Browsing by Author "Wex, H."
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- ItemAnnual variability of ice-nucleating particle concentrations at different Arctic locations(Göttingen : Copernicus GmbH, 2019) Wex, H.; Huang, L.; Zhang, W.; Hung, H.; Traversi, R.; Becagli, S.; Sheesley, R.J.; Moffett, C.E.; Barrett, T.E.; Bossi, R.; Skov, H.; Hünerbein, A.; Lubitz, J.; Löffler, M.; Linke, O.; Hartmann, M.; Herenz, P.; Stratmann, F.Number concentrations of ice-nucleating particles (NINP) in the Arctic were derived from ground-based filter samples. Examined samples had been collected in Alert (Nunavut, northern Canadian archipelago on Ellesmere Island), Utqiagvik, formerly known as Barrow (Alaska), Nyalesund (Svalbard), and at the Villum Research Station (VRS; northern Greenland). For the former two stations, examined filters span a full yearly cycle. For VRS, 10 weekly samples, mostly from different months of one year, were included. Samples from Ny-Alesund were collected during the months from March until September of one year. At all four stations, highest concentrations were found in the summer months from roughly June to September. For those stations with sufficient data coverage, an annual cycle can be seen. The spectra of NINP observed at the highest temperatures, i.e., those obtained for summer months, showed the presence of INPs that nucleate ice up to-5 °C. Although the nature of these highly ice-active INPs could not be determined in this study, it often has been described in the literature that ice activity observed at such high temperatures originates from the presence of ice-active material of biogenic origin. Spectra observed at the lowest temperatures, i.e., those derived for winter months, were on the lower end of the respective values from the literature on Arctic INPs or INPs from midlatitude continental sites, to which a comparison is presented herein. An analysis concerning the origin of INPs that were ice active at high temperatures was carried out using back trajectories and satellite information. Both terrestrial locations in the Arctic and the adjacent sea were found to be possible source areas for highly active INPs.
- ItemApplication of linear polarized light for the discrimination of frozen and liquid droplets in ice nucleation experiments(München : European Geopyhsical Union, 2013) Clauss, T.; Kiselev, A.; Hartmann, S.; Augustin, S.; Pfeifer, S.; Niedermeier, D.; Wex, H.; Stratmann, F.We report on the development and test results of the new optical particle counter TOPS-Ice (Thermo-stabilized Optical Particle Spectrometer for the detection of Ice). The instrument uses measurements of the cross-polarized scattered light by single particles into the near-forward direction (42.5° ± 12.7°) to distinguish between spherical and non-spherical particles. This approach allows the differentiation between liquid water droplets (spherical) and ice particles (non-spherical) having similar volume-equivalent sizes and therefore can be used to determine the fraction of frozen droplets in a typical immersion freezing experiment. We show that the numerical simulation of the light scattered on non-spherical particles (spheroids in random orientation) considering the actual scattering geometry used in the instrument supports the validity of the approach, even though the cross-polarized component of the light scattered by spherical droplets does not vanish in this scattering angle. For the separation of the ice particle mode from the liquid droplet mode, we use the width of the pulse detected in the depolarization channel instead of the pulse height. Exploiting the intrinsic relationship between pulse height and pulse width for Gaussian pulses allows us to calculate the fraction of frozen droplets even if the liquid droplet mode dominates the particle ensemble. We present test results obtained with TOPS-Ice in the immersion freezing experiments at the laminar diffusion chamber LACIS (Leipzig Aerosol Cloud Interaction Simulator) and demonstrate the excellent agreement with the data obtained in similar experiments with a different optical instrument. Finally, the advantages of using the cross-polarized light measurements for the differentiation of liquid and frozen droplets in the realistic immersion freezing experiments are discussed.
- ItemCalibration of LACIS as a CCN detector and its use in measuring activation and hygroscopic growth of atmospheric aerosol particles(München : European Geopyhsical Union, 2006) Wex, H.; Kiselev, A.; Ziese, M.; Stratmann, F.A calibration for LACIS (Leipzig Aerosol Cloud Interaction Simulator) for its use as a CCN (cloud condensation nuclei) detector has been developed. For this purpose, sodium chloride and ammonium sulfate particles of known sizes were generated and their grown sizes were detected at the LACIS outlet. From these signals, the effective critical super-saturation was derived as a function of the LACIS wall temperature. With this, LACIS is calibrated for its use as a CCN detector. The applicability of LACIS for measurements of the droplet activation, and also of the hygroscopic growth of atmospheric aerosol particles was tested. The activation of the urban aerosol particles used in the measurements was found to occur at a critical super-saturation of 0.46% for particles with a dry diameter of 75 nm, and at 0.42% for 85 nm, respectively. Hygroscopic growth was measured for atmospheric aerosol particles with dry diameters of 150, 300 and 350 nm at relative humidities of 98 and 99%, and it was found that the larger dry particles contained a larger soluble volume fraction of about 0.85, compared to about 0.6 for the 150 nm particles.
