5 results
Search Results
Now showing 1 - 5 of 5
- ItemAtmospheric black carbon and warming effects influenced by the source and absorption enhancement in central Europe(München : European Geopyhsical Union, 2014) Nordmann, S.; Cheng, Y.F.; Carmichael, G.R.; Yu, M.; van der Gon, H.A.C.Denier; Zhang, Q.; Saide, P.E.; Pöschl, U.; Su, H.; Birmili, W.; Wiedensohler, A.Particles containing black carbon (BC), a strong absorbing substance, exert a rather uncertain direct and indirect radiative forcing in the atmosphere. To investigate the mass concentration and absorption properties of BC particles over central Europe, the model WRF-Chem was used at a resolution of 12 km in conjunction with a high-resolution BC emission inventory (EUCAARI 42-Pan-European Carbonaceous Aerosol Inventory; 1/8° × 1/16°). The model simulation was evaluated using measurements of equivalent soot carbon, absorption coefficients and particle number concentrations at seven sites within the German Ultrafine Aerosol Network, PM mass concentrations from the dense measurement network of the German Federal Environmental Agency at 392 monitoring stations, and aerosol optical depth from MODIS and AERONET. A distinct time period (25 March to 10 April 2009) was chosen, during which the clean marine air mass prevailed in the first week and afterwards the polluted continental air mass mainly from the southeast dominated with elevated daily average BC concentration of up to 4 μ g m−3. The simulated PM mass concentration, aerosol number concentration and optical depth were in good agreement with the observations, while the modelled BC mass concentrations were found to be a factor of 2 lower than the observations. Together with back trajectories, detailed model bias analyses suggested that the current BC emission in countries to the east and south of Germany might be underestimated by a factor of 5, at least for the simulation period. Running the model with upscaled BC emissions in these regions led to a smaller model bias and a better correlation between model and measurement. In contrast, the particle absorption coefficient was positively biased by about 20% even when the BC mass concentration was underestimated by around 50%. This indicates that the internal mixture treatment of BC in the WRF-Chem optical calculation is unrealistic in our case, which overamplifies the light absorption by BC-containing particles. By adjusting the modelled mass absorption cross-section towards the measured values, the simulation of particle light absorption of BC was improved as well. Finally, the positive direct radiative forcing of BC particles at the top of the atmosphere was estimated to be in the range of 0 to +4 W m−2 over Germany for the model run with improved BC mass concentration and adjusted BC light absorption cross-section. This adjustment lowered the positive forcing of BC by up to 70%, compared with the internal mixing treatment of BC in the model simulation.
- ItemLong-term measurements of aerosol and carbon monoxide at the ZOTTO tall tower to characterize polluted and pristine air in the Siberian taiga(München : European Geopyhsical Union, 2013) Chi, X.; Winderlich, J.; Mayer, J.-C.; Panov, A.V.; Heimann, M.; Birmili, W.; Heintzenberg, J.; Cheng, Y.; Andreae, M.O.Siberia is one of few continental regions in the Northern Hemisphere where the atmosphere may sometimes approach pristine background conditions. We present the time series of aerosol and carbon monoxide (CO) measurements between September 2006 and December 2011 at the Zotino Tall Tower Observatory (ZOTTO) in Central Siberia (61° N; 89° E). We investigate the seasonal, weekly and diurnal variations of aerosol properties (including absorption and scattering coefficients and derived parameters, such as equivalent black carbon (BCe), Ångström exponent, single scattering albedo, and backscattering ratio) and the CO mixing ratios. Criteria were established to distinguish polluted from near-pristine air masses, providing quantitative characteristics for each type. Depending on the season, 23–36% of the sampling time at ZOTTO was found to be representative of a clean atmosphere. The summer pristine data indicate that primary biogenic and secondary organic aerosol formation are quite strong particle sources in the Siberian taiga. The summer seasons 2007–2008 were dominated by an Aitken mode around 80 nm size, whereas the summer 2009 with prevailing easterly winds produced particles in the accumulation mode around 200 nm size. We found these differences to be