The Kleiner Feldberg Cloud Experiment 1990. An overview

Journal of Atmospheric Chemistry 19: 3-35, 1994. @ 1994 Kluwer Academic Publishers. Printed in the Netherlands'.

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The Kleiner Feldberg Cloud Experiment 1990. An Overview W. W O B R O C K 1' a, D. S C H E L L 1, R. M A S E R 1, W. J A E S C H K E t, H.-W. G E O R G I I l, W. W I E P R E C H T I' b, B. G. A R E N D S 2, J. J. M O L S 2, G. R A. K O S 2, S. F U Z Z I 3, M. C. F A C C H I N I 3'c, G. O R S I 3, A. B E R N E R 4, I. S O L L Y 4, C. K R U I S Z 4, I. B. S V E N N I N G S S O N s, A. W I E D E N S O H L E R 5' h, H.-C. H A N S S O N 5, J. A. O G R E N 6' d, K. J. N O O N E 6~e, A. H A L L B E R G 6, S. P A H L 7, T. S C H N E I D E R 7, R W I N K L E R 7' f, W. W I N I W A R T E R 8' g, R. N. C O L V I L E 9, T. W. C H O U L A R T O N 9, A. I. F L O S S M A N N l°'a and S. B O R R M A N N 1° ~Zentrum far Umweltforschung und Institut fi~r Meteorologie und Geophysik, Johann Wolfgang Goethe-Universit~t Frankfurt, Georg-Voigt-Str. 14, D-60325 Frankfurt a.M., Germany 2Nethertand Energy Research Foundation, P.O. Box t, 1755 ZG Petten, The Netherlands 31stituto FISBAT- C.N.R., via Gobetti t01, 1-40129 Bologna, Italy Qnstitut f~r Experimentatphysik, Universiti~t Wien, Strudlhofgasse 4, A-1090 Wien, Austria 5Department of Nuclear Physics, University of Lund, S-223 62 Lund, Sweden 6Department of Meteorology, Stockholm University, S-106 91 Stockholm, Sweden 7Deutscher Wetterdienst, Meteorologisches Observatorium Hamburg, Frahmredder 95, D-22361 Hamburg, Germany 8Institut fiir Analytische Chemic, Technische Universitat Wien, Getreidemarkt 9/151, A-1060 Vienna, Austria 9Department of Pure and Applied Physics, UMIST, PO Box 88, Manchester M60 1QD, United Kingdom t°lnstitut fiir Physik der Atmosphdre, Universitiit Mainz, Saarstr. 21, D-55122 Mainz, Germany

(Received: 13 July 1993; in final form: 19 May 1994) Abstract. 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

Present affiliations: Laboratoire de Mdt6orologie Physique, Universit6 Blaise Pascal, 24, Avenue des Landais, F-63177 Aubi~re Cedex, France bFraunhofer Institut fiir AtmosphOxische Umweltforschung, Forschungseinrichtung for Luftchemie, Rudower Chaussee 5, D-12484 Berlin, Germany c Presidio Multizonale di Prevenzione, Settore Chimico, Via Triachini 17, 40138 Bologna, Italy NOAA/CMDL/R/E/CG, 325 Broadway, Boulder, CO 80303-3328, U.S.A. Center for Atmospheric Chemistry Studies, Graduate School of Oceanography, Narragansett, RI 02882-1197, U.S.A. f Deutscher Wetterdienst, Meteorologisches Observatorium Hohenpeigenberg, Albin Schwaiger Weg 10, D-82383 Hohenpeigenberg, Germany g Forschungszentrum Seibersdorf, A-2444 Seibersdorf, Austria hInstitut ftir Troposph~irenforschung, Permoserstrasse 15, D-04303 Leipzig, Germany

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w. WOBROCK ET AL.

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.

Key words: orographicclouds,cloudchemistryand microphysics.

1. Introduction

Clouds are important for both the Earth's biosphere and atmosphere under various different aspects. Cloud cover influences the Earth's climate by regulating the radiative heating of surface and atmosphere. Clouds are a part of the hydrological cycle via the processes of condensation/evaporation and precipitation and clouds can take up, transport, transform, and remove atmospheric trace constituents stemming from natural causes and human activities. Up to now, various different cloud observations have been performed (e.g. Chandler et al., 1988; Mohnen and Kadlecek, 1989; Liu et al., 1993) addressing different aspects of clouds. The specific scope of the Ground-based Cloud Experiment (GCE) of EL~.OTRAC is the investigation of the interactions of cloud and fog droplets with atmospheric trace constituents, i.e. gaseous and particulate pollutants. In order to study the uptake of atmospheric trace constituents and their transformation processes by cloud droplets, the GCE investigated different clouds over Europe. These are ground fogs in an agricultural site and continental frontal clouds, both strongly influenced by industries and other human activities, as well as orographic clouds in a relatively clean maritime environment. Ground fog observations were performed in November 1989 in the Po Valley, near Bologna, Italy and the results of the campaign are published in Tellus 44B (5), (1992). The continental frontal clouds, which are the subject of this issue of the Journal of Atmospheric Chemistry, were observed in fall 1990 on the mountain Kleiner Feldberg (Taunus), near Frankfurt-am-Main, Germany. A field experiment for maritime orographic clouds was performed in spring 1993 in the north of England. The three sites were chosen to cover a wide range of possible meteorological and air chemical situations. The objectives of these experiments required the observation of the dynamical meteorological situation, which determines the formation of the cloud droplets, the time evolution of the gaseous species of interest, of aerosol particles as well as droplet spectra, and subsequent analysis of the collected liquid and aerosol particle samples. In order to tackle this task, numerous disciplines of atmospheric sciences had to be combined in a joint field enterprise. After describing, in Section 2, the general GCE objectives, the field site conditions and the typical cloud types immersing the observational site are presented in order to formulate the more specific objectives of this experiment. The resulting

THE KLEINER FELDBERG CLOUD EXPERIMENT 1990. AN OVERVIEW

5

experimental design and a list of the subsequent measurements performed are given in Sections 3.3 and 3.4. After a short meteorological description various observations are displayed to illustrate the general conditions of the pollutant levels in the gas- and liquid phases and for aerosol particles. Here, also some general findings resulting from the individual investigations will be summarized. By means of a cloud model, we compare the observed cloud droplet spectra and the observed concentrations of S(IV) and S(VI) in the liquid phase with the modelling results for a meteorological situation where the cloud is believed to have been formed by adiabatic lifting at the mountain. As a final remark, a short comparison of the observational results between this experiment and the preceding GCE experiment for ground fogs will be presented. 2. G C E - General Objectives

The Kleiner Feldberg Cloud Experiment took place from 27 October until 13 November 1990. Similar to the previous field campaign in 1989, the problems of interest were: How are the pollutant concentrations partitioned between gas phase, aerosol particles, and cloud water? What kind of processes cause changes in this partitioning and which interactions can be found between the microphysical/dynamical processes and the chemical composition of the cloud water? Is the chemical composition of the droplets size dependent as can be expected by the different scavenging processes7 Which influence do chemical reactions have on cloud water composition and are they detectable? Does the gas/liquid equilibrium observed in the laboratory experiments correspond to field observations? Are there significant differences in the chemical composition in different types of clouds over Europe and how strong are the deposition rates of cloud water? Are the sampling techniques for cloud water reliable with respect to its chemical composition? These objectives and the related problem areas (e.g. aerosol and gas scavenging, liquid phase reactions, etc.) are discussed also in Fuzzi et aL (1992). 3. Description of the Experiment

3.1. OBSERVATIONALSITE The Kleiner Feldberg (KF) belongs to a chain of mountains which form the socalled Taunus highlands. The Taunus ranges from southwest to northeast (Figure 1) and gives the northern barrier of the Upper Rhein Valley. The atmospheric pollutant levels in the south of the Taunus are rather high due to the dense industry m the Rhein-Main area as well as human activities by a population of about 2.5 million people. In contrast to this, the next 50-100 km in northern directions are almost free of industrial influences and also the population in this recreational area (rural conditions, low pollutant levels) is rather low.

