G. Orsi

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.

Cloud processing of soluble gases

Abstract Experimental data from the Great Dun Fell Cloud Experiment 1993 were used to investigate interactions between soluble gases and cloud droplets. Concentrations of H 2 O 2 , SO 2 , CH 3 COOOH, HCOOH, and HCHO were monitored at different sites within and downwind of a hill cap cloud and their temporal and spatial evolution during several cloud events was investigated. Significant differences were found between in-cloud and out-of-cloud concentrations, most of which could not be explained by simple dissolution into cloud droplets. Concentration patterns were analysed in relation to the chemistry of cloud droplets and the gas/liquid equilibrium. Soluble gases do not undergo similar behaviour: CH 3 COOH simply dissolves in the aqueous phase and is outgassed upon cloud dissipation; instead, SO 2 is consumed by its reaction with H 2 O 2 . The behaviour of HCOOH is more complex because there is evidence for in-cloud chemical production. The formation of HCOOH interferes with the odd hydrogen cycle by enhancing the liquid-phase production of H 2 O 2 . The H 2 O 2 concentration in cloud therefore results from the balance of consumption by oxidation of SO 2 in-cloud production, and the rate by which it is supplied to the system by entrainment of new air into the clouds.

The NEVALPA project: A regional network for fog chemical climatology over the PO Valley basin

Abstract This paper describes the NEVALPA network, aimed at the study of fog water chemical composition in the Po Valley (northern Italy), its variability and the contribution of fog droplets to the total chemical deposition in the valley. The NEVALPA network is comprised of seven sites representative of the whole Po river basin. The results of the fog campaign carried out during the fall-winter season 1992–1993 are presented here. Higher fog occurrence was recorded in the central and eastern part of the valley, with January being the month with the overall highest fog frequency. An overall high pollutant loading of fog droplets was measured at all locations of the network with a maximum value of fog water ionic strength of 78,500 μeq l −1 at a liquid water content of 45 mg m−3. NH4+, SO42− and NO3− were the major components of fog water, accounting on average for 91 ± 4% of total ionic strength. Overall, the acid content of fog water in the Po Valley was not high, although pH values as low as 3 were sometimes recorded. The median pH value of fog water increased from west to east, with maximum values in the central part of the valley. Median ionic concentration values in the different stations differ within a factor of 2, an indication of the relative homogeneity of pollution throughout the Po Valley during the fall-winter season. No definite seasonal trend of fog water chemical composition was observed at any of the sites.

An automated fog water collector suitable for deposition networks: Design, operation and field tests

The study of fog water chemical composition and the contribution of fog droplets to totalchemical deposition has become a relevant environmental subject over the past few years. Thispaper describes a fog water collector suitable for deposition network operation, due to itscomplete automation and to the facility of remote acquisition of sampling information. Samplingof fog droplets on teflon strings is activated by an optical fog detector according to a particularprotocol operated by a microprocessor controller. Multiple sample collection, alsomicroprocessor controlled, is possible with this instrument. The problem of fog droplet samplingin sub-freezing conditions is overcome using a sampling schedule implemented by themicroprocessor controller which alternates between sampling periods and stand-by periods duringwhich melting of the rime collected on the strings is allowed. Field tests on the reliability andreproducibility of the sampling operations are presented in the paper. Side by side operation of thefog collector with a PVM-100 fog liquid water content meter shows that the amount of water perunit volume of air collected by the sampling instrument is proportional to the fog liquid watercontent averaged over the period of an entire fog event.

Radiation fog liquid water acidity at a field station in the po valley

Abstract The problem of fog water acidity is examined in this paper. The few data on this subject in literature concern only bulk samples of entire fog events; no data are reported on the acidity trend during a single event. The fog occurrence in some sites of the Po Valley (northern Italy) during the winter months is very high (about 30% of the time), so that the problem of fog acidity is very important in the degradation of materials, agriculture and human health. The analysis of the pH trend of three individual fog events is presented. From these first data it appears that, although pH follows the same trend of fog liquid water content, the evolving microphysical fog structure cannot alone account for pH variations during a fog event. The analysis of SO 4 2− , NO 3 − and Cl − concentration in fog water by means of the ion chromatograph, suggests that SO 2 and NO x oxidation to H 2 SO 4 and HNO 3 is responsible for the excess acidity not accounted for by condensation nuclei chemical composition and microphysical fog structure evolution (condensation/evaporation processes).