mainly related to air temperature, through its effect on the production rates of biogenic volatile organic compounds (VOC) precursor gases. In winter, the particle size distribution peaked at 160 nm, and the footprint of clean background air was characteristic for aged particles from anthropogenic sources at great distances from ZOTTO and diluted biofuel burning emissions from domestic heating. The wintertime polluted air originates mainly from large cities south and southwest of the site; these particles have a dominant mode around 100 nm, and the ΔBCe / ΔCO ratio of 7–11 ng m−3 ppb−1 suggests dominant contributions from coal and biofuel burning for heating. During summer, anthropogenic emissions are the dominant contributor to the pollution particles at ZOTTO, while only 12% of the polluted events are classified as biomass-burning-dominated, but then often associated with extremely high CO concentrations and aerosol absorption coefficients. Two biomass-burning case studies revealed different ΔBCe / ΔCO ratios from different fire types, with the agricultural fires in April~2008 yielding a very high ratio of 21 ng m−3 ppb−1. Overall, we find that anthropogenic sources dominate the aerosol population at ZOTTO most of the time, even during nominally clean episodes in winter, and that near-pristine conditions are encountered only in the growing season and then only episodically.
- ItemHygroscopic properties of atmospheric aerosol particles over the Eastern Mediterranean: Implications for regional direct radiative forcing under clean and polluted conditions(München : European Geopyhsical Union, 2011) Stock, M.; Cheng, Y.F.; Birmili, W.; Massling, A.; Wehner, B.; Müller, T.; Leinert, S.; Kalivitis, N.; Mihalopoulos, N.; Wiedensohler, A.This work examines the effect of direct radiative forcing of aerosols in the eastern Mediterranean troposphere as a function of air mass composition, particle size distribution and hygroscopicity, and relative humidity (RH). During intensive field measurements on the island of Crete, Greece, the hygroscopic properties of atmospheric particles were determined using a Hygroscopicity Tandem Differential Mobility Analyzer (H-TDMA) and a Hygroscopicity Differential Mobility Analyzer-Aerodynamic Particle Sizer (H-DMA-APS). Similar to former studies, the H-TDMA identified three hygroscopic sub-fractions of particles in the sub-μm range: a more hygroscopic group, a less hygroscopic group and a nearly hydrophobic particle group. The average hygroscopic particle growth factors at 90 % RH were a significant function of particle mobility diameter (Dp): 1.42 (± 0.05) at 30 nm compared to 1.63 (± 0.07) at 250 nm. The H-DMA-APS identified up to three hygroscopic sub-fractions at mobility diameters of 1.0 and 1.2 μm. The data recorded between 12 August and 20 October 2005 were classified into four distinct synoptic-scale air mass types distinguishing between different regions of origin (western Mediterranean vs. the Aegean Sea) as well as the degree of continental pollution (marine vs. continentally influenced). The hygroscopic properties of particles with diameter Dp≥150 nm showed the most pronounced dependency on air mass origin, with growth factors in marine air masses exceeding those in continentally influenced air masses. Particle size distributions and hygroscopic growth factors were used to calculate aerosol light scattering coefficients at ambient RH using a Mie model. A main result was the pronounced enhancement of particle scattering over the eastern Mediterranean due to hygroscopic growth, both in the marine and continentally influenced air masses. When RH reached its summer daytime values around 70–80 %, up to 50–70 % of the calculated visibility reduction was due to the hygroscopic growth of the particles by water compared to the effect of the dry particles alone. The estimated aerosol direct radiative forcings for both, marine and continentally influenced air masses were negative indicating a net cooling of the atmosphere due to the aerosol. The radiative forcing ΔFr was nevertheless governed by the total aerosol concentration most of the time: ΔFr was typically more negative for continentally influenced aerosols (ca. −4 W m−2) compared to rather clean marine aerosols (ca. −1.5 W m−2). When RH occasionally reached 90 % in marine air masses, ΔFr even reached values down to −7 W m−2. Our results emphasize, on the basis of explicit particle hygroscopicity measurements, the relevance of ambient RH for the radiative forcing of regional atmospheres.