6

w. WOBROCK ET AL.

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10kin ! I Fig. 1. Map of Taunus, Rhein-Main area, and Upper Rhein Valley. The triangle gives the location of the Kleiner Feldberg. The Taunus is illustrated by an isopleteof 350, the Rhein and Main Valley by 100 m.

Fig. 2.

Orography of the Taunus mountains next to the Kleiner Feldberg (dimension: 20 x

20 km2).

As can be seen from Figure 2, the terrain around the Kleiner Feldberg, mainly in the northern surroundings, is rather irregular. Another two high-reaching summits are 1.3 km northeast (Groger Feldberg, 878 m) and 2.7 km ESE (Altk6nig, 798 m) of the KF.


(kin) O

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Fig. 3. Topographical map of the Kleiner Feldberg and the vicinity. The numbers give the positions of the three observational sites.

All Taunus mountains are almost completely wooded with predominantly 1520 m high reaching conifers. This is also tree for the Kleiner Feldberg up to 30 m below the summit, where only grass and a few bushes cover the rocky soil. The field site, the 'Taunus Observatorium' of the University of Frankfurt a.M., hosted the main measuring platform (site 1) of the KF '90 experiment (see Figure 3). Two additional observational points were available: The first one, 1.5 km north of the summit station in 660 m (site 2), where a laboratory van of ZUF (for acronyms, see Table I) was sited, and the second one (site 3, a monitoring station of the local environmental protection agency) 1.7 km south of the summit in 520 m. Both sites were also located in forest free regions, however, the clearing of site 3 was too small for reasonable wind observations. The measuring sites used during KF '90 are indicated in the contour map (see Figure 3) for the Kleiner Feldberg and its vicinity. Most experimental measurements were made on the summit station. For this reason, ZUF and METFFM, who hosted this second GCE experiment, erected a cloud research laboratory. It consists of four containers on the uppermost point of

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W. WOBROCK ET AL.

TABLE I. Acronyms of the GCE groups Research group

Acronym

Netherlands Energy Research Foundation, Petten (the Netherlands) Institut fur Experimentalphysik, Universitfit Wien (Austria) Istituto per lo Studio dei fenomeni Fisici e Chimici della Bassa e Alta Atmosfera, C.N.R., Bologna (Italy) Department of Nuclear Physics, University of Lund (Sweden) Department of Meteorology, Stockholm University (Sweden) Deutscher Wetterdienst, Meteorologisches Observatorium Hamburg (Germany) Institut far Analytische Chemie, Technische Universitgt Wien (Austria) Department of Pure and Applied Physics, University of Manchester (United Kingdom) Institut ftir Meteorologie und Geophysik, J. Wl Goethe-Universit~t, Frankfurt a.M. (Germany) Zentrum far Umweltforschung, J. W. Goethe-Universitfit, Frankfurt a.M. (Germany)

ECN IEP FISBAT LUND MISU MOH TUV UMIST METFFM ZUF

the mountain. On the top of the containers, a platform was timbered large enough for the instrumentation (Figure 4).

3.2. CLOUDS OCCURRED AT KLEINER FELDBERG Clouds immersing the summit of KF are mostly observed before, during, and after the passage of fronts over central Germany. The frontal systems mainly come in from the Atlantic Ocean and cross Central Europe from south to northwest. As the front approaches, the associated cloud base comes down. Consequently, the summit can be immersed e.g., in stratocumulus and stratus clouds. This lowering of cloud base enhances when the frontal zone reaches the mountains. The terrain enhances the upward-going vertical motion and air is lifted almost moist adiabatically over the mountain barrier. This results in an even lower cloud base. Naturally, front passages are often accompanied by rain. Next to the stratus and stratocumulus clouds, cumulus and cumuluscongestus can also occur, especially if the passing front has the character of a cold front. Apart from the clouds which accompany passing fronts, clouds and cloud formation observed at the observational site also happen in a cyclonic large scale airflow, wherein vortexes or troughs of smaller scales develop.

9

THE KLEINER FELDBERG CLOUD EXPERIMENT 1990, AN OVERVIEW

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1. FSSP-100 (0.547 nun) (Z'UFIMETFt:M). 2. PVM-100, optical LWC-measurement(ECN). 3. Heated AssmanPsychrometer, global and UV radiation (MOIl). 4. Multistagesampling of wet and dry particulate (IEP). 5. PVM-100, optical LWC-mcasurement0VIOH). 6. PVM-100, optical LWC-measurement(FISBAT). 7. Trace gas measurements(HNO3,NI-I3)with denuders (ECN). 8. Two FSSP-100 (0.5-47 and 1-95 nun) (ECN). 9. CVI and interstitial air inlet for MISU and LUND aerosol investigations. 10, Cloud water sampling (IEP and MOH). 11. pH, conductivity and PAN gas phase measurements, data acquisition (ZUFmmTFFM). 12. Data acquisition(MOH, ECN, ZUF/METFFM). 13. Instrumentation(NflSU). 14. Insirmnentation(LUND). 15. Horizontalwindspeedand direction in 12 m (ZUF/METFFM). 16. Temperatureand relative humidityin 9 m (ZUFfMETFFM). 17. 3-dimensionalwindspeedand direction in 10 m (ZUF/METFFM). 18. Temperatureand relative humidityin 2 m (ZUF/METFFM). Further activities: MeasurementsofH202 and S(IV) in the liquid phase (ZUF/METFFM). - Trace gas measurements(SOs,03, NO, NO2)(ZUF/METFFM). Measurementsof organic acids in the gas phase O'UV). Cloud water collection: two-stagecollector (ZUF/METFFM). Laser hologramcamera (Universityof Mainz). I-I20:and formaldehydegas phase measurements(FISBAT). Doppler SODARmeasurements. -

-

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Fig. 4. Main observational platform at KF summit (site 1).

A third p h e n o m e n o n is the formation o f clouds in lower levels and their rise up to the K F summit. This can be low stratus or elevated fog, which have formed in an inversion layer over some hundred meter above the ground.

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3.3.

W. WOBROCK ET AL. EXPERIMENTAL DESIGN AND FIELD EXPERIMENT OBJECTIVES

The variety of cloud type occurring at the KF summit does not allow us to design an experiment for the investigation of a single cloud or cloud evolution process. To add a further complexity to the problem, only some clouds were formed directly at the mountain by adiabatic lifting. Others were formed elsewhere and were transported to the observational site, whereby pollutants had already been taken up, transformed, and eventually released from the liquid phase on places far away. Consequently notable featm'es of this GCE campaign are the number of different cloud types, different associated microphysics, and pollutant levels observed. In order to still pursue the main objectives of GCE (see Section 2) for both situations, i.e. clouds formed at the mountain and clouds advected to the site, the design of the individual experiments was chosen in such a way that for both cases the partitioning of the pollutant concentrations in gas, liquid and 'particulate' phases could be investigated. Therefore, measurements of these species were performed continuously during cloud events on the summit station (site 1). Most observations also ran in the periods before and after the cloud events. The following individual observations were performed in detail on the summit site: continuous gas measurements of SO2, NO, NO2, 03, H202, HCHO, HNO3, NH3 and organic acids, measurements of the particle spectra of interstitial and residual aerosols, the determination of their growth factor in subsaturated air and the size segregated collection of the aerosol particle mass for a subsequent chemical analysis, as well as observations of the cloud droplet spectra and cloud water collection (bulk and size segregated) for chemical analysis of S and N compounds, organic acids, H202, and HCHO. These observations were accompanied by the measurement of meteorological parameters (see Section 3.4). This experimental set-up allows us to investigate numerous aspects of the multiphase partitioning of pollutants in clouds: A comparison of the spectra of cloud droplet residuals with those of the interstitial aerosol particles (HaUberg et al,, 1994a) provides us with insights on the particle scavenging. The processes responsible for the particle scavenging, i.e. the phase partitioning of aerosols in clouds are, apart from the dynamical processes, the number and shape of the particle spectra, and their growth behaviour, i.e. the composition of the particles in terms of soluble/insoluble substances. These microphysical and chemical features were investigated by selecting monodisperse aerosol particles, drying them, and imposing them to a high relative humidity in order to reproduce their growth properties (Svenningsson et aL, 1992, 1994). Aerosol particles collected on impactor filters were apart from the standard IC analysis, also treated by a single particle analysis (Hallberg et al., 1994b) to distinguish soluble, mixed and nonsoluble particles over the size range from 0.07 to 1 #m diameter. The simultaneous observation -

