Aerosol Interaction And Dynamics In The Atmosphere Research Articles

Influence of surface morphology on the immersion mode ice nucleation efficiency of hematite particles

Abstract. In this paper, the effect of the morphological modification of aerosol particles with respect to heterogeneous ice nucleation is comprehensively investigated for laboratory-generated hematite particles as a model substrate for atmospheric dust particles. The surface-area-scaled ice nucleation efficiencies of monodisperse cubic hematite particles and milled hematite particles were measured with a series of expansion cooling experiments using the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) cloud simulation chamber. Complementary offline characterization of physico-chemical properties of both hematite subsets were also carried out with scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy, dynamic light scattering (DLS), and an electro-kinetic particle charge detector to further constrain droplet-freezing measurements of hematite particles. Additionally, an empirical parameterization derived from our laboratory measurements was implemented in the single-column version of the Community Atmospheric Model version 5 (CAM5) to investigate the model sensitivity in simulated ice crystal number concentration on different ice nucleation efficiencies. From an experimental perspective, our results show that the immersion mode ice nucleation efficiency of milled hematite particles is almost an order of magnitude higher at −35.2 °C < T < −33.5 °C than that of the cubic hematite particles, indicating a substantial effect of morphological irregularities on immersion mode freezing. Our modeling results similarly show that the increased droplet-freezing rates of milled hematite particles lead to about one order magnitude higher ice crystal number in the upper troposphere than cubic hematite particles. Overall, our results suggest that the surface irregularities and associated active sites lead to greater ice activation through droplet freezing.

Integrative fitting of absorption line profiles with high accuracy, robustness, and speed

The principle of the integrative evaluation of absorption line profiles relies on the numeric integration of absorption line signals to retrieve absorber concentrations, e.g., of trace gases. Thus, it is a fast and robust technique. However, previous implementations of the integrative evaluation principle showed shortcomings in terms of accuracy and the lack of a fit quality indicator. This has motivated the development of an advanced integrative (AI) fitting algorithm. The AI fitting algorithm retains the advantages of previous integrative implementations—robustness and speed—and is able to achieve high accuracy by introduction of a novel iterative fitting process. A comparison of the AI fitting algorithm with the widely used Levenberg–Marquardt (LM) fitting algorithm indicates that the AI algorithm has advantages in terms of robustness due to its independence from appropriately chosen start values for the initialization of the fitting process. In addition, the AI fitting algorithm shows speed advantages typically resulting in a factor of three to four shorter computational times on a standard personal computer. The LM algorithm on the other hand retains advantages in terms of a much higher flexibility, as the AI fitting algorithm is restricted to the evaluation of single absorption lines with precomputed line width. Comparing both fitting algorithms for the specific application of in situ laser hygrometry at 1,370 nm using direct tunable diode laser absorption spectroscopy (TDLAS) suggests that the accuracy of the AI algorithm is equivalent to that of the LM algorithm. For example, a signal-to-noise ratio of 80 and better typically yields a deviation of <1 % between both fitting algorithms. The properties of the AI fitting algorithm make it an interesting alternative if robustness and speed are crucial in an application and if the restriction to a single absorption line is possible. These conditions are fulfilled for the 1,370 nm TDLAS hygrometry at the aerosol and cloud chamber aerosol interactions and dynamics in the atmosphere (AIDA)—a unique large-scale facility to study atmospheric processes. The robustness of the AI fitting algorithm has been validated for typical AIDA conditions encompassing strong transmission fluctuations during the formation of droplet or ice clouds inside AIDA. Under these conditions, the stability of the AI algorithm remained virtually unaffected. Thus, the AI algorithm presents an alternative technique for a fast, reliable, and accurate online data evaluation of the humidity measurements at AIDA.

