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- ItemTowards closing the gap between hygroscopic growth and activation for secondary organic aerosol - Part 2: Theoretical approaches(München : European Geopyhsical Union, 2009) Wex, H.; Petters, M.D.; Carrico, C.M.; Hallbauer, E.; Massling, A.; McMeeking, G.R.; Poulain, L.; Wu, Z.; Kreidenweis, S.M.; Stratmann, F.We examine the hygroscopic properties of secondary organic aerosol particles generated through the reaction of α-pinene and ozone using a continuous flow reaction chamber. The water activity versus composition relationship is calculated from measurements of growth factors at relative humidities up to 99.6% and from measurements of cloud condensation nuclei activity. The observed relationships are complex, suggesting highly non-ideal behavior for aerosol water contents at relative humidities less than 98%. We present two models that may explain the observed water activity-composition relationship equally well. The first model assumes that the aerosol is a pseudo binary mixture of infinitely water soluble compounds and sparingly soluble compounds that gradually enter the solution as dilution increases. The second model is used to compute the Gibbs free energy of the aerosol-water mixture and shows that the aerosol behaves similarly to what can be expected for single compounds that contain a certain fraction of oxygenated and non-polar functional groups.
- ItemLACIS-measurements and parameterization of sea-salt particle hygroscopic growth and activation(München : European Geopyhsical Union, 2008) Niedermeier, D.; Wex, H.; Voigtländer, J.; Stratmann, F.; Brüggemann, E.; Kiselev, A.; Henk, H.; Heintzenberg, J.The Leipzig Aerosol Cloud Interaction Simulator (LACIS) was used to investigate the hygroscopic growth and activation of sea-salt particles which were generated from three different sea-water samples. The measurements showed that the sea-salt particles exhibit a slightly reduced hygroscopic growth compared to pure NaCl particles. Köhler theory was utilized to model the hygroscopic growth of these particles. Some parameters used in this model are unknown for sea-salt. These parameters are combined in an "ionic density" ρion. For each sea-salt sample an average ρion was determined by fitting the Köhler equation to the data from the hygroscopic growth measurements. LACIS was also used to measure the activation of the sea-salt particles at three different supersaturations: 0.11%, 0.17% and 0.32%. A CCN-closure was tested by calculating the critical diameters Dcrit for the sea-salt particles at these supersaturations, using the Köhler model and the corresponding ρion as derived from the hygroscopic growth data. These calculated critical diameters were compared to the measured ones. Measured and calculated values of Dcrit agree within the level of uncertainty. Based on this successful closure, a new parameterization to describe sea-salt-particle hygroscopic growth (at RH>95%) and activation has been developed.
- ItemSO2 oxidation products other than H2SO4 as a trigger of new particle formation. Part 1: Laboratory investigations(München : European Geopyhsical Union, 2008) Berndt, T.; Stratmann, F.; Bräsel, S.; Heintzenberg, J.; Laaksonen, A.; Kulmala, M.Mechanistic investigations of atmospheric H2SO4 particle formation have been performed in a laboratory study taking either H2SO4 from a liquid reservoir or using the gas-phase reaction of OH radicals with SO2. Applying both approaches for H2SO4 generation simultaneously it was found that H2SO4 evaporated from the liquid reservoir acts considerably less effective for the process of particle formation and growth than the products originating from the reaction of OH radicals with SO2. Furthermore, for NOx concentrations >5×1011 molecule cm−3 the formation of new particles from the reaction of OH radicals with SO2 is inhibited. This suggests that substances other than H2SO4 (potentially products from sulphur-containing peroxy radicals) trigger lower tropospheric new particle formation and growth. The currently accepted mechanism for SO2 gas-phase oxidation does not consider the formation of such substances. The analysis of new particle formation for different reaction conditions in our experiment suggests that a contribution of impurities to the nucleation process is unlikely.
- ItemHydration increases the lifetime of HSO5 and enhances its ability to act as a nucleation precursor – a computational study(München : European Geopyhsical Union, 2009) Kurtén, T.; Berndt, T.; Stratmann, F.Recent experimental findings indicate that HSO5 radicals may play a key role in the nucleation of atmospheric SO2 oxidation products. HSO5 radicals are metastable intermediates formed in the SO2 oxidation process, and their stability and lifetime are, at present, highly uncertain. Previous high-level computational studies have predicted rather low stabilities for HSO5 with respect to dissociation into SO3+HO2, and have predicted the net reaction HSO3+OH→SO3+HO2 to be slightly exothermal. However, these studies have not accounted for hydration of HSO5 or its precursor HSO3. In this study, we have estimated the effect of hydration on the stability and lifetime of HSO5 using the advanced quantum chemical methods CCSD(T) and G3B3. We have computed formation energies and free energies for mono- and dihydrates of OH, HSO3, HSO5, SO3 and HO2, and also reanalyzed the individual steps of the HSO3+O2→HSO5→SO3+HO2 reaction at a higher level of theory than previously published. Our results indicate that hydration is likely to significantly prolong the lifetime of the HSO5 intermediate in atmospheric conditions, thus increasing the probability of reactions that form products with more than one sulfur atom. Kinetic modeling indicates that these results may help explain the experimental observations that a mixture of sulfur-containing products formed from SO2 oxidation by OH radicals nucleates much more effectively than sulfuric acid taken from a liquid reservoir.
- 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.
- 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.