-

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11

of the cloud droplet spectra, its associated LWC, and the spectra of the predominant interstitial aerosol particles, further allow us to study the influence of aerosol characteristics on the microphysics of the clouds. Apart from tile aerosol particles, also gases are partitioned between the different phases. Here, special emphasis was put on chemical species which affect the acidity of the cloud system (Fuzzi et al., 1994), i.e. sulphur and nitrogen compounds. In connection with this, the amount of gases in the liquid and gaseous phases observed, allowed us to study tile deviation from Henry's law equilibrium (Winiwarter et al., 1994). In order to obtain information on the size dependent chemical composition of cloud drops, measurements with a two stage cloud water collector (Schelt and Georgii, 1989) were also made. These results will be discussed in Section 4.1. Furthermore, observations of the size-dependent solute concentration of droplets were performed with a CVI (Ogren et al., 1992). However, the results of these measurements have not yet been evaluated. As already mentioned above, the partitioning of the chemical species among the gas, liquid, and 'particulate' phases is significantly determined by the prevailing dynamics. Most of the time measurements in clouds at one single observational site are insufficient to enlighten the effects of dynamics on the investigated phase partitioning. However, for some encountered weather situations, the dynamical evolution is rather simple and allows the study of processes like entrainment and turbulence on the microphysics and chemistry. We are referring to all weather situations where the cloud forms by adiabatic lifting of moist air along the slopes of the KE This can occur, e.g., during frontal passages with the wind coming from SE to SW directions where practically no orographic obstacle hinders the air in the uppermost 400-500 m from the free uphill flow. Figures 2 and 3 show that this is also true for most other inflowing directions (e.g. from the east and northwest), apart from the directions directly to the west (260 °) where a 690 m mountain (Glaskopf) is deflecting the flow slightly. Consequently, in the cases with air flowing in and rising at the KF, the clouds formed nearby the observation point and were thus characterised by the local surroundings. They were investigated more thoroughly in the GCE campaign with the help of models under the assumption of a moist adiabatic lifting. In order to obtain information about the air flow close to cloud base, two additional sites, 2 and 3, were chosen. Site 2 gives the conditions for the air flowing uphill from the NW directions, site 3 for all southerly directions. The assumption of cloud formation by lifting processes can be tested as mentioned above by running an adiabatic air parcel model and comparing these results with the observations obtained (see Section 4.2). A more extensive modelling investigation is given by Colvile et al. (1994), taking into account the airflow in the complex terrain of the Taunus mountains, as well as cloud microphysics and gas and aqueous-phase chemistry.

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W.WOBROCKET AL.

3.4. OBSERVATIONS PERFORMED As the major part of the GCE instrumentation during KF '90 was almost identical with that used during the Po Valley Fog experiment '89, we restrict its description to the short schematic overview given in Table II and refer for a detailed information to Fuzzi et al. (1992) and to the individual papers in this journal. Consequently, only significant experimental differences, technical changes or improvements, and newly included measurements will be presented here. 3.4.1. Meteorological Measurements The observational sites 1 and 2 (Figure 3) measured dry and wet bulb temperature, wind speed and direction, site 3 only dry and wet bulb temperature. In order to obtain additional information on the vertical movement of the airflow, a Gill-type propeller anemometer was run on the KF summit. Vertical soundings of temperature and humidity were provided once a day during most of the time of the observational period. These measurements were performed by the German weather service at Offenbach, 24 km southeast of the Kleiner Feldberg (as vertical soundings closer to the field site were technically not possible). 3.4.2. Gas Measurements All observational sites were equipped with instrumentation for gas phase measurements of NO, NO2, 03 and SO2. The complete set of gas phase measurements at the summit station is given in Table II. 3.4.3. Measurements of Aerosol Particles As in the Po Valley campaign, two different inlets were used by MISU and LUND to sample cloud droplets and interstitial air separately. The cloud droplets sampled (diameter > 5 #m), evaporated leaving behind residual particles. Attached to each inlet, several instruments were used to characterise the particles. The particle size distribution and the particle number concentration of the interstitial aerosol was measured using a Differential Mobility Particle Sizer (DMPS) (Fissan et aL, 1983). Interstitial particles (diameter < 5 #m) were dried in cloud periods as well as out of cloud periods before they reached the instrument. The DMPS system employed on Kleiner Feldberg gave the particle size distribution from 0.017 to 1.056 #m in 34 size channels with a time resolution of 10 minutes. Results of the DMPS measurements during KF '90 are presented in Section 4.lb. Filter samples of the particles were also taken. In this campaign, one-stage impactors with a single jet were included to obtain samples of aerosol particles during short time periods (Hallberg et aL, 1994b). Samples were taken of the dry residual particles yielded by the cloud droplets and the interstitial particles. By a

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T - I M P = Twin Impactors; T D M A = Tandem Differential Mobility Analyzer.

Particle Counter; P I X E = Proton Induced X - R a y Emission; P V M = Particulate Volume Monitor; R - I M P = R o u n d nozzle-Impactor; S - I M P = Slit Impactor;

C h r o m a t o g r a p h y ; H P L C = H i g h - p e r f o r m a n c e liquid c h r o m a t o g r a p h y ; I C = Ion C h r o m a t o g r a p h y ; I M P = Impactor; L y a = L y m a n - a l p h a Detector; O P C = Optical

Particle Spectrometer; E L E C = Specific Electrode; F = Filter; F L U = Fluorescence Methods; F S S P = F o r w a r d Scattering Spectrometer Probe; G C = G a s

Methods; C N C = Condensation Nucleus Counter; C o n d = el. conductivity; C V I = Counterflow Virtual Impactor; D E N = Denuder; D M P S = Differential Mobility

2 S T = two-stage cloud collector; A B S = Photometric absorption o f elemental carbon; C F C L = C o n t i n u o u s F l o w C h e m i l u m i n e s c e n c e ; C H L = C h e m o l u m i n e s c e u c e

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temperature,

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wind,

EC- ABS

sampling - F anions - I C

total numb. - C N C n u m b e r distr. - O P C

sampling - IMP metals, S - P I X E

sampling - IMP

cations - I C

hyg, g r o w t h - T D M A

organic acids -

Particle s a m p l i n g a n d analysis

num. distr. - D M P S

(wet and dry)

