng, Yafang
    Wildfires inject large amounts of black carbon (BC) particles into the atmosphere, which can reach the lowermost stratosphere (LMS) and cause strong radiative forcing. During a 14-month period of observations on board a passenger aircraft flying between Europe and North America, we found frequent and widespread biomass burning (BB) plumes, influencing 16 of 160 flight hours in the LMS. The average BC mass concentrations in these plumes (∼140 ng·m−3, standard temperature and pressure) were over 20 times higher than the background concentration (∼6 ng·m−3) with more than 100-fold enhanced peak values (up to ∼720 ng·m−3). In the LMS, nearly all BC particles were covered with a thick coating. The average mass equivalent diameter of the BC particle cores was ∼120 nm with a mean coating thickness of ∼150 nm in the BB plume and ∼90 nm with a coating of ∼125 nm in the background. In a BB plume that was encountered twice, we also found a high diameter growth rate of ∼1 nm·h−1 due to the BC particle coatings. The observed high concentrations and thick coatings of BC particles demonstrate that wildfires can induce strong local heating in the LMS and may have a significant influence on the regional radiative forcing of climate.
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    Knowledge Transfer with Citizen Science: Luft-Leipzig Case Study
    (Basel : MDPI, 2021) Tõnisson, Liina; Voigtländer, Jens; Weger, Michael; Assmann, Denise; Käthner, Ralf; Heinold, Bernd; Macke, Andreas
    Community-based participatory research initiatives such as “hackAir”, “luftdaten.info”, “senseBox”, “CAPTOR”, “CurieuzeNeuzen Vlaanderen”, “communityAQ”, and “Healthy Air, Healthier Children” campaign among many others for mitigating short-lived climate pollutants (SLCPs) and improving air quality have reported progressive knowledge transfer results. These research initiatives provide the research community with the practical four-element state-of-the-art method for citizen science. For the preparation-, measurements-, data analysis-, and scientific support-elements that collectively present the novel knowledge transfer method, the Luft-Leipzig project results are presented. This research contributes to science by formulating a novel method for SLCP mitigation projects that employ citizen scientists. The Luft-Leipzig project results are presented to validate the four-element state-of-the-art method. The method is recommended for knowledge transfer purposes beyond the scope of mitigating short-lived climate pollutants (SLCPs) and improving air quality.
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    How organized is deep convection over Germany?
    (Weinheim [u.a.] : Wiley, 2019) Pscheidt, Ieda; Senf, Fabian; Heinze, Rieke; Deneke, Hartwig; Trömel, Silke; Hohenegger, Cathy
    Deep moist convection shows a tendency to organize into mesoscale structures. To be able to understand the potential effect of convective organization on the climate, one needs first to characterize organization. In this study, we systematically characterize the organizational state of convection over Germany based on two years of cloud-top observations derived from the Meteosat Second Generation satellite and of precipitation cores detected by the German C-band radar network. The organizational state of convection is characterized by commonly employed organization indices, which are mostly based on the object numbers, sizes and nearest-neighbour distances. According to the organization index Iorg, cloud tops and precipitation cores are found to be in an organized state for 69% and 92% of the time, respectively. There is an increase in rainfall when the number of objects and their sizes increase, independently of the organizational state. Case-studies of specific days suggest that convectively organized states correspond to either local multi-cell clusters, with less numerous, larger objects close to each other, or to scattered clusters, with more numerous, smaller organized objects spread out over the domain. For those days, simulations are performed with the large-eddy model ICON with grid spacings of 625, 312 and 156 m. Although the model underestimates rainfall and shows a too large cold cloud coverage, the organizational state is reasonably well represented without significant differences between the grid spacings.