- ItemCCN measurements at the Princess Elisabeth Antarctica research station during three austral summers(Göttingen : Copernicus GmbH, 2019) Herenz, P.; Wex, H.; Mangold, A.; Laffineur, Q.; Gorodetskaya, I.V.; Fleming, Z.L.; Panagi, M.; Stratmann, F.For three austral summer seasons (2013-2016, each from December to February) aerosol particles arriving at the Belgian Antarctic research station Princess Elisabeth (PE) in Dronning Maud Land in East Antarctica were characterized. This included number concentrations of total aerosol particles (N CN ) and cloud condensation nuclei (N CCN ), the particle number size distribution (PNSD), the aerosol particle hygroscopicity, and the influence of the air mass origin on N CN and N CCN . In general N CN was found to range from 40 to 6700cm -3 , with a median of 333cm -3 , while N CCN was found to cover a range between less than 10 and 1300cm-3 for supersaturations (SSs) between 0.1% and 0.7%. It is shown that the aerosol is dominated by the Aitken mode, being characterized by a significant amount of small, and therefore likely secondarily formed, aerosol particles, with 94% and 36% of the aerosol particles smaller than 90 and ≈35nm, respectively. Measurements of the basic meteorological parameters as well as the history of the air masses arriving at the measurement station indicate that the station is influenced by both marine air masses originating from the Southern Ocean and coastal areas around Antarctica (marine events - MEs) and continental air masses (continental events - CEs). CEs, which were defined as instances when the air masses spent at least 90% of the time over the Antarctic continent during the last 10 days prior to arrival at the measurements station, occurred during 61% of the time during which measurements were done. CEs came along with rather constant N CN and N CCN values, which we denote as Antarctic continental background concentrations. MEs, however, cause large fluctuations in N CN and N CCN , with low concentrations likely caused by scavenging due to precipitation and high concentrations likely originating from new particle formation (NPF) based on marine precursors. The application of HYSPLIT back trajectories in form of the potential source contribution function (PSCF) analysis indicate that the region of the Southern Ocean is a potential source of Aitken mode particles. On the basis of PNSDs, together with N CCN measured at an SS of 0.1%, median values for the critical diameter for cloud droplet activation and the aerosol particle hygroscopicity parameter ° were determined to be 110nm and 1, respectively. For particles larger than ĝ‰110nm the Southern Ocean together with parts of the Antarctic ice shelf regions were found to be potential source regions. While the former may contribute sea spray particles directly, the contribution of the latter may be due to the emission of sea salt aerosol particles, released from snow particles from surface snow layers, e.g., during periods of high wind speed, leading to drifting or blowing snow. The region of the Antarctic inland plateau, however, was not found to feature a significant source region for aerosol particles in general or page276 for cloud condensation nuclei measured at the PE station in the austral summer.
- ItemCharacterization of aerosol properties at Cyprus, focusing on cloud condensation nuclei and ice-nucleating particles(Göttingen : Copernicus GmbH, 2019) Gong, X.; Wex, H.; Müller, T.; Wiedensohler, A.; Höhler, K.; Kandler, K.; Ma, N.; Dietel, B.; Schiebel, T.; Möhler, O.; Stratmann, F.As part of the A-LIFE (Absorbing aerosol layers in a changing climate: aging, LIFEtime and dynamics) campaign, ground-based measurements were carried out in Paphos, Cyprus, to characterize the abundance, properties, and sources of aerosol particles in general and cloud condensation nuclei (CCN) and ice-nucleating particles (INP) in particular. New particle formation (NPF) events with subsequent growth of the particles into the CCN size range were observed. Aitken mode particles featured k values of 0.21 to 0.29, indicating the presence of organic materials. Accumulation mode particles featured a higher hygroscopicity parameter, with a median k value of 0.57, suggesting the presence of sulfate and maybe sea salt particles mixed with organic carbon. A clear downward trend of k with increasing supersaturation and decreasing dcrit was found. Super-micron particles originated mainly from sea-spray aerosol (SSA) and partly from mineral dust. INP concentrations (NINP) were measured in the temperature range from-6:5 to-26:5 °C, using two freezing array-type instruments. NINP at a particular temperature span around 1 order of magnitude below-20 °C and about 2 orders of magnitude at warmer temperatures (T >-18 °C). Few samples showed elevated concentrations at temperatures >-15 °C, which suggests a significant contribution of biological particles to the INP population, which possibly could originate from Cyprus. Both measured temperature spectra and NINP probability density functions (PDFs) indicate that the observed INP (ice active in the temperature range between-15 and-20 °C) mainly originate from long-range transport. There was no correlation between NINP and particle number concentration in the size range> 500 nm (N>500 nm). Parameterizations based on N>500 nm were found to overestimate NINP by about 1 to 2 orders of magnitude. There was also no correlation between NINP and particle surface area concentration. The ice active surface site density (ns) for the polluted aerosol encountered in the eastern Mediterranean in this study is about 1 to 3 orders of magnitude lower than the ns found for dust aerosol particles in previous studies. This suggests that observed NINP PDFs such as those derived here could be a better choice for modeling NINP if the aerosol particle composition is unknown or uncertain.