Heterogeneous ice nucleation ability of crystalline sodium chloride dihydrate particles

AbstractThe aerosol and cloud chamber AIDA (Aerosol Interactions and Dynamics in the Atmosphere) of the Karlsruhe Institute of Technology has been used to quantify the deposition mode ice nucleation ability of airborne crystalline sodium chloride dihydrate (NaCl ∙ 2H2O) particles with median diameters between 0.06 and 1.1 µm. For this purpose, expansion cooling experiments with starting temperatures from 235 to 216 K were conducted. Recently, supermicron‐sized NaCl ∙ 2H2O particles deposited onto a surface have been observed to be ice‐active in the deposition mode at temperatures below 238 K, requiring a median threshold ice saturation ratio of only 1.02 in the range from 238 to 221 K. In AIDA, heterogeneous ice nucleation by NaCl ∙ 2H2O was first detected at a temperature of 227.1 K with a concomitant threshold ice saturation ratio of 1.25. Above that temperature, the crystallized salt particles underwent a deliquescence transition to form aqueous NaCl solution droplets upon increasing relative humidity. At nucleation temperatures below 225 K, the inferred threshold ice saturation ratios varied between 1.15 and 1.20. The number concentration of the nucleated ice crystals was related to the surface area of the seed aerosol particles to deduce the ice nucleation active surface site (INAS) density of the aerosol population as a function of the ice supersaturation. Maximum INAS densities of about 6 ⋅ 1010 m−2 at an ice saturation ratio of 1.20 were found for temperatures below 225 K. These INAS densities are similar to those recently derived for deposition mode ice nucleation on mineral dust particles.

The accommodation coefficient of water molecules on ice – cirrus cloud studies at the AIDA simulation chamber

Abstract. Cirrus clouds and their impact on the Earth's radiative budget are subjects of current research. The processes governing the growth of cirrus ice particles are central to the radiative properties of cirrus clouds. At temperatures relevant to cirrus clouds, the growth of ice crystals smaller than a few microns in size is strongly influenced by the accommodation coefficient of water molecules on ice, αice, making this parameter relevant for cirrus cloud modeling. However, the experimentally determined magnitude of αice for cirrus temperatures is afflicted with uncertainties of almost three orders of magnitude, and values for αice derived from cirrus cloud data lack significance so far. This has motivated dedicated experiments at the cloud chamber AIDA (Aerosol Interactions and Dynamics in the Atmosphere) to determine αice in the cirrus-relevant temperature interval between 190 K and 235 K under realistic cirrus ice particle growth conditions. The experimental data sets have been evaluated independently with two model approaches: the first relying on the newly developed model SIGMA (Simple Ice Growth Model for determining Alpha), the second one on an established model, ACPIM (Aerosol-Cloud-Precipitation Interaction Model). Within both approaches a careful uncertainty analysis of the obtained αice values has been carried out for each AIDA experiment. The results show no significant dependence of αice on temperature between 190 K and 235 K. In addition, we find no evidence for a dependence of αice on ice particle size or on water vapor supersaturation for ice particles smaller than 20 μm and supersaturations of up to 70%. The temperature-averaged and combined result from both models is αice = 0.7−0.5+0.3, which implies that αice may only exert a minor impact on cirrus clouds and their characteristics when compared to the assumption of αice =1. Impact on prior calculations of cirrus cloud properties, e.g., in climate models, with αice typically chosen in the range 0.2–1 is thus expected to be negligible. In any case, we provide a well-constrained αice which future cirrus model studies can rely on.

A Novel Adiabatic-Expansion-Type Cloud Simulation Chamber

A new cloud simulation chamber was built at the Meteorological Research Institute (MRI) to investigate the details of the fundamental processes of cloud formation. The MRI cloud chamber was designed as an adiabatic-expansion-type cloud chamber covering temperatures from 30 to −100°C, pressures from 1030 to 30 hPa, and an evacuation rates corresponding to ascent rates from 0 to 30 m s−1. Improvements to the cooling system and cloud characterization instrumentation distinguish the new facility from past devices of this type that are no longer functional (e.g., the Colorado State University dynamic cloud chamber), and the capabilities exceed those of any other active facility (e.g., the Aerosol Interactions and Dynamics in the Atmosphere (AIDA) chamber) for covering a range of atmospheric conditions while reproducing approximately adiabatic parcel conditions. Results from the preliminary experiments demonstrate the accuracy of coordinated pressure and temperature controls to reproduce cloud formation processes (both dry and wet adiabatic expansion processes) and the ability of the chamber's instrumentation to measure aerosol, cloud droplet, and ice crystal characteristics. Performance tests demonstrate the chamber's usefulness as a facility to investigate cloud droplet and ice crystal formation processes through the activation of various types of aerosol particles.