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    Identification of dust sources in a Saharan dust hot-spot and their implementation in a dust-emission model
    (Basel : MDPI, 2019) Feuerstein, Stefanie; Schepanski, Kerstin
    Although mineral dust plays a key role in the Earth’s climate system and in climate and weather prediction, models still have difficulties in predicting the amount and distribution of mineral dust in the atmosphere. One reason for this is the limited understanding of the distribution of dust sources and their behavior with respect to their spatiotemporal variability in activity. For a better estimation of the atmospheric dust load, this paper presents an approach to localize dust sources and thereby estimate the sediment supply for a study area centered on the Aïr Massif in Niger with a north–south extent of 16 ∘ –22 ∘ N and an east–west extent of 4 ∘ –12 ∘ E. This approach uses optical Sentinel-2 data at visible and near infrared wavelengths together with HydroSHEDS flow accumulation data to localize ephemeral riverbeds. Visible channels from Sentinel-2 data are used to detect sand sheets and dunes. The identified sediment supply map was compared to the dust source activation frequency derived from the analysis of Desert-Dust-RGB imagery from the Meteosat Second Generation series of satellites. This comparison reveals the strong connection between dust activity, prevailing meteorology and sediment supply. In a second step, the sediment supply information was implemented in a dust-emission model. The simulated emission flux shows how much the model results benefit from the updated sediment supply information in localizing the main dust sources and in retrieving the seasonality of dust activity from these sources. The described approach to characterize dust sources can be implemented in other regional model studies, or even globally, and can thereby help to get a more accurate picture of dust source distribution and a more realistic estimation of the atmospheric dust load.
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    Advection of Biomass Burning Aerosols towards the Southern Hemispheric Mid-Latitude Station of Punta Arenas as Observed with Multiwavelength Polarization Raman Lidar
    (Basel : MDPI AG, 2021) Floutsi, Athena Augusta; Baars, Holger; Radenz, Martin; Haarig, Moritz; Yin, Zhenping; Seifert, Patric; Jimenez, Cristofer; Ansmann, Albert; Engelmann, Ronny; Barja, Boris; Zamorano, Felix; Wandinger, Ulla
    In this paper, we present long-term observations of the multiwavelength Raman lidar PollyXT conducted in the framework of the DACAPO-PESO campaign. Regardless of the relatively clean atmosphere in the southern mid-latitude oceans region, we regularly observed events of long-range transported smoke, originating either from regional sources in South America or from Australia. Two case studies will be discussed, both identified as smoke events that occurred on 5 February 2019 and 11 March 2019. For the first case considered, the lofted smoke layer was located at an altitude between 1.0 and 4.2 km, and apart from the predominance of smoke particles, particle linear depolarization values indicated the presence of dust particles. Mean lidar ratio values at 355 and 532 nm were 49 ± 12 and 24 ± 18 sr respectively, while the mean particle linear depolarization was 7.6 ± 3.6% at 532 nm. The advection of smoke and dust particles above Punta Arenas affected significantly the available cloud condensation nuclei (CCN) and ice nucleating particles (INP) in the lower troposphere, and effectively triggered the ice crystal formation processes. Regarding the second case, the thin smoke layers were observed at altitudes 5.5–7.0, 9.0 and 11.0 km. The particle linear depolarization ratio at 532 nm increased rapidly with height, starting from 2% for the lowest two layers and increasing up to 9.5% for the highest layer, indicating the possible presence of non-spherical coated soot aggregates. INP activation was effectively facilitated. The long-term analysis of the one year of observations showed that tropospheric smoke advection over Punta Arenas occurred 16 times (lasting from 1 to 17 h), regularly distributed over the period and with high potential to influence cloud formation in the otherwise pristine environment of the region.
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    Millennial-scale fluctuations in Saharan dust supply across the decline of the African Humid Period
    (Amsterdam [u.a.] : Elsevier, 2017) Zielhofer, Christoph; von Suchodoletz, Hans; Fletcher, William J.; Schneider, Birgit; Dietze, Elisabeth; Schlegel, Michael; Schepanski, Kerstin; Weninger, Bernhard; Mischke, Steffen; Mikdad, Abdeslam
    The Sahara is the world's largest dust source with significant impacts on trans-Atlantic terrestrial and large-scale marine ecosystems. Contested views about a gradual or abrupt onset of Saharan aridity at the end of the African Humid Period dominate the current scientific debate about the Holocene Saharan desiccation. In this study, we present a 19.63 m sediment core sequence from Lake Sidi Ali (Middle Atlas, Morocco) at the North African desert margin. We reconstruct the interaction between Saharan dust supply and Western Mediterranean hydro-climatic variability during the last 12,000 yr based on analyses of lithogenic grain-sizes, XRF geochemistry and stable isotopes of ostracod shells. A robust chronological model based on AMS 14C dated pollen concentrates supports our multi-proxy study. At orbital-scale there is an overall increase in southern dust supply from the Early Holocene to the Late Holocene, but our Northern Saharan dust record indicates that a gradual Saharan desiccation was interrupted by multiple abrupt dust increases before the ‘southern dust mode‘ was finally established at 4.7 cal ka BP. The Sidi Ali record features millennial peaks in Saharan dust increase at about 11.1, 10.2, 9.4, 8.2, 7.3, 6.6, 6.0, and 5.0 cal ka BP. Early Holocene Saharan dust peaks coincide with Western Mediterranean winter rain minima and North Atlantic cooling events. In contrast, Late Holocene dust peaks correspond mostly with prevailing positive phases of the North Atlantic Oscillation. By comparing with other North African records, we suggest that increases in Northern Saharan dust supply do not solely indicate sub-regional to regional aridity in Mediterranean Northwest Africa but might reflect aridity at a trans-Saharan scale. In particular, our findings support major bimillennial phases of trans-Saharan aridity at 10.2, 8.2, 6.0 and 4.2 cal ka BP. These phases coincide with North Atlantic cooling and a weak African monsoon.