H202 - FLU HCHO - FLU

Z U F and

TUV

LWC - PVM

trajectory model,

MOH

U V B + solar radiation

LWC - CVI/Lya

LWC - PVM

NH3 - D E N + C o n d HNO3 - DEN + IC

Gas m e a s u r e m e n t s

droplet s p e c ~ a - F S S P

m a s s spectra - T - I M P

Aerosol particles

LWC - PVM

MISU

LUND

IEP

FISBAT

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group

Main experimental investigations at the s u m m i t site

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14

W. WOBROCK ET AL.

dialysis technique (Okada, 1983; Mossop, 1963) the soluble vs. insoluble fraction of each single particle was determined. We compared that information with the difference between particles forming cloud droplets and particles that remained in the interstitial air, in terms of their hygroscopic properties. Aerosol particles were also sampled with low pressure cascade impactors of different types (compare Fuzzi et al., 1994). A new experimental investigation for GCE was the use of the so-called twin impactor set-up. In this technique (Berner, 1988), two identical cascade impactors are operated side by side and simultaneously with one impactor ('wet') sampling the aerosol directly from the atmosphere and the other one ('dry') through a warmed inlet in order to dry the aerosol particles or cloud droplets before collection. The 'wet' impactor collects the aerosol particles and cloud droplets with their correct ambient sizes. With each of both impactors (vol. flow rate = 30 llmin) the aerosol mass was collected in eight size ranges from 0.06 to 16 # m diameter continually on a 12 hour basis. This technique allows us to determine the mass distribution of the aerosol particles as a function of particle diameter (dry impactor) as well as the mass distribution of aerosol particles in the droplets as a function of the droplet diameter. The comparison of both spectra can give valuable information about the size dependency of droplet growth and scavenging ratios. 3.4.4. Droplet Spectra and LWC A newly developed Holographic Droplet and Aerosol Recording system (HODAR) (Borrmann and Jaenicke, 1993) was operated to measure in-situ cloud droplet size distributions by recording Fraunhofer in-line holograms of small cloud sample volumes. The holographic images taken in the field site were subsequently analyzed in the laboratory. In a technical note (Borrmann et al., 1994), the resulting size distributions are compared with those obtained from a FSSP-100 laser optical particle counter. Similar to the first GCE experiment in the Po Valley, also during this experiment two fog collectors were operated to investigate the chemical composition of cloud droplets: a slit impactor operated by MOH and a round jet impactor operated by IEP. Some changes were made to improve the performance of the two collectors, since remarkable differences in cloud water chemical composition occurred during the Po Valley fog experiment 1990 (see Schell et al., 1992). As the collection efficiency of the IEP collector was very sensitive to changes in wind velocity a bended wind shield was fixed behind the four sampling cups, in order to render a stagnation zone above the vertically oriented wind inlets. The wind shield is tumable equipped with a wind vane and a cover at a distance of about 40 cm to avoid rain contamination. Some improvements were also made for the MOH collector. The new instrument was manufactured from one piece of Perspex with a new design of the impaction body; first one causing better and sharper edges, and second one improving the water transport into the vials.


15

3.4.5. Cloud Water Deposition In order to investigate the deposition of cloud water due to impaction processes of droplets (interception) by coniferous trees, MOH developed a monitoring apparatus for drip water. The instrumentation, which was operated in a coniferous forest next to the summit, consists of a 1 m 2 collection surface and a light gate counting the number of droplets dripping down from the canopy. The main advantage of this technique is the time resolved observation of the cloud water deposition. Results can be used for the verification of models for cloud water deposition (see Pahl et al., 1994). 3.5.

METEOROLOGICAL SITUATION DURING KF ' 9 0

The weather situation during the observational period of the Kleiner Feldberg campaign from 27 October to 13 November can be subdivided in periods with different characteristics. Between 27 and 30 October, several fronts passed the observational site accompanied by cumulus and stratocumulus clouds. The low passing from 28 October, 18:00 to 29 October, 3:00 showed the well defined warm front, warm sector, and cold front of an ideal cyclone. Also for the following cyclone, moving in very fast from the West, the passages of warm and cold front could be registered after midnight on 30 October. This series of cyclones coming in from the Atlantic Ocean shifted more and more to the south, whereby the frontal zone finally reached a stationary position over the Mediterranean region. In the following time period, the weather situation over central Europe became determined by short-wave, secondary troughs, which were responsible for cloud and rain formation from 31 October to 2 November. After 3 November, the transport of cold polar air from Scandinavia became dominant. This resulted in snow and ice at levels above 700 m and, consequently, the observations had to be interrupted until 9 November. Figure 5 illustrates the time evolution of temperature, liquid water content, relative humidity, and marks periods of rain observed during the KF'90 experiment. The cold period was terminated by a new Atlantic perturbation advecting air from southwest to central Europe. Two occluded fronts passed the observational site on 10 and 11 November bringing mainly stratus clouds and rain. The last 30 hours of the observational period were determined by a small high pressure wedge causing the formation of low stratus clouds during the night of t2/13 November, which ascended in the morning to the KF summit. With tile onset of rain at 16:00, 13 November, due to a newly approaching front, the campaign was terminated. For the detailed analysis of the meteorological situation, see Winkler et al. (1994). A number of significant differences could be encountered between the two main observational periods: during 27 October until 2 November 1990 the air masses had a maritime character with low pollutant concentrations in all three phases. In contrast to that the pollutant concentrations were much higher from 10 to 13

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November, when continental air masses dominated. In the unstable stratification of the first period, cumulus-type clouds were investigated while stratus clouds prevailed in the more stable second period. Consequently, notable differences in the cloud dynamics and microphysics could be found in the two periods.


17

4. General Results

The following section gives an overview of the observational results and presents some major findings with respect to the experiment objectives for the different types of clouds investigated. In the first part (Section 4.1) we will report some of the measurements of pollutants in the different phases and point out the major results mainly with regard to their partitioning and the processes responsible. The detailed discussion and interpretation of the observations leading to the final conclusions will be the scope of the individual papers of this issue and will not be presented in this overview. However, as mentioned earlier, additional information can be obtained by model calculations supporting the interpretation of data in the case of cloud formation by adiabatic lifting. They help to gain insights into the importance of chemical reactions for the cloud water chemical composition, reasons for the sub- or supersaturation of dissolved gases in the liquid phase, as well as the rote of the dynamics on the observed partitioning. Those results are given in Section 4.2. 4.1.

CONCENTRATION LEVELS OF THE CHEMICAL COMPONENTS DURING KF ' 9 0

4.1.1. In the Gas Phase

As the meteorological conditions were different for most cloud events, it is not surprising that also the pollutant concentrations in gas, aerosol, and liquid phase vary considerably between single events. The temporal evolution of the gas phase concentrations will be discussed here as an example. Figure 6 shows the observation of the nitrogen oxide concentration at the three observational sites, which are marked in Figure 3. We can detect that the NO concentration is generally low (2-3 ppbv), but significantly increases when the wind direction is around 150-170 °, i.e., when the air masses come from the SSE-direction. The wind direction is displayed in Figure 6 by the dotted curve. The increase of NO during phases with southern or south-eastern winds are evident, as for these cases the air was transported from the next city (about 5 km south east of the summit) to the observation points. Furthermore, Figure 6 indicates that the increase in NO concentration in general took place simultaneously at all three observational sites. This behaviour becomes more pronounced for the time evolution of the NO2 concentrations at the different sites, illustrated in Figure 7. Hereby, the lowest observation point at 520 m asl (site 3) always registered the highest increase in pollutants while the summit station, at 820 m, registered the lowest due to the stronger dispersion at higher altitudes. In general, the lowest concentrations for NO and NO2 were found in periods of westerly and north-westerly winds. In these cases, the air masses were transported over wide forest areas which were nearly free of pollutant sources. For situations with southern wind directions, however, again the influence of the industrialized and highly polluted Rhein-Main area became obvious.

18

W. WOBROCK ET AL,

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Fig. 6. NO concentrations at the three observational sites as welt as the wind direction at the summit station during KF '90.

