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- 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.
- 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.
- 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.
- 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.
- 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.
- 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.