- ItemSouth African EUCAARI measurements: Seasonal variation of trace gases and aerosol optical properties(München : European Geopyhsical Union, 2012) Laakso, L.; Vakkari, V.; Virkkula, A.; Laakso, H.; Backman, J.; Kulmala, M.; Beukes, J.P.; van Zyl, P.G.; Tiitta, P.; Josipovic, M.; Pienaar, J.J.; Chiloane, K.; Gilardoni, S.; Vignati, E.; Wiedensohler, A.; Tuch, T.; Birmili, W.; Piketh, S.; Collett, K.; Fourie, G.D.; Komppula, M.; Lihavainen, H.; de Leeuw, G.; Kerminen, V.-M.In this paper we introduce new in situ observations of atmospheric aerosols, especially chemical composition, physical and optical properties, on the eastern brink of the heavily polluted Highveld area in South Africa. During the observation period between 11 February 2009 and 31 January 2011, the mean particle number concentration (size range 10–840 nm) was 6310 cm3 and the estimated volume of sub-10 μm particles 9.3 μm3 m−3. The aerosol absorption and scattering coefficients at 637 nm were 8.3 Mm−1 and 49.5 Mm−1, respectively. The mean single-scattering albedo at 637 nm was 0.84 and the Ångström exponent of scattering was 1.5 over the wavelength range 450–635 nm. The mean O3, SO2, NOx and H2S-concentrations were 37.1, 11.5, 15.1 and 3.2 ppb, respectively. The observed range of concentrations was large and attributed to the seasonal variation of sources and regional meteorological effects, especially the anticyclonic re-circulation and strong winter-time inversions. In a global context, the levels of gases and particulates were typical for continental sites with strong anthropogenic influence, but clearly lower than the most polluted areas of south-eastern Asia. Of all pollutants observed at the site, ozone is the most likely to have adverse environmental effects, as the concentrations were high also during the growing season. The measurements presented here will help to close existing gaps in the ground-based global atmosphere observation system, since very little long-term data of this nature is available for southern Africa.
- ItemSize-segregated chemical, gravimetric and number distribution-derived mass closure of the aerosol in Sagres, Portugal during ACE-2(Milton Park : Taylor & Francis, 2016) Neusüß, C.; Weise, D.; Birmili, W.; Wex, H.; Wiedensohler, A.; Covert, D.S.During the ACE-2 field campaign in the summer of 1997 an intensive, ground-based physical and chemical characterisation of the clean marine and continentally polluted aerosol was performed at Sagres, Portugal. Number size distributions of the dry aerosol in the size range 3–10 000 nm were continuously measured using DMPS and APS systems. Impactor samples were regularly taken at 60% relative humidity (RH) to obtain mass size distributions by weighing the impactor foils, and to derive a chemical mass balance by ion and carbon analysis. Hygroscopic growth factors of the metastable aerosol at 60% RH were determined to estimate the number size distribution at a relative humidity of 60%. A size segregated 3-way mass closure study was performed in this investigation for the first time. Mass size distributions at 60% RH derived from number size distribution measurements and impactors samples (weighing and chemical analysis) are compared. A good agreement was found for the comparison of total gravimetrically-determined mass with both number distribution-derived (slope=1.23/1.09; R2>0.97; depending on the parameters humidity growth and density) and chemical mass concentration (slope=1.02; R2=0.79) for particles smaller than 3 mm in diameter. Except for the smallest impactor size range relatively good correlations (slope=0.86–1.42) with small deviations (R2=0.76–0.98) for the different size fractions were found. Since uncertainties in each of the 3 methods are about 20% the observed differences in the size-segregated mass fractions can be explained by the measurement uncertainties. However, the number distributionderived mass is mostly higher than the chemically and gravimetrically determined mass, which can be explained by sampling losses of the impactor, but as well with measurement uncertainties as, e.g., the sizing of the DMPS/APS.