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    Preventing airborne transmission of SARS-CoV-2 in hospitals and nursing homes
    (Basel : MDPI AG, 2020) Ahlawat, Ajit; Mishra, Sumit Kumar; Birks, John W.; Costabile, Francesca; Wiedensohler, Alfred
    [No abstract available]
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    CRAAS: A European Cloud Regime dAtAset Based on the CLAAS-2.1 Climate Data Record
    (Basel : MDPI, 2022) Tzallas, Vasileios; Hünerbein, Anja; Stengel, Martin; Meirink, Jan Fokke; Benas, Nikos; Trentmann, Jörg; Macke, Andreas
    Given the important role of clouds in our planet’s climate system, it is crucial to further improve our understanding of their governing processes as well as the resulting spatio-temporal variability of their properties. This co-variability of different cloud optical properties is adequately represented through the well-established concept of cloud regimes. The focus of the present study lies on the creation of a cloud regime dataset over Europe, named “Cloud Regime dAtAset based on the CLAAS-2.1 climate data record” (CRAAS), in order to analyze their variability and their changes at different spatio-temporal scales. In addition, co-occurrences between the cloud regimes and large-scale weather patterns are investigated. The CLoud property dAtAset using Spinning Enhanced Visible and Infrared (SEVIRI) edition 2.1 (CLAAS-2.1) data record, which is produced by the Satellite Application Facility on Climate Monitoring (CM SAF), was used as the basis for the derivation of the cloud regimes over Europe for a 14-year period (2004–2017). In particular, the cloud optical thickness (COT) and cloud top pressure (CTP) products of CLAAS-2.1 were used in order to compute 2D histograms. Then, the k-means clustering algorithm was applied to the generated 2D histograms in order to derive the cloud regimes. Eight cloud regimes were identified, which, along with the geographical distribution of their frequency of occurrence, assisted in providing a detailed description of the climate of the cloud properties over Europe. The annual and diurnal variabilities of the eight cloud regimes were studied, and trends in their frequency of occurrence were also examined. Larger changes in the frequency of occurrence of the produced cloud regimes were found for a regime associated to alto- and nimbo-type clouds and for a regime connected to shallow cumulus clouds and fog (−0.65% and +0.70% for the time period of the study, respectively).
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    Enhanced tenacity of mycobacterial aerosols from necrotic neutrophils
    ([London] : Macmillan Publishers Limited, part of Springer Nature, 2020) Pfrommer, E.; Dreier, C.; Gabriel, G.; Dallenga, T.; Reimer, R.; Schepanski, K.; Scherließ, R.; Schaible, U.E.; Gutsmann, T.
    The tuberculosis agent Mycobacterium tuberculosis is primarily transmitted through air, but little is known about the tenacity of mycobacterium-containing aerosols derived from either suspensions or infected neutrophils. Analysis of mycobacterial aerosol particles generated from bacterial suspensions revealed an average aerodynamic diameter and mass density that may allow distant airborne transmission. The volume and mass of mycobacterial aerosol particles increased with elevated relative humidity. To more closely mimic aerosol formation that occurs in active TB patients, aerosols from mycobacterium-infected neutrophils were analysed. Mycobacterium-infected intact neutrophils showed a smaller particle size distribution and lower viability than free mycobacteria. In contrast, mycobacterium-infected necrotic neutrophils, predominant in M. tuberculosis infection, revealed particle sizes and viability rates similar to those found for free mycobacteria, but in addition, larger aggregates of viable mycobacteria were observed. Therefore, mycobacteria are shielded from environmental stresses in multibacillary aggregates generated from necrotic neutrophils, which allows improved tenacity but emphasizes short distance transmission between close contacts.