A. Berner

A new electromobility spectrometer for the measurement of aerosol size distributions in the size range from 1 to 1000 nm

Abstract The Electromobility Spectrometer is an automated measurement system for the size analysis of fine and ultrafine aerosols using Differential Mobility Analysers (DMA) for the classification of particles and an electrical sensor for their detection. To cover a particle size range from 1 to 1000 nm, new DMAs with different geometries have been designed to optimize performance for small and large particles. Key problems in the size characterization of particles smaller than 20 nm have been addressed. A Faraday cup electrometer with a sensitivity of 10 −16 A is used as a particle sensor to avoid the deficiency of condensation nucleus counters with counting efficiencies decreasing with particle size. A new flow control technique allows the generation of stable air flows to further improve system performance. Additional improvements include the use of a multi-stage cascade impactor at the aerosol inlet, an instrument background correction mechanism and a refined data reduction algorithm. The computer controlled measurement program allows for variable size resolution, parallel operation of two DMAs and a time resolution for the measurement of size distributions of the order of 1 min.

The size distribution of the urban aerosol in Vienna

Abstract Two instruments have been developed recently which permit to measure the size distribution of urban aerosols in the size range from 0.002 to 16 μm, a special cloud chamber and a low pressure cascade impactor. The cloud chamber is a computer-controlled condensation nuclei counter which measures aerosols in the range between 0.002 and 0.1 μm by exposing the aerosol stepwise to well-defined supersaturations; the instrument is capable of measuring the nucleation mode. The cascade impactor samples the aerosol through nine stages. The stages for the smaller sizes operate between pressure considerably below atmospheric pressure and permit the extension of the size range down to 0.06 μm providing in this way a sufficient overlap to the cloud chamber. The impactor measures the accumulation mode completely. Both instruments have been applied to the Viennese urban aerosol and the accumulation mode as well as the nucleation mode were measured.

A closure study of sub-micrometer aerosol particle hygroscopic behaviour

Abstract The hygroscopic properties of sub-micrometer aerosol particles were studied in connection with a ground-based cloud experiment at Great Dun Fell, in northern England in 1995. Hygroscopic diameter growth factors were measured with a Tandem Differential Mobility Analyser (TDMA) for dry particle diameters between 35 and 265 nm at one of the sites upwind of the orographic cloud. An external mixture consisting of three groups of particles, each with different hygroscopic properties, was observed. These particle groups were denoted less-hygroscopic, more-hygroscopic and sea spray particles and had average diameter growth factors of 1.11–1.15, 1.38–1.69 and 2.08–2.21 respectively when taken from a dry state to a relative humidity of 90%. Average growth factors increased with dry particle size. A bimodal hygroscopic behaviour was observed for 74–87% of the cases depending on particle size. Parallel measurements of dry sub-micrometer particle number size distributions were performed with a Differential Mobility Particle Sizer (DMPS). The inorganic ion aerosol composition was determined by means of ion chromatography analysis of samples collected with Berner-type low pressure cascade impactors at ambient conditions. The number of ions collected on each impactor stage was predicted from the size distribution and hygroscopic growth data by means of a model of hygroscopic behaviour assuming that only the inorganic substances interacted with the ambient water vapour. The predicted ion number concentration was compared with the actual number of all positive and negative ions collected on the various impactor stages. For the impactor stage which collected particles with aerodynamic diameters between 0.17–0.53 μm at ambient relative humidity, and for which all pertinent data was available for the hygroscopic closure study, the predicted ion concentrations agreed with the measured values within the combined measurement and model uncertainties for all cases but one. For this impactor sampling occasion, the predicted ion concentration was significantly higher than the measured. The air mass in which this sample was taken had undergone extensive photochemical activity which had probably produced hygroscopically active material other than inorganic ions, such as organic oxygenated substances.

Modal character of atmospheric black carbon size distributions

Samples of atmospheric aerosols, collected with cascade impactors in the urban area of Vienna (Austria) and at a coastal site on the North Sea, were investigated for black carbon (BC) as the main component of absorbing material and for mass. The size distributions are structured. The BC distributions of these samples show a predominant mode, the accumulation aerosol, in the upper submicron size range, a less distinct finer mode attributable to fresh emissions from combustion sources, and a distinct coarse mode of unclear origin. It is important to note that some parameters of the accumulation aerosol are related statistically, indicating the evolution of the atmospheric accumulation aerosol.

CCN activation of oxalic and malonic acid test aerosols with the University of Vienna cloud condensation nuclei counter

Abstract The cloud droplet activation of monodisperse laboratory aerosols consisting of single organic and inorganic substances as well as a mixture of several substances was investigated using the University of Vienna cloud condensation nuclei counter (CCNC). The CCNC operates on the principle of a static thermal diffusion chamber. Water vapour supersaturations can be set in the range from 0.1% to 2%. Aqueous solutions of oxalic acid and malonic acid as well as solutions of inorganic compounds (NaCl and (NH4)2SO4) were nebulized in a Collison atomizer and then passed through a closed-loop differential mobility particle spectrometer to produce monodispersed particles. An internally mixed aerosol consisting of ammonium sulphate, oxalic acid and malonic acid with relative concentrations resembling those found in cloud water at a mountain station [Loflund, Kasper-Giebl, Schuster, Giebl, Hitzenberger, Reischl et al. (2002) Atmos. Environ. 36, 1553] was also investigated for cloud condensation nuclei (CCN) activation. All these particles were activated at supersaturations expected from Kohler theory. Oxalic and malonic acid particles are therefore expected to be good atmospheric CCN both as pure particles and as internally mixed particles containing other chemical compounds.