Computer modelling of clouds at Kleiner Feldberg

The airflow, cloud microphysics and gas- and aqueous-phase chemistry on Kleiner Feldberg have been modelled for the case study of the evening of 1 November 1990, in order to calculate parameters that are not easily measured in the cloud and thus to aid the interpretation of the GCE experimental data-set. An airflow model has been used to produce the updraught over complex terrain for the cloud model, with some care required to ensure realistic modelling of the strong stable stratification of the atmosphere. An extensive set of measurements has been made self-consistent and used to calculate gas and aerosol input parameters for the model. A typical run of the cloud model has calculated a peak supersaturation of 0.55% which occurs about 20 s after entering cloud where the updraught is 0.6 m s−1. This figure has been used to calculate the efficiency with which aerosol particles were scavenged; it is higher than that calculated by other methods, and produces a cloud with slightly too many droplets. A broad cloud droplet size spectrum has been produced by varying the model inputs to simulate turbulent mixing and fluctuations in cloud parameters in space and time, and the ability of mixing processes near cloud-base to produce a lower peak supersaturation is discussed. The scavenging of soluble gases by cloud droplets has been observed and departures from Henrys Law in bulk cloud-water samples seen to be caused by variation of pH across the droplet spectrum and the inability of diffusion to adjust initial distributions of highly soluble substances across the spectrum in the time available. Aqueous-phase chemistry has been found to play a minor role in the cloud as modelled, but circumstances in which these processes would be more important are identified.

The Cloud Ice Mountain Experiment (CIME) 1998: experiment overview and modelling of the microphysical processes during the seeding by isentropic gas expansion

The second field campaign of the Cloud Ice Mountain Experiment (CIME) project took place in February 1998 on the mountain Puy de Dome in the centre of France. The content of residual aerosol particles, of H2O2 and NH3 in cloud droplets was evaluated by evaporating the drops larger than 5 μm in a Counterflow Virtual Impactor (CVI) and by measuring the residual particle concentration and the released gas content. The same trace species were studied behind a round jet impactor for the complementary interstitial aerosol particles smaller than 5 μm diameter. In a second step of experiments, the ambient supercooled cloud was converted to a mixed phase cloud by seeding the cloud with ice particles by the gas release from pressurised gas bottles. A comparison between the physical and chemical characteristics of liquid drops and ice particles allows a study of the fate of the trace constituents during the presence of ice crystals in the cloud. In the present paper, an overview is given of the CIME 98 experiment and the instrumentation deployed. The meteorological situation during the experiment was analysed with the help of a cloud scale model. The microphysics processes and the behaviour of the scavenged aerosol particles before and during seeding are analysed with the detailed microphysical model ExMix. The simulation results agreed well with the observations and confirmed the assumption that the Bergeron–Findeisen process was dominating during seeding and was influencing the partitioning of aerosol particles between drops and ice crystals. The results of the CIME 98 experiment give an insight on microphysical changes, redistribution of aerosol particles and cloud chemistry during the Bergeron–Findeisen process when acting also in natural clouds.

Wet deposition due to fog in the Po Valley, Italy

Wet deposition due to radiation fog is examined in this paper. The area where the reported measurements were performed, the Po Valley, northern Italy, is characterized by both a high fog occurrence during the fall-winter months and fog water solutions of high ionic concentration and acidity.Estimated wet deposition for NH4+, NOinf3sup-and SOinf4sup2-ions due to fog droplet settling to the ground accounts for 13.2, 12.1 and 5.3 percent with respect to bulk precipitations over the same period: January–March and October–December (fog season).Fog deposition rates show that this process can be an important pathway of trace gases and particles loss from the air. First indicative results of fog removal efficiency with respect to air particulate matter are presented.Dry deposition parameters should be taken into account in evaluating the potential effect of fog droplet deposition on vegetation.