Figure 8 illustrates the time evolution of 03, SO2, and NO2 obselwed at the summit station. The course of 03 and NO2 concentrations often displays an inverse correlation. This can be caused by the oxidation of NO by 03, which results in an NO2 increase and a decrease in 03. This process can be detected, especially when

19


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air m a s s e s with higher N O concentrations were advected to the Kleiner Feldberg ( c o m p a r e Figure 6). Furthermore, the gas phase observations show no diurnal trend as is c o m m o n l y o b s e r v e d at mountain sites. Figure 8 also illustrates the time evolution o f the SO2 concentrations o b s e r v e d on the K F summit. C o m p a r i n g the curves of SO2 and 03, it is m o s t prominent that

20

W.WOBROCKETAL. '

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21

Considering the gas-phase observations of the first experimental period (27 October to 2 November) and the second (9-13 November) no differences in the pollutant levels can be detected for most gas species (i.e. 03, SO2, and NO2). Only the concentrations of NO show an increase from the first to the second period with a shift of the average values over each individual period from 2.3 to 3.8 ppb~. However, a pronounced difference in the vertical distribution of the pollutants can be seen in Figure 7 for the NO2 concentrations. Reflected here is the fact that during the first period the stratification was slightly unstable, resulting in almost identical NO2 concentrations in all three heights, while in the second period, the stratification was stable which is reflected in the pronounced gradients between the three different stations. This is in agreement with the stability analysis of Winkler et al. (1994). As NO, NO2, and 03 are quite insoluble, no differences between phases with and without clouds could be detected. The same was tree for the moderately soluble gases like SO2, which might partly be caused by transport processes replenishing the SO2 taken up by the liquid phase. The behavior of the highly soluble gases NH3 and HNO3 is discussed in Fuzzi et aI. (1994). It was found that almost no NH3 was present during the campaign (< 0.3 ppbv). However, HNO3 was found to range between 0.2 and 1.4 ppb~, in cloud-free periods. During clouds, the HNO3 level was most of the time below the detection limits (0.07 ppbv), apart from some short periods between 12 and 13 November. 4.1.2. A e r o s o l Particles

The number distribution of aerosol particles observed with a DMPS are displayed in Figure 9 for most observational periods. The periods of observations varied considerably during the campaign: from 27 to 30 October observations were performed generally from 10:00-15:00. The first part of the observations on 31 October was monitored from 13:30-17:00, the second part of the observations where the aerosol distribution increases considerably in small sizes was monitored from 21:00 until midnight. On 1 November, observations displayed are from 17:00-24:00, on 2 November from 9:30-18:00. In the time interval from t0 November, 19:00 until 12 November, 7:40 the spectra were observed almost continuously with a time resolution of 10 minutes. Measurements on 13 November started at 4:50 and ended at 11:00. The size range observed by the DMPS is 0.017-1.06 #m diameter (Figure 9) (corresponding to 34 size bins). The widths of the size bins vary from dlgD = 0.045 for the smallest particles to 0.076 for the largest. The particle spectra of the DMPS were gained by sampling the interstitial air (i.e. only particles < 5 #m) during in cloud or out of cloud periods. One apparent set of information coming from this figure is the pronounced difference in the shape of the number distribution between the first and the second observational period. From 27 October to 2 November

22

W. WOBROCK ET AL.

Fig. 9. Time evolution of the number distribution of the interstitial aerosol particles aurmg the observational periods measured with the DMPS system. For a detailed explanation see section 4.1.2).

the size distribution is dominated by the nucleation mode, after 10 November, a clear maximum is displayed in the accumulation mode. The corresponding mean modal diameters are, in the first case, 0.025-0.04 #m, and in the second, 0.07-0.09 #m. This implies an increase in the total particle volume concentration by a factor of 2 during the second period as compared to the first. Another consequence of the dominant accumulation mode is an enhanced liquid phase pollutant level in the second period as these particles serve as CCN (discussed in Section 4.1.3). For the first period, Hallberg et aL (1994a) found that 87% of the volume (73% of the number) of all accumulation mode particles (i.e. particles in the 0.1-1 # m diameter range) were incorporated in the cloud droplets. However, in the second period, this volume fraction only counted for 42% (the number fraction was 11%). These significant differences can be explained by considering the size-dependent partitioning of aerosol particles between cloud droplets and interstitial air. From Hallberg et al. (1994a) we can see that in the first period, particles down to 0.05 # m were incorporated into droplets, while in the second period, only particles larger than 0.1 #m. The fraction of particles scavenged by droplets increases quite monotonously from these cut-on values to a plateau with values of 0.8-0.9 for larger particles (comp. Figures 5 and 7 in Hallberg et aL, 1994a). This plateau starts during the first period already at 0.2 #m, in the second at 0.4 #m. Consequently, the accumulation mode particles were much more efficiently scavenged in the first period, as mentioned above. In accordance with the results from the first GCE campaign in the Po Valley (Noone et aL, 1992) larger aerosol particles were never scavenged to 100%. An explanation for this last finding could be obtained by the growth studies on interstitial and residual aerosol particles. Svenningsson et al. (1994) found that


23

atmospheric particles of a unique size preferably have two different growth factors, indicating that one part of the particulate matter is less and the other part is more hygroscopic. This was confirmed by the single particle analysis of Hallberg et al. (1994b) who found that, over the entire accumulation mode, particles of the same size could consist completely of insoluble or highly soluble material. The main conclusion from these investigations is that the aerosol mass on KF was an external mixture which significantly influenced the aerosol partitioning in the clouds during this experiment. Another process responsible for the partitioning of the pollutant in the clouds is the dynamics which determines the strength and time evolution of the supersaturation. The different scavenging results already presented above, clearly indicate that the supersaturation during the first period must have been higher than in the second. The simple observational result that in the first period Cu and Sc were present (more unstable stratification), in contrast to the Stratus cloud (stable stratification) during the second period, also supports this evidence. Furthermore, the observed microphysical properties give a hint on the differences in maximum supersaturation between the two periods: two typical droplet spectra are displayed in Figure 10. The higher number of droplets for the spectrum on 2 November (first period) compared to the spectrum on 13 November again indicates the presumably higher supersaturation in the first period. For the Cu-types of clouds in the first period, Arends et al. (1994) found that the number of droplets decreases strongly within the first 100-200 m distance from the cloud base. This drop in number concentration is accompanied with an increase in LWC. Some 80-90% of the observed LWC is due to droplets larger than 10 # m which form a stable mode at a diameter of 13-15 #m. No change of droplet number with increasing distance from cloud base was observed for the stratiform clouds during the second period. In general, droplet numbers were higher during the periods with cumulus clouds than in the second period for the stratus clouds. However, in the first period, typical LWCs were lower with 150-200 mg m -3, while in the second period, they were normally around 300 mg m -3. This result, as well as the spectra in Figure 10, support the idea that the life history of the droplets in the stratus clouds was longer, i.e. the droplets of these clouds formed in places far away, so that despite lower supersaturations, larger drop diameter could be reached. 4.1.3. In the Liquid Phase

To summarize the results of the cloud water chemical analyses, the temporal variations of pH, conductivity as well as the concentrations of NH +, SO]-, and NO 3 in #mol/1 are shown in Figures 1 la (first period) and 1 lb (second period). Comparing the two periods, we find the concentrations in the second period to be nearly a factor of 2-3 above those in the first period. This is in agreement with the dominant accumulation mode of the particle spectra in the second period, as

24

w. WOBROCKET AL.