- ItemCoal fly ash: Linking immersion freezing behavior and physicochemical particle properties(Göttingen : Copernicus GmbH, 2018) Grawe, S.; Augustin-Bauditz, S.; Clemen, H.-C.; Ebert, M.; Eriksen Hammer, S.; Lubitz, J.; Reicher, N.; Rudich, Y.; Schneider, J.; Staacke, R.; Stratmann, F.; Welti, A.; Wex, H.To date, only a few studies have investigated the potential of coal fly ash particles to trigger heterogeneous ice nucleation in cloud droplets. The presented measurements aim at expanding the sparse dataset and improving process understanding of how physicochemical particle properties can influence the freezing behavior of coal fly ash particles immersed in water. Firstly, immersion freezing measurements were performed with two single particle techniques, i.e., the Leipzig Aerosol Cloud Interaction Simulator (LACIS) and the SPectrometer for Ice Nuclei (SPIN). The effect of suspension time on the efficiency of the coal fly ash particles when immersed in a cloud droplet is analyzed based on the different residence times of the two instruments and employing both dry and wet particle generation. Secondly, two cold-stage setups, one using microliter sized droplets (Leipzig Ice Nucleation Array) and one using nanoliter sized droplets (WeIzmann Supercooled Droplets Observation on Microarray setup) were applied. We found that coal fly ash particles are comparable to mineral dust in their immersion freezing behavior when being dry generated. However, a significant decrease in immersion freezing efficiency was observed during experiments with wet-generated particles in LACIS and SPIN. The efficiency of wet-generated particles is in agreement with the cold-stage measurements. In order to understand the reason behind the deactivation, a series of chemical composition, morphology, and crystallography analyses (single particle mass spectrometry, scanning electron microscopy coupled with energy dispersive X-ray microanalysis, X-ray diffraction analysis) were performed with dry- and wet-generated particles. From these investigations, we conclude that anhydrous CaSO4 and CaO - which, if investigated in pure form, show the same qualitative immersion freezing behavior as observed for dry-generated coal fly ash particles - contribute to triggering heterogeneous ice nucleation at the particle-water interface. The observed deactivation in contact with water is related to changes in the particle surface properties which are potentially caused by hydration of CaSO4 and CaO. The contribution of coal fly ash to the ambient population of ice-nucleating particles therefore depends on whether and for how long particles are immersed in cloud droplets.
- ItemA comprehensive laboratory study on the immersion freezing behavior of illite NX particles: A comparison of 17 ice nucleation measurement techniques(München : European Geopyhsical Union, 2015) Hiranuma, N.; Augustin-Bauditz, S.; Bingemer, H.; Budke, C.; Curtius, J.; Danielczok, A.; Diehl, K.; Dreischmeier, K.; Ebert, M.; Frank, F.; Hoffmann, N.; Kandler, K.; Kiselev, A.; Koop, T.; Leisner, T.; Möhler, O.; Nillius, B.; Peckhaus, A.; Rose, D.; Weinbruch, S.; Wex, H.; Boose, Y.; DeMott, P.J.; Hader, J.D.; Hill, T.C.J.; Kanji, Z.A.; Kulkarn, G.; Levin, E.J.T.; McCluskey, C.S.; Murakami, M.; Murray, B.J.; Niedermeier, D.; Petters, M.D.; O'Sullivan, D.; Saito, A.; Schill, G.P.; Tajiri, T.; Tolbert, M.A.; Welti, A.; Whale, T.F.; Wright, T.P.; Yamashita, K.Immersion freezing is the most relevant heterogeneous ice nucleation mechanism through which ice crystals are formed in mixed-phase clouds. In recent years, an increasing number of laboratory experiments utilizing a variety of instruments have examined immersion freezing activity of atmospherically relevant ice-nucleating particles. However, an intercomparison of these laboratory results is a difficult task because investigators have used different ice nucleation (IN) measurement methods to produce these results. A remaining challenge is to explore the sensitivity and accuracy of these techniques and to understand how the IN results are potentially influenced or biased by experimental parameters associated with these techniques. Within the framework of INUIT (Ice Nuclei Research Unit), we distributed an illite-rich sample (illite NX) as a representative surrogate for atmospheric mineral dust particles to investigators to perform immersion freezing experiments using different IN measurement methods and to obtain IN data as a function of particle concentration, temperature (T), cooling rate and nucleation time. A total of 17 measurement methods were involved in the data intercomparison. Experiments with seven instruments started with the test sample pre-suspended in water before cooling, while 10 other instruments employed water vapor condensation onto dry-dispersed particles followed by immersion freezing. The resulting comprehensive immersion freezing data set was evaluated using the ice nucleation active surface-site density, ns, to develop a representative ns(T) spectrum that spans a wide temperature range (−37 °C < T < −11 °C) and covers 9 orders of magnitude in ns. In general, the 17 immersion freezing measurement techniques deviate, within a range of about 8 °C in terms of temperature, by 3 orders of magnitude with respect to ns. In addition, we show evidence that the immersion freezing efficiency expressed in ns of illite NX particles is relatively independent of droplet size, particle mass in suspension, particle size and cooling rate during freezing. A strong temperature dependence and weak time and size dependence of the immersion freezing efficiency of illite-rich clay mineral particles enabled the ns parameterization solely as a function of temperature. We also characterized the ns(T) spectra and identified a section with a steep slope between −20 and −27 °C, where a large fraction of active sites of our test dust may trigger immersion freezing. This slope was followed by a region with a gentler slope at temperatures below −27 °C. While the agreement between different instruments was reasonable below ~ −27 °C, there seemed to be a different trend in the temperature-dependent ice nucleation activity from the suspension and dry-dispersed particle measurements for this mineral dust, in particular at higher temperatures. For instance, the ice nucleation activity expressed in ns was smaller for the average of the wet suspended samples and higher for the average of the dry-dispersed aerosol samples between about −27 and −18 °C. Only instruments making measurements with wet suspended samples were able to measure ice nucleation above −18 °C. A possible explanation for the deviation between −27 and −18 °C is discussed. Multiple exponential distribution fits in both linear and log space for both specific surface area-based ns(T) and geometric surface area-based ns(T) are provided. These new fits, constrained by using identical reference samples, will help to compare IN measurement methods that are not included in the present study and IN data from future IN instruments.