- ItemSO2 oxidation products other than H2SO4 as a trigger of new particle formation. Part 2: Comparison of ambient and laboratory measurements, and atmospheric implications(München : European Geopyhsical Union, 2008) Laaksonen, A.; Kulmala, M.; Bernd, T.; Stratmann, F.; Mikkonen, S.; Ruuskanen, A.; Lehtinen, K.E.J.; Dal Maso, M.; Aalto, P.; Petäjä, T.; Riipinen, I.; Sihto, S.-L.; Janson, R.; Arnold, F.; Hanke, M.; Ücker, J.; Umann, B.; Sellegri, K.; O'Dowd, C.D.; Viisanen, Y.Atmospheric new particle formation is generally thought to occur due to homogeneous or ion-induced nucleation of sulphuric acid. We compare ambient nucleation rates with laboratory data from nucleation experiments involving either sulphuric acid or oxidized SO2. Atmospheric nucleation occurs at H2SO4 concentrations 2–4 orders of magnitude lower than binary or ternary nucleation rates of H2SO4 produced from a liquid reservoir, and atmospheric H2SO4 concentrations are very well replicated in the SO2 oxidation experiments. We hypothesize these features to be due to the formation of free HSO5 radicals in pace with H2SO4 during the SO2 oxidation. We suggest that at temperatures above ~250 K these radicals produce nuclei of new aerosols much more efficiently than H2SO4. These nuclei are activated to further growth by H2SO4 and possibly other trace species. However, at lower temperatures the atmospheric relative acidity is high enough for the H2SO4–H2O nucleation to dominate.
- ItemTowards closing the gap between hygroscopic growth and activation for secondary organic aerosol: Part 1 – Evidence from measurements(München : European Geopyhsical Union, 2009) Wex, H.; Petters, M.D.; Carrico, C.M.; Hallbauer, E.; Massling, A.; McMeeking, G.R.; Poulain, L.; Wu, Z.; Kreidenweis, S.M.; Stratmann, F.Secondary Organic Aerosols (SOA) studied in previous laboratory experiments generally showed only slight hygroscopic growth, but a much better activity as a CCN (Cloud Condensation Nucleus) than indicated by the hygroscopic growth. This discrepancy was examined at LACIS (Leipzig Aerosol Cloud Interaction Simulator), using a portable generator that produced SOA particles from the ozonolysis of α-pinene, and adding butanol or butanol and water vapor during some of the experiments. The light scattering signal of dry SOA-particles was measured by the LACIS optical particle spectrometer and was used to derive a refractive index for SOA of 1.45. LACIS also measured the hygroscopic growth of SOA particles up to 99.6% relative humidity (RH), and a CCN counter was used to measure the particle activation. SOA-particles were CCN active with critical diameters of e.g. 100 nm and 55 nm at super-saturations of 0.4% and 1.1%, respectively. But only slight hygroscopic growth with hygroscopic growth factors ≤1.05 was observed at RH<98% RH. At RH>98%, the hygroscopic growth increased stronger than would be expected if a constant hygroscopicity parameter for the particle/droplet solution was assumed. An increase of the hygroscopicity parameter by a factor of 4–6 was observed in the RH-range from below 90% to 99.6%, and this increase continued for increasingly diluted particle solutions for activating particles. This explains an observation already made in the past: that the relation between critical super-saturation and dry diameter for activation is steeper than what would be expected for a constant value of the hygroscopicity. Combining measurements of hygroscopic growth and activation, it was found that the surface tension that has to be assumed to interpret the measurements consistently is greater than 55 mN/m, possibly close to that of pure water, depending on the different SOA-types produced, and therefore only in part accounts for the discrepancy between hygroscopic growth and CCN activity observed for SOA particles in the past.
- ItemNew-particle formation events in a continental boundary layer: First results from the SATURN experiment(München : European Geopyhsical Union, 2003) Stratmann, F.; Siebert, H.; Spindler, G.; Wehner, B.; Althausen, D.; Heintzenberg, J.; Hellmuth, O.; Rinke, R.; Schmieder, U.; Seidel, C.; Tuch, T.; Uhrner, U.; Wiedensohler, A.; Wandinger, U.; Wendisch, M.; Schell, D.; Stohl, A.During the SATURN experiment, which took place from 27 May to 14 June 2002, new particle formation in the continental boundary layer was investigated. Simultaneous ground-based and tethered-balloon-borne measurements were performed, including meteorological parameters, particle number concentrations and size distributions, gaseous precursor concentrations and SODAR and LIDAR observations. Newly formed particles were observed inside the residual layer, before the break-up process of the nocturnal inversion, and inside the mixing layer throughout the break-up of the nocturnal inversion and during the evolution of the planetary boundary layer.
- ItemParticle formation at a continental background site: Comparison of model results with observations(München : European Geopyhsical Union, 2003) Uhrner, U.; Birmili, W.; Stratmann, F.; Wilck, M.; Ackermann, I.J.; Berresheim, H.At Hohenpeissenberg (47° 48' N, 11° 07' E, 988 m asl), a rural site 200--300~m higher than the surrounding terrain, sulphuric acid concentrations, particle size distributions, and other trace gas concentrations were measured over a two and a half year period. Measured particle number concentrations and inferred particle surface area concentrations were compared with box-model simulations for 12 carefully selected data sets collected during the HAFEX experiment (Birmili et al., 2003). The 12 cases were selected after meteorological and aerosol dynamical criteria in order to justify the use of a box-model. The aerosol model included a binary sulphuric acid water nucleation scheme. Calculated nucleation rates were corrected with a factor to match measured and calculated particle number concentrations. For the investigated 12 data sets, the correction factors were smallest for measurements made under stable thermal stratification and low wind conditions, i.e. conditions that are frequently encountered during winter. Correction factors were largest for measurements made under strong convective conditions. Our comparison of measured and simulated particle size distributions suggests that the particle formation process maybe strongly influenced by mixing processes driven by thermal convection and/or wind sheer.