28 Oct. 1 7 : 0 0 ( 0 . 2 6 g m - 3 )

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Droplet spectra of a sc-cloud (28 October) and of a st-cloud (13 November).

discussed in 4.1.2), which is responsible for some of the differences in the chemical composition of the cloud water. We also see that the pH in the first period is between 4 and 5, while in the second period the pH is below 4. This is caused by the fact that the clouds in the second period had a longer lifetime (see Winkler et al., 1994) and, consequently, a longer time for chemical reactions in the liquid phase leading to acidification. Furthermore, we have found that the air masses arriving at the observational site during the first period originated from the North Atlantic and moved in with high velocities, while in the second period, the air masses travelled slower, coming from a more continental origin. This is also reflected in the analysis of the Na + ions which contributed 12% to the total ion concentration of the LWC during the first period and only 3% in the second. Table III represents a statistical summary of cloud water chemical composition during the experiment. The data are presented in units of nmol m -3, so that the different LWC values throughout the campaign are accounted for. As described in Schell et al. (1992), the two GCE cloud water impactors operated during the Po Valley Fog Experiment 1989 showed considerable differences in the


25

TABLE III. Statistical summary of the concentration (nmol m -3) of different components measured in cloud water during the experiment

H+ NH + Na + K+ Ca 2+ Mg 2+ CINO~NO 3 SO42HCOOCH3COOHMSA Fe Mn

min.

25 perc.

median

75 perc.

max

1.4 7.3 DL DL DL 0.5 0.8 DL 6.5 3.3 DL DL DL 0.12 0.01

16.0 30.8 5.3 1.8 0.8 1.6 11.5 DL 30.6 12.6 0.6 0.2 0.6 0.19 0.03

50.4 55.7 13.9 2.6 1.8 2,6 25.4 0. i 55.1 18.5 0.9 0.4 3.3 0.26 0.06

108.1 86.8 26,4 3.8 2.9 4.1 39.8 0.4 105.4 34.1 1.4 0.5 5.4 0.38 0.09

282.9 292.6 83.5 31.8 10.5 11.3 88.6 5.4 222.2 98.5 7.6 12.6 18.8 0.92 0.31

analytical results as well as in the amount of collected LWC. Thus, MOH and IEP modified the design of both instruments to improve their performance (see Section 3.4). Consequently, the results of the chemical analysis of liquid water of both impactors, as displayed in Figure 11, agree very well with slightly higher concentrations in the samples of the IEP impactor, disregarding only a few samples where the discrepancies were somewhat larger. The periods of high discrepancies are connected to periods of patchy clouds. Then, the MOH impactor switched off automatically once the LWC was below 20 mg m -3, while the IEP impactor ran continuously for technical reasons. In spite of the generally good agreement of the chemical analyses of the samples, the total amount of collected water for both samplers still differed by up to 2030%. This gives evidence for persisting differences in the collection efficiencies of both collectors. Concerning the results of the chemical analysis the following conclusions are possible: 1. Both instruments collect the same size range interval of the cloud droplet spectrum and the differences of the collection efficiencies affect all droplet size ranges in the same way, so that no instalment excludes certain droplet size intervals from sampling. 2. The size dependence of the chemical concentrations of cloud droplets was small in these events so that, unless differences in collection efficiencies occurred, only small differences in the concentrations could be observed.

W.WOBROCKETAL

26

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28

w. WOBROCK ET AL.

Size-resolved chemical measurements of the cloud water composition indicate that explanation (2) is the more likely one. To obtain those, a two-stage fog water collector was operated (Schell and Georgii, 1989) during some events, which collects droplets in the size ranges 2 < d < 4 #m and d > 4 #m with a time resolution of 1-2 hours, depending on the amount of water collected by the second stage (small droplets). The samples were analyzed for the major ions, pH and conductivity. The results are also included in Figures 1 la, b. For most cases they show no significant differences in concentration between both size ranges. Only in some cases the measurements showed higher or lower concentrations in the large droplet range (Figures 11 a, b). In the relatively clean stratocumulus clouds over Sweden (Ogren et al., 1989) the behaviour of increasing solute concentrations with increasing droplet size was normally observed. The opposite, i.e. decreasing concentrations with increasing droplet diameters, was observed during the Po Valley Fog Experiment, 1989 (Ogren et al., 1992; Schell et al., 1992). In both locations, the resulting size dependency was much more prominent than in any of the events encountered during KF'90. An explanation of the first behaviour can easily be given by the theory of diffusional growth (Pruppacher and Klett, 1978). As long as only supersaturation periods prevail, the water uptake of cloud condensation nuclei (of identical chemical composition) decreases with increasing size of the nucleus, resulting in an increasing solute concentration with increasing droplet diameter (Ogren and Charlson, 1992). Dynamical processes in clouds, however, can also effect subsaturation phases, wherein the droplet evaporation process is again strongly size dependent. Furthermore, as we have seen in Section 4.1.2, cloud condensation nuclei can be mixed externally, which also allows the larger aerosol particles to have better growth properties than the smaller ones. Thus, also a decrease in concentration with increasing droplet size can result. The occurrence, magnitude, and slope of the size dependence of solute concentrations seems to be mainly determined by the microphysical and chemical conditions at the individual sites. Important governing parameters are the size distribution of the pre-existing aerosol particles, the amount of supersaturation, and its history, i.e. the age of droplets. Another interesting finding related to the pollutant concentrations in liquid phase concerns the deposition or interception of cloud droplets by a coniferous forest close to the measuring site. Results from a one-dimensional resistance model (Pahl et al., 1994) were compared with continuous measurements of the intercepted (impacted) cloud water dripping from the trees. Therefor, the modelled cloud water deposition agreed quite well with observed values. Although the deposition rates (0.3-0.6 mm h -1) were rather low (compared to the precipitation rates of rain), the chemical analysis of the drip water gave concentrations for NO 3, NH + and SO ] - which were 3 to 6 times higher than those of the rainwater samples taken during the experiment. This result gives a clear hint on the important role of the cloud-water deposition responsible for the damage to the subalpine forests in Germany and other parts of Europe.


29

As already mentioned in the previous sections and illustrated in Figures 5 and 11 during some observational periods rain showers also were encountered. As these showers had, in most cases, only very low precipitation rates (< 2 mm h -1) and most measurements (e.g. the chemical analysis of the cloud water) did not show significant differences to the periods before or after the rain events, we refrain from further discussion of these situations.

4.2.

CLOUD FORMATION BY MOIST ADIABATIC LIFTING

In order to test the experimental approach, that cloud formation at the KF in some cases can occur due to adiabatic lifting a simple air parcel model (Flossmann et aL, 1985; Flossmann, 1994) was used to make some comparison with the observational results. For the comparison, we chose the night of 1 to 2 November. Clouds appeared (Figure 5) after 20:00, 1 November, which initially immersed the summit only 50100 m until 2:00, 2 November. Thereafter the observed cloud base dropped to an altitude of 550 m (see Winkler et aI., 1994) where it remained until 6:00. As a first step to check the assumption of pure moist adiabatic lapse rate, the height of cloud base was recalculated iteratively from temperature, pressure, and LWC observations at the summit assuming a wet bulb temperature profile. The results are in good agreement with the observed cloud bases around 750800 m from 20:00-2:00. Afterwards, however, when the observed cloud base dropped to altitudes between 550 and 600 m, the calculations only gave 650670 m (compare also Winkler et aL, 1994) which could have been caused by entrainment processes. For the application of the parcel model to the period after 2:00, where the cloud had a depth to enable a sufficient microphysical and chemical evolution for comparison with observation, we assumed a wet bulb temperature profile between cloud base at 550 m (960 hPa, 6 °C) and the summit (approx.) at 850 m (930 hPa, 4.3 °C). For the initialization of the microphysics, a distribution for the aerosol number was prescribed which resulted from DMPS measurements at 23:47, 1 November (as during 2:00-6:00 no measurements were made). The aerosol particles were assumed to consist of (NH4)2SO4, 35% soluble. The LWC of 0.56 g m -3, resulting from these moist adiabatic conditions is rather high compared with the observed LWC which was in the range from 0.20 to 0.35 g m -3 (see Table IV). By adding a homogeneous entrainment parametrization to the parcel model, the exchange processes with the environment were included resulting in a significantly smaller LWC of 0.27 g m -3. For a more detailed discussion of entrainment processes occurring during KF '90, see Arends et al. (1994). Another valuable hint for the validity of the assumption of an adiabatically rising air parcel is given by the droplet spectra (Figure 12). In the case of pure adiabatic lifting, the model already produces the typical narrow shape of the drop number distribution which was observed. Including the entrainment, even results

30

w. WOBROCK ET AL.

