C. Kruisz

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.

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.

Henry’s Law and the Behavior of Weak Acids and Bases in Fog and Cloud

Experimental data from two field experiments on ground based clouds were used to study the distribution of formic acid, acetic acid, ammonia and S(IV) species between liquid and gas phase. The ratio of the concentrations of these compounds between the phases during concurrent measurements was compared to ratios expected according to Henrys law (considering the pH influence). Large discrepancies of several orders of magnitude were seen. Three hypotheses have been investigated to explain the observed discrepancies: The existence of a microscale equilibrium which does not persist in a bulk sample, a thermodynamic shift of the equilibrium due to competing reactions, and nonequilibrium conditions due to mass transfer limitations. Approximate quantitative calculations show that none of these hypotheses is sufficient to explain all of the discrepancies, so a combination of different effects seems to be responsible for this observation. The same theoretical considerations also suggest that mass transfer limitation may be an important factor for highly soluble compounds. The data presented here indicates that it is not possible to simply extrapolate interstitial gas phase composition from measured bulk liquid phase concentrations of a fog or cloud.

Multiphase chemistry and acidity of clouds at Kleiner Feldberg

The chemistry of cloud multiphase systems was studied within the Kleiner Feldberg Cloud Experiment 1990. The clouds encountered during this experimental campaign could be divided into two categories according to the origin of air masses in which the clouds formed. From the chemical point of view, clouds passing the sampling site during the first period of the campaign (26 October-4 November) were characterized by lower pollutant loading and higher pH, as compared to clouds during the final period of the experimental campaign (10–13 November). The study of multiphase partitioning of the main chemical constituents of the cloud systems and of atmospheric acidity within the multiphase systems themselves (gas + interstitial aerosol + liquid droplets) are presented in this paper. A general lack of gaseous NH3 was found in these cloud systems, which caused a lack of buffer capacity toward acid addition. Evidence supports the hypothesis that the higher acidity of the cloud systems during this final period of the campaign was due to input of HNO3. Our measurements, however, could not determine whether the observed input was due to scavenging of gaseous HNO3 from the air feeding into the cloud, or to heterogeneous HNO3 formation via NO2 oxidation by O3 to NO3 and N2O5. Sulfate in cloud droplets mainly originated from aerosol SO42− scavenging, since S(IV) to S(VI) liquid phase conversion was inhibited due to both lack of H2O2 and low pH of cloud droplets, which made O3 and metal catalyzed S(IV) oxidation inefficient.

METEOROLOGY OF THE GREAT DUN FELL CLOUD EXPERIMENT 1993

Synoptic and local meteorological conditions during the Spring 1993 Ground-based Cloud Experiment on Great Dun Fell are described, including cloud microphysics, general pollution levels and sources of air, especially for five case studies selected for detailed analysis. Periods when air was flowing across the hill are identified and the extent to which air mixed into the cloud from above reached the ground is estimated. To aid the interpretation of cloud chemistry and microphysics measurements, the horizontal and vertical extent of the cloud are used to estimate droplet lifetimes and to comment on the influence of complex terrain on peak supersaturation.

Microphysics of clouds at Kleiner Feldberg

During a field measuring campaign at Kleiner Feldberg (Taunus) in 1990, microphysical characteristics of clouds have been measured by Forward Scattering Spectrometer Probes (FSSP). The aim was to study the influence of aerosol and meteorological factors on droplet size and number. The results are: More mass in the accumulation size range of the aerosol leads to more droplets in stratocumulus clouds and to higher soluble masses in droplets of stratus clouds. However, the aerosol distribution was coarser in the stratus clouds compared to the stratocumulus clouds. Within the first 200 m from cloud base, the droplets grow while their number decreases. The growth results in a stable size of about 14 µm diameter over a large distance from cloud base in many stratocumulus clouds. Two types of mixing processes were observed: processes with reductions in the number of droplets (inhomogeneous mixing) and with reductions in the size of the droplets (homogeneous mixing).