- ItemDesign and performance of an automatic regenerating adsorption aerosol dryer for continuous operation at monitoring sites(München : European Geopyhsical Union, 2009) Tuch, T.M.; Haudek, A.; Müller, T.; Nowak, A.; Wex, H.; Wiedensohler, A.Sizes of aerosol particles depend on the relative humidity of their carrier gas. Most monitoring networks require therefore that the aerosol is dried to a relative humidity below 50% r.H. to ensure comparability of measurements at different sites. Commercially available aerosol dryers are often not suitable for this purpose at remote monitoring sites. Adsorption dryers need to be regenerated frequently and maintenance-free single column Nafion dryers are not designed for high aerosol flow rates. We therefore developed an automatic regenerating adsorption aerosol dryer with a design flow rate of 1 m3/h. Particle transmission efficiency of this dryer has been determined during a 3 week experiment. The lower 50% cut-off was found to be smaller than 3 nm at the design flow rate of the instrument. Measured transmission efficiencies are in good agreement with theoretical calculations. One dryer has been successfully deployed in the Amazon river basin. We present data from this monitoring site for the first 6 months of measurements (February 2008–August 2008). Apart from one unscheduled service, this dryer did not require any maintenance during this time period. The average relative humidity of the dried aerosol was 27.1+/−7.5% r.H. compared to an average ambient relative humidity of nearly 80% and temperatures around 30°C. This initial deployment demonstrated that these dryers are well suitable for continuous operation at remote monitoring sites under adverse ambient conditions.
- ItemEvolution of particle composition in CLOUD nucleation experiments(München : European Geopyhsical Union, 2013) Keskinen, H.; Virtanen, A.; Joutsensaari, J.; Tsagkogeorgas, G.; Duplissy, J.; Schobesberger, S.; Gysel, M.; Riccobono, F.; Slowik, J.G.; Bianchi, F.; Yli-Juuti, T.; Lehtipalo, K.; Rondo, L.; Breitenlechner, M.; Kupc, A.; Almeida, J.; Amorim, A.; Dunne, E.M.; Downard, A.J.; Ehrhart, S.; Franchin, A.; Kajos, M.K.; Kirkby, J.; Kürten, A.; Nieminen, T.; Makhmutov, V.; Mathot, S.; Miettinen, P.; Onnela, A.; Petäjä, T.; Praplan, A.; Santos, F.D.; Schallhart, S.; Sipilä, M.; Stozhkov, Y.; Tomé, A.; Vaattovaara, P.; Wimmer, D.; Prevot, A.; Dommen, J.; Donahue, N.M.; Flagan, R.C.; Weingartner, E.; Viisanen, Y.; Riipinen, I.; Hansel, A.; Curtius, J.; Kulmala, M.; Worsnop, D.R.; Baltensperger, U.; Wex, H.; Stratmann, F.; Laaksonen, A.Sulphuric acid, ammonia, amines, and oxidised organics play a crucial role in nanoparticle formation in the atmosphere. In this study, we investigate the composition of nucleated nanoparticles formed from these compounds in the CLOUD (Cosmics Leaving Outdoor Droplets) chamber experiments at CERN (Centre européen pour la recherche nucléaire). The investigation was carried out via analysis of the particle hygroscopicity, ethanol affinity, oxidation state, and ion composition. Hygroscopicity was studied by a hygroscopic tandem differential mobility analyser and a cloud condensation nuclei counter, ethanol affinity by an organic differential mobility analyser and particle oxidation level by a high-resolution time-of-flight aerosol mass spectrometer. The ion composition was studied by an atmospheric pressure interface time-of-flight mass spectrometer. The volume fraction of the organics in the particles during their growth from sizes of a few nanometers to tens of nanometers was derived from measured hygroscopicity assuming the Zdanovskii–Stokes–Robinson relationship, and compared to values gained from the spectrometers. The ZSR-relationship was also applied to obtain the measured ethanol affinities during the particle growth, which were used to derive the volume fractions of sulphuric acid and the other inorganics (e.g. ammonium salts). In the presence of sulphuric acid and ammonia, particles with a mobility diameter of 150 nm were chemically neutralised to ammonium sulphate. In the presence of oxidation products of pinanediol, the organic volume fraction of freshly nucleated particles increased from 0.4 to ~0.9, with an increase in diameter from 2 to 63 nm. Conversely, the sulphuric acid volume fraction decreased from 0.6 to 0.1 when the particle diameter increased from 2 to 50 nm. The results provide information on the composition of nucleated aerosol particles during their growth in the presence of various combinations of sulphuric acid, ammonia, dimethylamine and organic oxidation products.