TABLE IV. Comparison of calculated and observed pollutant levels in cloud water Calculated Case I Case 2

Vertical wind LWC SO2+ (via gas uptake and (S(IV)-S(VI) conversion) SO] + (via uptake of aerosol particles) S(IV) (in the drops) H202 (in the drops) pH

0.25 0.56 4.6

0.5 0,56 3.3

Case 3

Case 4 entrain.

1.0 0.56 2.0

0.5 0.27 3.4

17.6

18.9

19.8

28.5

2.5 0.8 4.9

3.5 1.9 5.0

3.9 2.9 5.t

3.3 5.6 5.0

Case 5 5 ppb SO2 0.5 0,56 6.0

Observed range (mean value)

Units

0.25-0.7 (0.5) 0.2-0.35 (0.27)

m s- 1 g m -3

25-106 (50)

#rnol 1-1

5-39 (11) 0.5-1 (0.68) 3.9-4.6 (4.1)

/,moI 1-i /zmol 1-t

18.9 13.4 0.0006 4.5

in a more realistic spectrum as the gap in the small drop sizes is bridged by newly activated aerosol particles from the freshly entrained air. From this, we can conclude that the assumption of the cloud at 2 November, 2:00-6:00 being generated from the almost moist adiabatic lifting of air up the KF is applicable. Consequently, we feel confident that also the chemical features of these types of observed clouds can be compared with chemical calculations within an adiabatic air parcel model. Therefor, we will focus on S(IV) to S(VI) conversion by H202 and 03 as described in Flossmann (1993). In Table IV, the calculated and the observed concentrations in the liquid phase are given for 5 different boundary conditions. In the rightmost columns, the observed range of values as well as the mean value for the time period from 2:00 to 6:00 are given. The initial SO2 concentration in the gas phase was chosen to be 0.5 ppb~ in cases 1, 2, 3, and 4, and 5 ppb~ in case 5 to cover the range of SO2 concentrations observed (compare Figure 8). H202 measurements in gas phase were generally below the detection limit of 70 ppt0, thus we chose this value as model input. 03 was set to 10 ppb~ in accordance with the observations. In cases 1-3, the influence of the vertical velocity on the drop chemistry was tested and in case 4 the effect of entrainment was included. In general, we can conclude that the total sulphate in the drops lies at the lower end of the range of observed values. This might be partly caused by the fact that the LWC was for all pure adiabatic cases (1, 2, 3, 5) too high. The majority of the sulphate was contributed via the aerosol phase and only 10-20% was produced by the conversion of S(IV) to S(VI) via gas uptake. This small contribution of sulphate production via oxidation is a result of the low H202 concentrations which never exceeded 150 ppbv even in completely cloud free periods (Fuzzi et al., 1994). Furthermore, the assumed initial concentration of 70 ppt~ H202 (detection limit of the H202 gas analyzer) seems to be too high for the event considered since

31


4

10 - ....... ~- ....

a d i a b a t i c (0.58 g m - ° ) a d i a b a t i c + e n t r a i n m e n t (0.27 g m - 3 ) o b s e r v e d L~'C, 2 Nov. 2:00-3:00 (0.29 g m - 3 )

3

10

!

/

i i /

2

10

/

o

1

10

/

/

p/v

o

\\

/

Z 10

o

I j

\\ \

i t

\ \ \ \,

\,~, \'\, 10

",,~

-1

0 Fig. 12.

10

20 diameter

\,\

30 (/xm)

40

50

Observed and calculated droplet spectra.

the resulting H202 concentration in the liquid phase was still too high, while the S(IV) concentration was slightly too low, The values for the pH, on the contrary, do not agree with the observations. This is most likely caused by the fact that large amounts of HNO3 were observed (while practically no NH3 was present) resulting in a strong acidification of the drops (Fuzzi etal., 1994) which is not included in the model. As another process responsible for the acidification Fuzzi et al. also suggest heterogeneous HNO3 formation via NO2 oxidation by 03 to NO3 and N205. These chemical mechanisms, among others, are considered in the model evaluations of Colvile e t al. (1994). They confirm the chemical analysis of KF '90 as described in Fuzzi et aL (1994) and the drawn conclusions. Furthermore, they investigate the problem of the deviation of the measured liquid phase concentrations from Henry's law equilibrium, as was found, e.g., by Winiwarter e t al. (1994) for organic acids during KF '90.

32

w. WOBROCKET AL.

Thus, supporting model evaluations help to gain an insight into the partitioning of the polhttants among gas, liquid, and aerosol particle phase and give us evidence of the ongoing chemical reactions. 5. Final Remarks An overview was given of the Kleiner Feldberg Cloud Experiment. After a description of the general aims of the research, of the observational site, and the measurements which were performed, the general cloud formation mechanisms encountered were presented. Furthermore, the development of the pollutant levels in gas, particulate, and liquid phase was presented and the partitioning of the pollutants among the phases was discussed. However, the present paper was not meant to be exhaustive, instead it only summarizes major findings of this campaign. Also, it was aimed at providing the frame for the individual papers in this issue which will more closely focus on particular aspects of the KF clouds. As this field campaign was the second in a planned series of three, we will attempt to make a first comparison regarding the differences of the pollutant levels on the KF as compared to those in the Po Valley. A comparison of the gas phase observations is displayed in Table V. Most prominent are the differences in the SO2 concentrations. While the SO2 concentrations in the Po Valley range in general below 1 ppbv, the KF observations register mean values of 3 ppbv. The concentrations of NO2 and especially of NO observed in the Po Valley, however, exceeded the concentration at the mountain station by a factor of 5 to 10. Ozone levels for both experiments were quite the same. The reason for the differences mostly lies in the fact that the Po Valley experiment was located in an agricultural area, which can also be seen in the high NH3 value and the resulting high pH. TABLE V. Mean gas phase concentrations and cloud waterpH observedduringKF '90 and PO' 89 Component

Po Valley-fog KF-cloud (KF '90) (PO '89) (ppbv) (ppbv)

SO2 0.5 H202 < 0.025 03 15.0 NO 50.0 NO2 20.0 HNO3 0.25 NH3 2.5 pH (cloud water) 5.0-6.0

3.0 < 0.15 18.0 3.0 5,0 0.3 < 0.3 3.5-45

33

THE K L E I N E R F E L D B E R G C L O U D E X P E R I M E N T 1990, A N OVERVIEW

10

-1

0.318 g m -3 (12 Nov.89, fog) 0.355 g m -3 ( 2 Nov.90, St) ....j--:

"--:i.........