Hygroscopic growth and droplet activation of soot particles: uncoated, succinic or sulfuric acid coated

Abstract. The hygroscopic growth and droplet activation of uncoated soot particles and such coated with succinic acid and sulfuric acid were investigated during the IN-11 campaign at the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) facility. A GFG-1000 soot generator applying either nitrogen or argon as carrier gas and a miniCAST soot generator were utilized to generate soot particles. Different organic carbon (OC) to black carbon (BC) ratios were adjusted for the CAST-soot by varying the fuel to air ratio. The hygroscopic growth was investigated by means of the mobile Leipzig Aerosol Cloud Interaction Simulator (LACIS-mobile) and two different Hygroscopicity Tandem Differential Mobility Analyzers (HTDMA, VHTDMA). Two Cloud Condensation Nucleus Counter (CCNC) were applied to measure the activation of the particles. For the untreated soot particles neither hygroscopic growth nor activation was observed at a supersaturation of 1%, with exception of a partial activation of GFG-soot generated with argon as carrier gas. Coatings of succinic acid lead to a detectable hygroscopic growth of GFG-soot and enhanced the activated fraction of GFG- (carrier gas: argon) and CAST-soot, whereas no hygroscopic growth of the coated CAST-soot was found. Sulfuric acid coatings led to an OC-content dependent hygroscopic growth of CAST-soot. Such a dependence was not observed for activation measurements. Coating with sulfuric acid decreased the amount of Polycyclic Aromatic Hydrocarbons (PAH), which were detected by AMS-measurements in the CAST-soot, and increased the amount of substances with lower molecular weight than the initial PAHs. We assume that these reaction products increased the hygroscopicity of the coated particles in addition to the coating substance itself.

A Particle-Surface-Area-Based Parameterization of Immersion Freezing on Desert Dust Particles

Abstract In climate and weather models, the quantitative description of aerosol and cloud processes relies on simplified assumptions. This contributes major uncertainties to the prediction of global and regional climate change. Therefore, models need good parameterizations for heterogeneous ice nucleation by atmospheric aerosols. Here the authors present a new parameterization of immersion freezing on desert dust particles derived from a large number of experiments carried out at the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) cloud chamber facility. The parameterization is valid in the temperature range between −12° and −36°C at or above water saturation and can be used in atmospheric models that include information about the dust surface area. The new parameterization was applied to calculate distribution maps of ice nuclei during a Saharan dust event based on model results from the regional-scale model Consortium for Small-Scale Modelling–Aerosols and Reactive Trace Gases (COSMO-ART). The results were then compared to measurements at the Taunus Observatory on Mount Kleiner Feldberg, Germany, and to three other parameterizations applied to the dust outbreak. The aerosol number concentration and surface area from the COSMO-ART model simulation were taken as input to different parameterizations. Although the surface area from the model agreed well with aerosol measurements during the dust event at Kleiner Feldberg, the ice nuclei (IN) number concentration calculated from the new surface-area-based parameterization was about a factor of 13 less than IN measurements during the same event. Systematic differences of more than a factor of 10 in the IN number concentration were also found among the different parameterizations. Uncertainties in the modeled and measured parameters probably both contribute to this discrepancy and should be addressed in future studies.

Ice nucleation properties of fine ash particles from the Eyjafjallajökull eruption in April 2010