Composition/size of the light-scattering aerosol in the Netherlands

In 1992, 1993 and 1994 the size/composition of the aerosol in The Netherlands was measured in several measuring campaigns. The central aim of the study was the characterisation of those anthropogenic particles which most effectively scatter short-wave solar radiation. Since the largest effect of aerosol on radiation was expected at the times with the highest radiative flux, the measurements were made in the summer half-year around midday and under sunny conditions. Aerosol in arctic marine air served as the reference background. It contained as little as 0.l gm m−3 nitrate and non-sea-salt sulphate. In continental air some 75% of the aerosol mass was submicron. Ammonium nitrate and ammonium sulphate were the dominant (anthropogenic) aerosol species in the size range with maximum light-scattering (0.4–1.0 gm) and, with values up to 25 gmm−3, almost completely of a manmade origin. The ammonium nitrate concentrations were as high as or higher than those of ammonium sulphate, while the concentration of ammonium nitrate may have been underestimated because of evaporative losses during collection, of which examples are given. The sulphate size distribution was very similar to that in the period 1982–1984, which is indicative of stability of the distribution over time. Almost half of the submicron aerosol in the relevant size range could not be identified. Elemental-carbon contributed only an estimated 10% to this mass and the submicron dust content was even smaller. It was thus concluded by inference that most of the unidentified material was organic carbon. In marine air advected over the U.K. the submicron aerosol was manmade. In the particles which most effectively scatter solar radiation natural sea-salt-chloride is substituted by manmade sulphate. This substitution greatly changes the aerosol (radiative) properties: laboratory investigations, performed as part of this study, showed that sodium sulphate is a water-free crystal, while the original sea-salt aerosols are metastable saline droplets.

The Kleiner Feldberg Cloud Experiment 1990. An overview

An overview is given of the Kleiner Feldberg cloud experiment performed from 27 October until 13 November 1990. The experiment was carried out by numerous European research groups as a joint effort within the EUROTRAC-GCE project in order to study the interaction of cloud droplets with atmospheric trace constituents. After a description of the observational site and the measurements which were performed, the general cloud formation mechanisms encountered during the experiment are discussed. Special attention is given here to the process of moist adiabatic lifting. Furthermore, an overview is given regarding the pollutant levels in the gas phase, the particulate and the liquid phase, and some major findings are presented with respect to the experimental objectives. Finally, a first comparison attempts to put the results obtained during this campaign into perspective with the previous GCE field campaign in the Po Valley.

The great dun fell cloud experiment 1993: An overview

The 1993 Ground-based Cloud Experiment on Great Dun Fell used a wide range of measurements of trace gases, aerosol particles and cloud droplets at five sites to study their sources and sinks especially those in cloud. These measurements have been interpreted using a variety of models. The conclusions add to our knowledge of air pollution, acidification of the atmosphere and the ground, eutrophication and climate change. The experiment is designed to use the hill cap cloud as a flow-through reactor, and was conducted in varying levels of pollution typical of much of the rural temperate continental northern hemisphere in spring-time.

Experimental evidence for in-cloud production of aerosol sulphate

Abstract The modification of physical and chemical properties of aerosols passing through clouds has received considerable attention over recent years. Some of these transformations are related to in-cloud chemical reactions, particularly the oxidation of sulphur dioxide (SO 2 ) to sulphate (SO 4 2− . The Great Dun Fell experiment provided an opportunity to investigate the connection between the chemistry within cloud droplets and the processing of an aerosol population. We have noted significant increases in SO 4 2− in the aerosol population downstream of the cloud compared to the aerosol entering the cloud. These increases are connected to both S(IV) oxidation in the liquid phase and to the entrainment of new air into the cloud, supplying reactants such as H 2 O 2 to the system. The addition of SO 4 2− mass to the aerosol is also associated with changes in the NH 4 + aerosol concentrations, possibly as a result of neutralisation of the acidified cloud droplets by NH 3 . The study was performed taking into account dynamical mixing of air masses as well as possible sampling artefacts.

Observations and modelling of the processing of aerosol by a hill cap cloud

Abstract Observations are presented of the aerosol size distribution both upwind and downwind of the Great Dun Fell cap cloud. Simultaneous measurements of the cloud microphysics and cloud chemistry, and of the chemical composition of the aerosol both upwind and downwind of the hill were made along with measurements of sulphur dioxide, hydrogen peroxide and ozone. These observations are used for initialisation of, and for comparison with the predictions of a model of the air flow, cloud microphysics and cloud chemistry of the system. A broad droplet size distribution is often observed near to the hill summit, seemingly produced as a result of a complex supersaturation profile and by mixing between parcels with different ascent trajectories. The model generates several supersaturation peaks as the airstream ascends over the complex terrain, activating increasing numbers of droplets. In conditions where sulphate production in-cloud (due to the oxidation of S(IV) by hydrogen peroxide and ozone) is observed, there is a marked effect on the chemical evolution of the aerosol particles on which the droplets form. When sulphate production occurs, a significant modification of the aerosol size distribution and hygroscopic properties is both predicted and observed. The addition of sulphate mass to those aerosol particles nucleation scavenged by the cloud generally increases the ease with which they are subsequently able to act as cloud condensation nuclei (CCN). Often, this will lead to an increase in the number of CCN available for subsequent cloud formation, although this latter effect is shown to be strongly dependent upon the activation history of the droplets and the concentration of pollutant gases present in the interstitial air. Situations are also identified where cloud processing could lead to a reduction in the capacity of smaller aerosol to act as CCN.