- ItemExperimental study of the role of physicochemical surface processing on the IN ability of mineral dust particles(München : European Geopyhsical Union, 2011) Niedermeier, D.; Hartmann, S.; Clauss, T.; Wex, H.; Kiselev, A.; Sullivan, R.C.; DeMott, P.J.; Petters, M.D.; Reitz, P.; Schneider, J.; Mikhailov, E.; Sierau, B.; Stetzer, O.; Reimann, B.; Bundke, U.; Shaw, R.A.; Buchholz, A.; Mentel, T.F.; Stratmann, F.During the measurement campaign FROST 2 (FReezing Of duST 2), the Leipzig Aerosol Cloud Interaction Simulator (LACIS) was used to investigate the influence of various surface modifications on the ice nucleating ability of Arizona Test Dust (ATD) particles in the immersion freezing mode. The dust particles were exposed to sulfuric acid vapor, to water vapor with and without the addition of ammonia gas, and heat using a thermodenuder operating at 250 °C. Size selected, quasi monodisperse particles with a mobility diameter of 300 nm were fed into LACIS and droplets grew on these particles such that each droplet contained a single particle. Temperature dependent frozen fractions of these droplets were determined in a temperature range between −40 °C ≤T≤−28 °C. The pure ATD particles nucleated ice over a broad temperature range with their freezing behavior being separated into two freezing branches characterized through different slopes in the frozen fraction vs. temperature curves. Coating the ATD particles with sulfuric acid resulted in the particles' IN potential significantly decreasing in the first freezing branch (T>−35 °C) and a slight increase in the second branch (T≤−35 °C). The addition of water vapor after the sulfuric acid coating caused the disappearance of the first freezing branch and a strong reduction of the IN ability in the second freezing branch. The presence of ammonia gas during water vapor exposure had a negligible effect on the particles' IN ability compared to the effect of water vapor. Heating in the thermodenuder led to a decreased IN ability of the sulfuric acid coated particles for both branches but the additional heat did not or only slightly change the IN ability of the pure ATD and the water vapor exposed sulfuric acid coated particles. In other words, the combination of both sulfuric acid and water vapor being present is a main cause for the ice active surface features of the ATD particles being destroyed. A possible explanation could be the chemical transformation of ice active metal silicates to metal sulfates. The strongly enhanced reaction between sulfuric acid and dust in the presence of water vapor and the resulting significant reductions in IN potential are of importance for atmospheric ice cloud formation. Our findings suggest that the IN concentration can decrease by up to one order of magnitude for the conditions investigated.
- ItemThe fine-scale structure of the trade wind cumuli over Barbados – An introduction to the CARRIBA project(München : European Geopyhsical Union, 2013) Siebert, H.; Beals, M.; Bethke, J.; Bierwirth, E.; Conrath, T.; Dieckmann, K.; Ditas, F.; Ehrlich, A.; Farrell, D.; Hartmann, S.; Izaguirre, M.A.; Katzwinkel, J.; Nuijens, L.; Roberts, G.; Schäfer, M.; Shaw, R.A.; Schmeissner, T.; Serikov, I.; Stevens, B.; Stratmann, F.; Wehner, B.; Wendisch, M.; Werner, F.; Wex, H.The CARRIBA (Cloud, Aerosol, Radiation and tuRbulence in the trade wInd regime over BArbados) project, focused on high resolution and collocated measurements of thermodynamic, turbulent, microphysical, and radiative properties of trade wind cumuli over Barbados, is introduced. The project is based on two one-month field campaigns in November 2010 (climatic wet season) and April 2011 (climatic dry season). Observations are based on helicopter-borne and ground-based measurements in an area of 100 km2 off the coast of Barbados. CARRIBA is accompanied by long-term observations at the Barbados Cloud Observatory located at the East coast of Barbados since early in 2010 and which provides a longer-term context for the CARRIBA measurements. The deployed instrumentation and sampling strategy are presented together with a classification of the meteorological conditions. The two campaigns were influenced by different air masses advected from the Caribbean area, the Atlantic Ocean, and the African continent which led to distinct aerosol conditions. Pristine conditions with low aerosol particle number concentrations of ~100 cm3 were alternating with periods influenced by Saharan dust or aerosol from biomass burning resulting in comparably high number concentrations of ~ 500 cm3. The biomass burning aerosol was originating from both the Caribbean area and Africa. The shallow cumulus clouds responded to the different aerosol conditions with a wide range of mean droplet sizes and number concentrations. Two days with different aerosol and cloud microphysical properties but almost identical meteorological conditions have been analyzed in detail. The differences in the droplet number concentration and droplet sizes appear not to show any significant change for turbulent cloud mixing, but the relative roles of droplet inertia and sedimentation in initiating coalescence, as well as the cloud reflectivity, do change substantially.