Abstract. During the eruption of the Eyjafjallajökull volcano in the south of Iceland in April/May 2010, about 40 Tg of ash mass were emitted into the atmosphere. It was unclear whether volcanic ash particles with d &lt; 10 μm facilitate the glaciation of clouds. Thus, ice nucleation properties of volcanic ash particles were investigated in AIDA (Aerosol Interaction and Dynamics in the Atmosphere) cloud chamber experiments simulating atmospherically relevant conditions. The ash sample that was used for our experiments had been collected at a distance of 58 km from the Eyjafjallajökull during the eruption period in April 2010. The temperature range covered by our ice nucleation experiments extended from 219 to 264 K, and both ice nucleation via immersion freezing and deposition nucleation could be observed. Immersion freezing was first observed at 252 K, whereas the deposition nucleation onset lay at 242 K and RHice =126%. About 0.1% of the volcanic ash particles were active as immersion freezing nuclei at a temperature of 249 K. For deposition nucleation, an ice fraction of 0.1% was observed at around 233 K and RHice =116%. Taking ice-active surface site densities as a measure for the ice nucleation efficiency, volcanic ash particles are similarly efficient ice nuclei in immersion freezing mode (ns,imm ~ 109 m−2 at 247 K) compared to certain mineral dusts. For deposition nucleation, the observed ice-active surface site densities ns,dep were found to be 1011 m−2 at 224 K and RHice =116%. Thus, volcanic ash particles initiate deposition nucleation more efficiently than Asian and Saharan dust but appear to be poorer ice nuclei than ATD particles. Based on the experimental data, we have derived ice-active surface site densities as a function of temperature for immersion freezing and of relative humidity over ice and temperature for deposition nucleation.

Studies of propane flame soot acting as heterogeneous ice nuclei in conjunction with single particle soot photometer measurements

Abstract. The ice nucleation efficiency of propane flame soot particles with and without a sulphuric acid coating was investigated using the aerosol and cloud chamber facility AIDA (Aerosol Interaction and Dynamics in the Atmosphere). The test soot for cloud formation simulations was produced using a propane flame Combustion Aerosol Standard generator (CAST, Jing-CAST Technologies). The organic carbon content (OC) of the test soot was altered in a reproducible fashion by changing the fuel/air mixture of the generator. The soot content of ice nuclei was subsequently investigated using a combination of a pumped counterflow virtual impactor (PCVI) to separate and evaporate the ice crystals, and a DMT single particle soot photometer (SP2) to examine the mixing state of the BC containing ice residuals. Ice nucleation was found to be most efficient for uncoated soot of low organic carbon content (~5 % organic carbon content) where deposition freezing occurred at an ice saturation ratio Sice ~ 1.22 at a temperature T = 226.6 K with 25 % of the test soot becoming active as ice nuclei. Propane flame soot of higher organic carbon content (~30 % and ~70 % organic carbon content) showed significantly lower ice nucleation efficiency (an activated fraction of the order of a few percent in the experiments) than the low organic carbon content soot, with water saturation being required for freezing to occur. Ice nucleation occurred over the range Sice = 1.22–1.70, and T = 223.2–226.6 K. Analysis of the SP2 data showed that the 5 % organic carbon content soot had an undetectable OC coating whereas the 30 % organic carbon content soot had a thicker or less volatile OC coating. The application of a sulphuric acid coating to the flame soot shifted the threshold of the onset of freezing towards that of the homogeneous freezing of sulphuric acid; for the minimum OC flame soot this inhibited nucleation since the onset of freezing occurred at colder temperatures and required a greater ice saturation ratio; for the medium and maximum OC flame soot, the addition of a sulphuric acid significantly reduced the freezing threshold.

Particle Habit Imaging Using Incoherent Light: A First Step toward a Novel Instrument for Cloud Microphysics

Abstract The imaging unit of the novel cloud particle instrument Particle Habit Imaging and Polar Scattering (PHIPS) probe has been developed to image individual ice particles produced inside a large cloud chamber. The PHIPS produces images of single airborne ice crystals, illuminated with white light of an ultrafast flashlamp, which are captured at a maximum frequency of ∼5 Hz by a charge-coupled device (CCD) camera with microscope optics. The imaging properties of the instrument were characterized by means of crystalline sodium hexafluorosilicate ice analogs, which are stable at room temperature. The optical resolving power of the system is ∼2 μm. By using dedicated algorithms for image processing and analysis, the ice crystal images can be analyzed automatically in terms of size and selected shape parameters. PHIPS has been operated at the cloud simulation chamber facility Aerosol Interaction and Dynamics in the Atmosphere (AIDA) of the Karlsruhe Institute of Technology at different temperatures between −17° and −4°C in order to study the influence of the ambient conditions, that is, temperature and ice saturation ratio, on ice crystal habits. The area-equivalent size distributions deduced from the PHIPS images are compared with the retrieval results from Fourier transform infrared (FTIR) extinction spectroscopy in case of small (&amp;lt;20 μm) and with single particle data from the cloud particle imager in case of larger (&amp;gt;20 μm) ice particles. Good agreement is found for both particle size regimes.