I

lO

r-I

v

o

*,,4

i1

10

c' 1

-3

II

i: 1

10

....

f'

-4

i

a

0

5

"l

I0

15

20

25

30

35

40

45

50

diameter (/zm)

Fig. 13. Typicalmass distributionsof dropletspectraobservedduringPo '89 and KF '90 (the ordinate gives the mass in each size bin). Also, the magnitude of the observed concentrations in the cloud water differs considerably from the measurements obtained in the Po Valley. The results in Figure 1 lb for the time period from 10 to 13 November represent the highest pollution concentrations observed during the entire KF cloud campaign. If we multiply these data with the observed LWC, the resulting NH +, SO ] . , and NO 3 concentrations (in #g m -3) were still 2-3 times lower than the typical concentrations of the Po Valley fog water (Fuzziet al., 1992). This result is also valid for other fog water pollutants like S(IV), Na +, and CI-. A detailed analysis of the chemical composition of the cloud water is presented in Fuzziet al. (1994). Another interesting comparison between KF and Po data concerns the droplet spectra. In Figure 13, typical drop mass spectra, one for each location, are displayed. We can see here that the spectrum measured during the Po Valley experiment was much broader though it had nearly the same LWC as the KF spectrum. These two different types of spectra are caused by the different evolution of the relative humidity. In the Po Valley fog, the supersaturation was around 0.03% and persisted for hours (compare Ogren et al., 1992), while during the KF experiment the supersaturation was estimated to be one order of magnitude larger. However, the time scale was only on the order of some minutes (see Hallberg et al., 1994a).

34

W. WOBROCKET AL.

Acknowledgements Gas phase measurements at the observational site 'Billtalhthe', (HLFU MeBstation K6nigstein) were provided by the Hessisches Landesamt ffir Umwelt, Wiesbaden. The digitized terrain of Kleiner Feldberg was supplied by the Hessisches Landesvermessungsamt, Wiesbaden. The temperature and humidity soundings have been provided by Deutscher Wetterdienst, Wetteramt Frankfurt. The authors want to thank S. Vogt from the Institut ftir Meteorologie und Ktimaforschung, Karlsruhe, for providing measurements with a Doppler Sodar. Fundings for the experiment were provided by: Bundesministerium ftir Forschung und Technologie (Project 07EU773 and 07EU726), Commission of European Communities (Project EV4V-0084-C), Ministry of Economic'Affairs of the Netherlands, Convenzione ENEL-CNR (Sottoprogetto 4), Austrian Fonds zur F6rderung der Wissenschaftlichen Forschung (Projekt P7656TEC), Swedish Environmental Protection Board and Department of the Environment of the United Kingdom. The Kleiner Feldberg Cloud Experiment 1990 was carried out within the project EUROTRAC, subproject GCE (Ground-based Cloud Experiment).

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35

Fuzzi, S., Facchini, M. C., Orsi, G., Lind, J. A., Wobrock, W., Kessel, M., Maser, R., Jaeschke, W., Enderle, K.-H., Arends, B. G., Berner, A., Solly, I., Kruisz, C., Reischl, G., Pahl, S., Kaminski, U., Winkler, E, Ogren, J. A., Noone, K. J., Hallberg, A., Fierlinger-Oberlinninger, H., Puxbaum, H., Marzorati, A., Hansson, H.-C., Wiedensohler, A., Svenningsson, I, B., Martinsson, B. G., Schell, D., Georgii, H.-W. (1992). The Po Valley fog experiment 1989: An overview, TeIlus 44B, 448-468. Fuzzi, S., Facchini, M. C., Schell, D., Wobrock, W, Winkler, R, Arends, B. G., Kessel, M., MNs, J. J., Pahl, S., Schneider, T., Berner, A., Solly, I., Kmisz, C, Kalina, M, Fierlinger, H., Haltberg, A., Vitali, E, Santoli, L., Tigli, G. (1994). Multiphase chemistry and acidity of clouds at Kleiner Feldberg, J. Atmos. Chem. 19, 87-106 (this issue). Hallberg, A., Noone, K. J., Ogren, J. A., Svenningsson, I. B., Flossmann, A. I., Wiedensohler, A., Hansson, H.-C., Heintzenberg, J., Anderson, 1"., Arends, B. G., Maser, R. (1994a). Phase partitioning of aerosol particles in clouds at Kleiner Feldberg, J. Atmos. Chem. 19, 10%127 (this issue). Hallberg, A., Ogren, J. A., Noone, K. J., Okada, K., Heintzenberg, J., Svenningsson, I. B. (1994b). The influence of aerosol particle composition on cloud droplet formation, J. Atmos, Chem. 19, 153-171 (this issue). Mohnen, V. A. and Kadlecek, J. A. (1989). Cloud chemistry research at Whiteface Mountain, Tellus 41B, 79-91. Mossop, S. C. (1963). Stratospheric particles at 20 km, Nature 199, 35-326. Noone, K. J., Ogren, J. A., Hallberg, A., Str/Sm, J., Hansson, H.-C., Svenningsson, B., Wiedensohler, A., Fuzzi, S., Faccbini, M. C., Arends, B. G., and Berner, A. (1992). Changes in aerosol size and phase distribution due to physical and chemical processes in fog, TeIlus 44B, 470-489. Ogren, J. A., Heintzenberg, J., Zuber, A., Noone, K. J. (1989). Measurements of size dependence of solute concentrations in cloud droplets, Teltus 41B, 24-31. Ogren, J. A. and Charlson, R. J. (1992). Implications for models and measurements of chemical inhomogenities among cloud droplets, TeUus 44B, 208-225. Ogren, J. A., Noone, K. J., Hallberg, A., Heintzenberg, J., Schell, D., Bemer, A., Solly, I., Kruisz, C., Reischl, G., Arends, B. G., Wobrock, W. (1992). Measurements of the size dependence of the concentration of non-volatile material in fog droplets, TelIus 44B, 570--580. Okada, K. (1983). Nature of individual hygroscopic particles in the urban atmosphere, J. Meteorot. Soc. Japan 61, 727-736. Pahl, S., Winkler, E, Schneider, T., Arends, B. G., Schell, D., Maser, R., Wobrock, W. (1994). Deposition of trace substances via cloud interception to a coniferous forest at Kleiner Feldberg, J. Atmos. Chem. 19, 231-252 (this issue). Pruppacher, H. R, and Klett, J. D. (1978). Microphysics of Clouds, D. Reidel, Dordrecht. Schell, D. and Georgii, H.-W. (1989). Design and operation of a two-stage fogwater collector, in H, W. Georgii (ed.), Mechanisms and Effects of Pollutant Transfer into Forests, Kluwer Academic Publ., Dordrecht, pp. 221-229. Schell, D., Georgii, H.-W., Maser, R., Jaeschke, W., Arends, B. G., Kos, G. R A., Winkler, R, Schneider, T., Berner, A., Kruisz, C. (1992). Intercomparisnn of fog water samplers, Tellus 44B, 612-631. Svenningsson, I. B., Hansson, H.-C., Wiedensohler, A., Ogren, J. A., Noone, K. J., and Hallberg, A. (1992). Hygroscopic growth of aerosol particles in the Po Valley, TelIus 44B, 546-556. Svenningsson, I. B., Hansson, H.-C., Wiedensohler, A., Noone, K. J., Ogren, J. A., Hallberg, A., and Colvile, R. (1994). Hygroscopic growth of aerosol particles and its influence on nucleation scavenging in cloud: Experimental results from Kleiner Feldberg, J. Atmos. Chem. 19, t29-152 (this issue). Winiwarter, W., Fierlinger, H., Puxbaum, H., Facchini, M. C., Arends, B. G., Fuzzi, S., Schell, D., Kanfinski, U., Pahl, S., Schneider, T., Berner, A., Solly, I., Kruisz, C. (I994). Henry's law and the behavior of weak acids and bases in fog and cloud, J. Atmos. Chem. 19, 173-188 (this issue). Winkler, R, Wobrock, W., Colvile, R. N., Schell, D. (1994). The Influence of meteorology on clouds at Kleiner Feldberg, J. Atmos. Chem. 19, 37-58 (this issue).