- ItemThe Global Aerosol Synthesis and Science Project (GASSP): Measurements and Modeling to Reduce Uncertainty(Boston, Mass. : ASM, 2017) Reddington, C.L.; Carslaw, K.S.; Stier, P.; Schutgens, N.; Coe, H.; Liu, D.; Allan, J.; Browse, J.; Pringle, K.J.; Lee, L.A.; Yoshioka, M.; Johnson, J.S.; Regayre, L.A.; Spracklen, D.V.; Mann, G.W.; Clarke, A.; Hermann, M.; Henning, S.; Wex, H.; Kristensen, T.B.; Leaitch, W.R.; Pöschl, U.; Rose, D.; Andreae, M.O.; Schmale, J.; Kondo, Y.; Oshima, N.; Schwarz, J.P.; Nenes, A.; Anderson, B.; Roberts, G.C.; Snider, J.R.; Leck, C.; Quinn, P.K.; Chi, X.; Ding, A.; Jimenez, J.L.; Zhang, Q.The largest uncertainty in the historical radiative forcing of climate is caused by changes in aerosol particles due to anthropogenic activity. Sophisticated aerosol microphysics processes have been included in many climate models in an effort to reduce the uncertainty. However, the models are very challenging to evaluate and constrain because they require extensive in situ measurements of the particle size distribution, number concentration, and chemical composition that are not available from global satellite observations. The Global Aerosol Synthesis and Science Project (GASSP) aims to improve the robustness of global aerosol models by combining new methodologies for quantifying model uncertainty, to create an extensive global dataset of aerosol in situ microphysical and chemical measurements, and to develop new ways to assess the uncertainty associated with comparing sparse point measurements with low-resolution models. GASSP has assembled over 45,000 hours of measurements from ships and aircraft as well as data from over 350 ground stations. The measurements have been harmonized into a standardized format that is easily used by modelers and nonspecialist users. Available measurements are extensive, but they are biased to polluted regions of the Northern Hemisphere, leaving large pristine regions and many continental areas poorly sampled. The aerosol radiative forcing uncertainty can be reduced using a rigorous model–data synthesis approach. Nevertheless, our research highlights significant remaining challenges because of the difficulty of constraining many interwoven model uncertainties simultaneously. Although the physical realism of global aerosol models still needs to be improved, the uncertainty in aerosol radiative forcing will be reduced most effectively by systematically and rigorously constraining the models using extensive syntheses of measurements.
- ItemHeterogeneous freezing of droplets with immersed mineral dust particles – measurements and parameterization(München : European Geopyhsical Union, 2010) Niedermeier, D.; Hartmann, S.; Shaw, R.A.; Covert, D.; Mentel, T.F.; Schneider, J.; Mentel, T.F.; Poulain, L.; Reitz, P.; Spindler, C.; Clauss, T.; Kiselev, A.; Hallbauer, E.; Wex, H.; Mildenberger, K.; Stratmann, F.During the measurement campaign FROST (FReezing Of duST), LACIS (Leipzig Aerosol Cloud Interaction Simulator) was used to investigate the immersion freezing behavior of size selected, coated and uncoated Arizona Test Dust (ATD) particles with a mobility diameter of 300 nm. Particles were coated with succinic acid (C4H6O4), sulfuric acid (H2SO4) and ammonium sulfate ((NH4)2SO4). Ice fractions at mixed-phase cloud temperatures ranging from 233.15 K to 239.15 K (±0.60 K) were determined for all types of particles. In this temperature range, pure ATD particles and those coated with C4H6O4 or small amounts of H2SO4 were found to be the most efficient ice nuclei (IN). ATD particles coated with (NH4)2SO4 were the most inefficient IN. Since the supercooled droplets were highly diluted before freezing occurred, a freezing point suppression due to the soluble material on the particles (and therefore in the droplets) cannot explain this observation. Therefore, it is reasonable to assume that the coatings lead to particle surface alterations which cause the differences in the IN abilities. Two different theoretical approaches based on the stochastic and the singular hypotheses were applied to clarify and parameterize the freezing behavior of the particles investigated. Both approaches describe the experimentally determined results, yielding parameters that can subsequently be used to compare our results to those from other studies. However, we cannot clarify at the current state which of the two approaches correctly describes the investigated immersion freezing process. But both approaches confirm the assumption that the coatings lead to particle surface modifications lowering the nucleation efficiency. The stochastic approach interprets the reduction in nucleation rate from coating as primarily due to an increase in the thermodynamic barrier for ice formation (i.e., changes in interfacial free energies). The singular approach interprets the reduction as resulting from a reduced surface density of active sites.
- ItemHeterogeneous ice nucleation: Exploring the transition from stochastic to singular freezing behavior(München : European Geopyhsical Union, 2011) Niedermeier, D.; Shaw, R.A.; Hartmann, S.; Wex, H.; Clauss, T.; Voigtländer, J.; Stratmann, F.Heterogeneous ice nucleation, a primary pathway for ice formation in the atmosphere, has been described alternately as being stochastic, in direct analogy with homogeneous nucleation, or singular, with ice nuclei initiating freezing at deterministic temperatures. We present an idealized, conceptual model to explore the transition between stochastic and singular ice nucleation. This "soccer ball" model treats particles as being covered with surface sites (patches of finite area) characterized by different nucleation barriers, but with each surface site following the stochastic nature of ice embryo formation. The model provides a phenomenological explanation for seemingly contradictory experimental results obtained in our research groups. Even with ice nucleation treated fundamentally as a stochastic process this process can be masked by the heterogeneity of surface properties, as might be typical for realistic atmospheric particle populations. Full evaluation of the model findings will require experiments with well characterized ice nucleating particles and the ability to vary both temperature and waiting time for freezing.