Studies of heterogeneous freezing by three different desert dust samples

Abstract. We present results of experiments at the aerosol interactions and dynamics in the atmosphere (AIDA) chamber facility looking at the freezing of water by three different types of mineral particles at temperatures between −12°C and −33°C. The three different dusts are Asia Dust-1 (AD1), Sahara Dust-2 (SD2) and Arizona test Dust (ATD). The dust samples used had particle concentrations of sizes that were log-normally distributed with mode diameters between 0.3 and 0.5 μm and standard deviations, σg, of 1.6–1.9. The results from the freezing experiments are consistent with the singular hypothesis of ice nucleation. The dusts showed different nucleation abilities, with ATD showing a rather sharp increase in ice-active surface site density at temperatures less than −24°C. AD1 was the next most efficient freezing nuclei and showed a more gradual increase in activity than the ATD sample. SD2 was the least active freezing nuclei. We used data taken with particle counting probes to derive the ice-active surface site density forming on the dust as a function of temperature for each of the three samples and polynomial curves are fitted to this data. The curve fits are then used independently within a bin microphysical model to simulate the ice formation rates from the experiments in order to test the validity of parameterising the data with smooth curves. Good agreement is found between the measurements and the model for AD1 and SD2; however, the curve for ATD does not yield results that agree well with the observations. The reason for this is that more experiments between −20 and −24°C are needed to quantify the rather sharp increase in ice-active surface site density on ATD in this temperature regime. The curves presented can be used as parameterisations in atmospheric cloud models where cooling rates of approximately 1°C min−1 or more are present to predict the concentration of ice crystals forming by the condensation-freezing mode of ice nucleation. Finally a polynomial is fitted to all three samples together in order to have a parameterisation describing the average ice-active surface site density vs. temperature for an equal mixture of the three dust samples.

Single Particle Laser Mass Spectrometry Applied to Differential Ice Nucleation Experiments at the AIDA Chamber

Experiments conducted at the Aerosol Interactions and Dynamics in the Atmosphere (AIDA) chamber located in Karlsruhe, Germany permit investigation of particle properties that affect the nucleation of ice at temperature and water vapor conditions relevant to cloud microphysics and climate issues. Ice clouds were generated by heterogeneous nucleation of Arizona test dust (ATD), illite, and hematite and homogeneous nucleation of sulfuric acid. Ice crystals formed in the chamber were inertially separated from unactivated, or “interstitial” aerosol particles with a pumped counterflow virtual impactor (PCVI), then evaporated. The ice residue (i.e., the aerosol which initiated ice nucleation plus any material which was scavenged from the gas- and/or particle-phase), was chemically characterized at the single particle level using a laser ionization mass spectrometer. In this manner the species that first nucleated ice could be identified out of a mixed aerosol population in the chamber. Bare mineral dust particle...

Technical Note: A numerical test-bed for detailed ice nucleation studies in the AIDA cloud simulation chamber

Abstract. The AIDA (Aerosol Interactions and Dynamics in the Atmosphere) aerosol and cloud chamber of Forschungszentrum Karlsruhe can be used to test the ice forming ability of aerosols. The AIDA chamber is extensively instrumented including pressure, temperature and humidity sensors, and optical particle counters. Expansion cooling using mechanical pumps leads to ice supersaturation conditions and possible ice formation. In order to describe the evolving chamber conditions during an expansion, a parcel model was modified to account for diabatic heat and moisture interactions with the chamber walls. Model results are shown for a series of expansions where the initial chamber temperature ranged from −20°C to −60°C and which used desert dust as ice forming nuclei. During each expansion, the initial formation of ice particles was clearly observed. For the colder expansions there were two clear ice nucleation episodes. In order to test the ability of the model to represent the changing chamber conditions and to give confidence in the observations of chamber temperature and humidity, and ice particle concentration and mean size, ice particles were simply added as a function of time so as to reproduce the observations of ice crystal concentration. The time interval and chamber conditions over which ice nucleation occurs is therefore accurately known, and enables the model to be used as a test bed for different representations of ice formation.

Some ice nucleation characteristics of Asian and Saharan desert dust

Abstract. The large (7 m×4 m cylinder, 84 m3) AIDA (Aerosol Interactions and Dynamics in the Atmosphere) cloud chamber facility at Forschungszentrum, Karlsruhe, Germany was used to test the ice nucleating ability of two desert dust samples from the Sahara and Asia. Aerosol samples were lognormally distributed with a mode diameter of 0.4(±0.1) μm and geometric standard deviation of ~1.7(±0.2). At temperatures warmer than −40°C droplets were formed before ice crystals formed and there was generally no deposition nucleation observed. At temperatures colder than −40°C both dust samples exhibited dual nucleation events that were observed during the same expansion experiment. The primary nucleation event occurred at ice saturation ratios of 1.1 to 1.3 and is likely to be a deposition nucleation mode. The secondary nucleation event occurred at ice saturation ratios between 1.35 and 1.5. We cannot categorically determine whether this ice nucleation event is via a further deposition mode or a condensation mode, but the presence of some soluble material in the dust samples leads us to favour the latter process. The activated fractions of desert dust ranged from ~5–10% at −20°C to 20–40% at temperatures colder than −40°C. There was no obvious difference between the nucleation behaviour of the two dust samples.

Efficiency of the deposition mode ice nucleation on mineral dust particles

Abstract. The deposition mode ice nucleation efficiency of various dust aerosols was investigated at cirrus cloud temperatures between 196 and 223 K using the aerosol and cloud chamber facility AIDA (Aerosol Interaction and Dynamics in the Atmosphere). Arizona test dust (ATD) as a reference material and two dust samples from the Takla Makan desert in Asia (AD1) and the Sahara (SD2) were used for the experiments at simulated cloud conditions. The dust particle sizes were almost lognormally distributed with mode diameters between 0.3 and 0.5 μm and geometric standard deviations between 1.6 and 1.9. Deposition ice nucleation was most efficient on ATD particles with ice-active particle fractions of about 0.6 and 0.8 at an ice saturation ratio Si&lt;1.15 and temperatures of 223 and 209 K, respectively. No significant change of the ice nucleation efficiency was found in up to three subsequent cycles of ice activation and evaporation with the same ATD aerosol. This indicates that the phenomenon of preactivation does not apply to ATD particles. The desert dust samples SD2 and AD1 showed a significantly lower fraction of active deposition nuclei, about 0.25 at 223 K and Si&lt;1.35. For all samples the ice activated aerosol fraction could be approximated by an exponential equation as function of Si. This indicates that deposition ice nucleation on mineral particles may not be treated in the same stochastic sense as homogeneous freezing. The suggested formulation of ice activation spectra may be used to calculate the formation rate of ice crystals in models, if the number concentration of dust particles is known. More experimental work is needed to quantify the variability of the ice activation spectra as function of the temperature and dust particle properties.

Ice nucleation on flame soot aerosol of different organic carbon content

The aerosol chamber AIDA (Aerosol Interactions and Dynamics in the Atmosphere) was used as a moderate expansion cloud chamber to investigate the effect of the organic carbon (OC) content on the ice nucleation properties of soot aerosol particles. Two different soot samples with OC contents of 16 % (CS16) and 40 % (CS40) where produced with the CAST (Combustion Aerosol Standard) burner operated at different air/fuel (propane) ratios. In dynamic expansion experiments with about 30 %/min increase of relative humidity with respect to ice, the CS16 sample started to nucleate ice crystals at an ice saturation ratio S in of 1.45 (at a temperature of 207 K). This value is very close to the ice saturation ratio of ice nucleation onset on carbon spark generator soot particles coated with a significant amount of sulphuric acid investigated in previous AIDA expansion experiments. A second experiment with CS40 soot performed at almost identical thermodynamic conditions showed ice nucleation onset to occur at S in between 1.5 and 1.7. The formation rate of ice crystals was at least two orders of magnitude less than for CS16 soot, even at ice saturation ratios up to values of 1.9, which is very close to water saturation at a temperature of 207 K. Therefore, increasing the amount of OC seems to significantly suppress the ice nucleation on flame soot particles. In contrast, similar expansion experiments with dry and untreated mineral dust particles (Arizona test dust) in the temperature range 194 to 241 K showed ice nucleation to occur at much lower ice saturation ratios of only 1.05 to 1.15.