- ItemHomogeneous and heterogeneous ice nucleation at LACIS: Operating principle and theoretical studies(München : European Geopyhsical Union, 2011) Hartmann, S.; Niedermeier, D.; Voigtländer, J.; Clauss, T.; Shaw, R.A.; Wex, H.; Kiselev, A.; Stratmann, F.At the Leipzig Aerosol Cloud Interaction Simulator (LACIS) experiments investigating homogeneous and heterogeneous nucleation of ice (particularly immersion freezing in the latter case) have been carried out. Here both the physical LACIS setup and the numerical model developed to design experiments at LACIS and interpret their results are presented in detail. Combining results from the numerical model with experimental data, it was found that for the experimental parameter space considered, classical homogeneous ice nucleation theory is able to predict the freezing behavior of highly diluted ammonium sulfate solution droplets, while classical heterogeneous ice nucleation theory, together with the assumption of a constant contact angle, fails to predict the immersion freezing behavior of surrogate mineral dust particles (Arizona Test Dust, ATD). The main reason for this failure is the compared to experimental data apparently overly strong temperature dependence of the nucleation rate coefficient. Assuming, in the numerical model, Classical Nucleation Theory (CNT) for homogeneous ice nucleation and a CNT-based parameterization for the nucleation rate coefficient in the immersion freezing mode, recently published by our group, it was found that even for a relatively effective ice nucleating agent such as pure ATD, there is a temperature range where homogeneous ice nucleation is dominant. The main explanation is the apparently different temperature dependencies of the two freezing mechanisms. Finally, reviewing the assumptions made during the derivation of the CNT-based parameterization for immersion freezing, it was found that the assumption of constant temperature during ice nucleation and the chosen ice nucleation time were justified, underlining the applicability of the method to determine the fitting coefficients in the parameterization equation.
- ItemHygroscopic growth and activation of HULIS particles: Experimental data and a new iterative parameterization scheme for complex aerosol particles(München : European Geopyhsical Union, 2008) Ziese, M.; Wex, H.; Nilsson, E.; Salma, I.; Ocskay, R.; Hennig, T.; Massling, A.; Stratmann, F.The hygroscopic growth and activation of two HULIS (HUmic LIke Substance) and one Aerosol-Water-Extract sample, prepared from urban-type aerosol, were investigated. All samples were extracted from filters, redissolved in water and atomized for the investigations presented here. The hygroscopic growth measurements were done using LACIS (Leipzig Aerosol Cloud Interaction Simulator) together with a HH-TDMA (High Humidity Tandem Differential Mobility Analyzer). Hygroscopic growth was determined for relative humidities (RHs) up to 99.75%. The critical diameters for activation were measured for supersaturations between 0.2 and 1%. All three samples showed a similar hygroscopic growth behavior, and the two HULIS samples also were similar in their activation behavior, while the Aerosol-Water-Extract turned out to be more CCN active than the HULIS samples. The experimental data was used to derive parameterizations for the hygroscopic growth and activation of HULIS particles. The concept of ρion (Wex et al., 2007a) and the Szyszkowski-equation (Szyszkowski, 1908; Facchini, 1999) were used for parameterizing the Raoult and the Kelvin (surface tension) terms of the Köhler equation, respectively. This concept proved to be very successful for the HULIS samples in the saturation range from RHs larger than 98% up to activation. It was also shown to work well with data on HULIS taken from literature. Here, different atmospheric life-times and/or different sources for the different samples showed up in different coefficients for the parameterization. However, the parameterization did not work out well for the Aerosol-Water-Extract.
- ItemIce nucleation by water-soluble macromolecules(München : European Geopyhsical Union, 2015) Pummer, B.G.; Budke, C.; Augustin-Bauditz, S.; Niedermeier, D.; Felgitsch, L.; Kampf, C.J.; Huber, R.G.; Liedl, K.R.; Loerting, T.; Moschen, T.; Schauperl, M.; Tollinger, M.; Morris, C.E.; Wex, H.; Grothe, H.; Pöschl, U.; Koop, T.; Fröhlich-Nowoisky, J.Cloud glaciation is critically important for the global radiation budget (albedo) and for initiation of precipitation. But the freezing of pure water droplets requires cooling to temperatures as low as 235 K. Freezing at higher temperatures requires the presence of an ice nucleator, which serves as a template for arranging water molecules in an ice-like manner. It is often assumed that these ice nucleators have to be insoluble particles. We point out that also free macromolecules which are dissolved in water can efficiently induce ice nucleation: the size of such ice nucleating macromolecules (INMs) is in the range of nanometers, corresponding to the size of the critical ice embryo. As the latter is temperature-dependent, we see a correlation between the size of INMs and the ice nucleation temperature as predicted by classical nucleation theory. Different types of INMs have been found in a wide range of biological species and comprise a variety of chemical structures including proteins, saccharides, and lipids. Our investigation of the fungal species Acremonium implicatum, Isaria farinosa, and Mortierella alpina shows that their ice nucleation activity is caused by proteinaceous water-soluble INMs. We combine these new results and literature data on INMs from fungi, bacteria, and pollen with theoretical calculations to develop a chemical interpretation of ice nucleation and water-soluble INMs. This has atmospheric implications since many of these INMs can be released by fragmentation of the carrier cell and subsequently may be distributed independently. Up to now, this process has not been accounted for in atmospheric models.