Effect of sulfuric acid coating on heterogeneous ice nucleation by soot aerosol particles

The low‐temperature aerosol and cloud chamber AIDA (Aerosol Interactions and Dynamics in the Atmosphere) of Forschungszentrum Karlsruhe was used to investigate the effect of sulfuric acid coating on the ice nucleation efficiency of soot aerosol particles from a spark discharge generator. The uncoated (sulfuric acid–coated) soot aerosol showed a nearly lognormal size distribution with number concentrations of 300–5000 cm−3 (2500–56,000 cm−3), count median diameters of 70–140 nm (90–200 nm), and geometric standard deviation of 1.3–1.4 (1.5–1.6). The volume fraction of the sulfuric acid coating to the total aerosol volume concentration ranged from 21 to 81%. Ice activation was investigated in dynamic expansion experiments simulating cloud cooling rates between about −0.6 and −3.5 K min−1. At temperatures between 186 and ∼235 K, uncoated soot particles acted as deposition nuclei at very low ice saturation ratios between 1.1 and 1.3. Above 235 K, ice nucleation only occurred after approaching liquid saturation. Coating with sulfuric acid significantly increased the ice nucleation thresholds of soot aerosol to saturation ratios increasing from ∼1.3 at 230 K to ∼1.5 at 185 K. This immersion mode of freezing nucleates ice well below the thresholds for homogeneous freezing of pure sulfuric acid solution droplets measured in previous AIDA experiments. A case study indicated that in contrast to the homogeneous freezing the nucleation rate of the immersion freezing mechanism depends only weakly on relative humidity and thereby the solute concentration. These results show that it is important to know the mixing state of soot and sulfuric acid aerosol particles in order to properly assess their role in cirrus formation.

A quantitative test of infrared optical constants for supercooled sulphuric and nitric acid droplet aerosols

Abstract. In situ Fourier transform infrared (FTIR) extinction spectra of supercooled H2SO4/H2O and HNO3/H2O solution droplets were recorded in the large coolable aerosol chamber AIDA (Aerosol Interactions and Dynamics in the Atmosphere) of Forschungszentrum Karlsruhe for a range of aerosol compositions and at temperatures extending down to 192 K. The measured spectra were quantitatively analysed in terms of aerosol composition and mass concentration by using Mie theory in combination with published refractive index data as input parameters. Simultaneously, total sulphuric acid and nitric acid mass concentrations from filter analysis and total water concentrations measured with the Lyman-a hygrometer of Forschungszentrum Jülich were used to calculate the aerosol composition at thermodynamic equilibrium inside the aerosol chamber. By comparing these measured aerosol parameters with those retrieved from the analysis of the FTIR spectra, the accuracy of the literature data sets of refractive indices could be assessed. In summary, four data sets were tested in the H2SO4/H2O system as well as two data sets in the HNO3/H2O system, partly revealing significant discrepancies in the retrieved aerosol properties. Potential explanations for these differences are discussed in this paper.

Experimental investigation of homogeneous freezing of sulphuric acid particles in the aerosol chamber AIDA

Abstract. The homogeneous freezing of supercooled H2SO4/H2O solution droplets was investigated in the aerosol chamber AIDA (Aerosol Interactions and Dynamics in the Atmosphere) of Forschungszentrum Karlsruhe. 24 freezing experiments were performed at temperatures between 189 and 235 K with aerosol particles in the diameter range 0.05 to 1 µm. Individual experiments started at homogeneous temperatures and ice saturation ratios between 0.9 and 0.95. Cloud cooling rates up to -2.8 K min-1 were simulated dynamically in the chamber by expansion cooling using a mechanical pump. Depending on the cooling rate and starting temperature, freezing threshold relative humidities were exceeded after expansion time periods between about 1 and 10 min. The onset of ice formation was measured with three independent methods showing good agreement among each other. Ice saturation ratios measured at the onset of ice formation increased from about 1.4 at 231 K to about 1.75 at 189 K. The experimental data set including thermodynamic parameters as well as physical and chemical aerosol analysis provides a good basis for microphysical model applications.