- ItemImmersion freezing of birch pollen washing water(München : European Geopyhsical Union, 2013) Augustin, S.; Wex, H.; Niedermeier, D.; Pummer, B.; Grothe, H.; Hartmann, S.; Tomsche, L.; Clauss, T.; Voigtländer, J.; Ignatius, K.; Stratmann, F.Birch pollen grains are known to be ice nucleating active biological particles. The ice nucleating activity has previously been tracked down to biological macromolecules that can be easily extracted from the pollen grains in water. In the present study, we investigated the immersion freezing behavior of these ice nucleating active (INA) macromolecules. Therefore we measured the frozen fractions of particles generated from birch pollen washing water as a function of temperature at the Leipzig Aerosol Cloud Interaction Simulator (LACIS). Two different birch pollen samples were considered, with one originating from Sweden and one from the Czech Republic. For the Czech and Swedish birch pollen samples, freezing was observed to start at −19 and −17 °C, respectively. The fraction of frozen droplets increased for both samples down to −24 °C. Further cooling did not increase the frozen fractions any more. Instead, a plateau formed at frozen fractions below 1. This fact could be used to determine the amount of INA macromolecules in the droplets examined here, which in turn allowed for the determination of nucleation rates for single INA macromolecules. The main differences between the Swedish birch pollen and the Czech birch pollen were obvious in the temperature range between −17 and −24 °C. In this range, a second plateau region could be seen for Swedish birch pollen. As we assume INA macromolecules to be the reason for the ice nucleation, we concluded that birch pollen is able to produce at least two different types of INA macromolecules. We were able to derive parameterizations for the heterogeneous nucleation rates for both INA macromolecule types, using two different methods: a simple exponential fit and the Soccer ball model. With these parameterization methods we were able to describe the ice nucleation behavior of single INA macromolecules from both the Czech and the Swedish birch pollen.
- ItemThe immersion mode ice nucleation behavior of mineral dusts: A comparison of different pure and surface modified dusts(Hoboken, NJ : Wiley, 2014) Augustin-Bauditz, S.; Wex, H.; Kanter, S.; Ebert, M.; Niedermeier, D.; Stolz, F.; Prager, A.; Stratmann, F.In this study we present results from immersion freezing experiments with size-segregated mineral dust particles. Besides two already existing data sets for Arizona Test Dust (ATD), and Fluka kaolinite, we show two new data sets for illite-NX, which consists mainly of illite, a clay mineral, and feldspar, a common crustal material. The experiments were carried out with the Leipzig Aerosol Cloud Interaction Simulator. After comparing the different dust samples, it became obvious that the freezing ability was positively correlated with the K-feldspar content. Furthermore, a comparison of the composition of the ATD, illite-NX, and feldspar samples suggests that within the K-feldspars, microcline is more ice nucleation active than orthoclase. A coating with sulfuric acid leads to a decrease in the ice nucleation ability of all mineral dusts, with the effect being more pronounced for the feldspar sample. Key Points The freezing ability of mineral dusts correlated with the K-feldspar contentAmong feldspars, microcline shows a better ice nucleation ability than orthoclaseAfter coating, all investigated dusts feature a similar ice nucleation ability.
- ItemInfluence of cloud processing on CCN activation behaviour in the Thuringian Forest, Germany during HCCT-2010(München : European Geopyhsical Union, 2014) Henning, S.; Dieckmann, K.; Ignatius, K.; Schäfer, M.; Zedler, P.; Harris, E.; Sinha, B.; van Pinxteren, D.; Mertes, S.; Birmili, W.; Merkel, M.; Wu, Z.; Wiedensohler, A.; Wex, H.; Herrmann, H.; Stratmann, F.Within the framework of the "Hill Cap Cloud Thuringia 2010" (HCCT-2010) international cloud experiment, the influence of cloud processing on the activation properties of ambient aerosol particles was investigated. Particles were probed upwind and downwind of an orographic cap cloud on Mt Schmücke, which is part of a large mountain ridge in Thuringia, Germany. The activation properties of the particles were investigated by means of size-segregated cloud condensation nuclei (CCN) measurements at 3 to 4 different supersaturations. The observed CCN spectra together with the total particle spectra were used to calculate the hygroscopicity parameter κ for the upwind and downwind stations. The upwind and downwind critical diameters and κ values were then compared for defined cloud events (FCE) and non-cloud events (NCE). Cloud processing was found to increase the hygroscopicity of the aerosol particles significantly, with an average increase in κ of 50%. Mass spectrometry analysis and isotopic analysis of the particles suggest that the observed increase in the hygroscopicity of the cloud-processed particles is due to an enrichment of sulfate and possibly also nitrate in the particle phase.