Experimental approach for studying cloud processes

Ž . calibration evidence is available Anderson et al., 1993 . Anderson et al. constructed a mini-CVI with minimum cut-size due to a variety of improvements compared to earlier Ž . CVI designs Ogren et al., 1985; Noone et al., 1988 and due to a superior dry calibration method having superior reliability than a former wet calibration method Ž . Noone et al., 1988 . Reported cut sizes in the range of 4–6 mm of ground-based CVIs Ž . used in campaigns during late Eighties and early Nineties e.g., Noone et al., 1992b were based on model estimates rather than experimental calibrations. A further advancement of this study, besides the development of an improved CVI, Ž . was the coupling of the CVI to a Differential Mobility Analyzer DMA for direct residual size measurements including the entire Aitken mode range. In earlier studies, optical particle spectrometers were interfaced to CVIs that performed at lower detection Ž . limits 100 nm e.g., Noone et al., 1992b; Hallberg et al., 1994 , and in a single Ž . experiment down to 80 nm Heintzenberg et al., 1989 . In the main part of the study, two cloud events will be presented, showing the combined data of the two samplers, with novel results of small activated particles in moderately polluted airmasses.

2. Experimental approach for studying cloud processes

One major task for the understanding of cloud processes is to determine the phase partitioning of volatile, non-volatile, soluble and insoluble material between the intersti- Ž . tial phase and the cloud phase liquid and ice phase . There are three frequently used approaches to study in-cloud processes: 1. Comparison of the concentration of species X in cloud water with that in pre-cloud air. 2. Comparison of the cloud water concentration of species X to that present as non-activated aerosols in the interstitial air. 3. Comparison of the concentration of species X in the interstitial aerosol to that in pre-cloud air. The experimental approach used in this study is represented by item 2. Items 1 and 3 have been pursued in studies elsewhere. For example, according to the third approach, Ž . Hallberg et al. 1994 derived the scavenging efficiency on Kleiner Feldberg comparing pre-cloud aerosol to in-cloud interstitial aerosol in a single point measurement. This approach is subject to errors with respect to the temporal variation of the aerosol Ž . population. Comparison of below-cloud to in-cloud post-cloud, respectively aerosol as Ž . Ž . performed, e.g., during Great Dun Fell GDF field campaigns Choularton et al., 1997 were conducted as two point measurements. The GDF, however, does not necessarily compare the same air-parcel, e.g., due to entrainment from layers above the sampling point. The non-identity of pre-cloud with in-cloud air parcel is indeed of no concern for Ž . Ž . our set-up, as it is for a setup according to methods 1 and 3 . For a detailed analysis of hydrometeors, the major question is how to extract the particulate and dissolved material for subsequent analysis. This kind of airborne collection of hydrometeors is accomplished by a CVI. It has the advantage of sampling Ž . all hydrometeors independently of their phase liquid or ice phase . 2.1. Instrumentation Ž . CVI and Round Jet Impactor RJI presented in this study were designed as the Ž . central instruments of the Cloud Ice Mountain Experiment CIME and installed in a horizontally oriented wind tunnel at the Observatoire du Physique du Globe de Cler- mont-Ferrand on top of the Puy de Dome, France. The CVI has been developed to ˆ Ž . sample cloud elements droplets andror crystals larger than a certain aerodynamic size Ž . while rejecting the interstitial aerosol Ogren et al., 1985 . The CVI cut size D is 50 defined as the diameter with 50 collection efficiency. For a ground-based CVI, a wind tunnel is needed to accelerate the cloudy air for the inertial separation. By varying wind tunnel velocity and CVI counterflow D can be controlled within a certain size range. 50 Ž . Up to now, 6.78 mm Anderson et al., 1993 was the lowest cut size for a ground-based Ž . CVI where calibration evidence is available Noone et al., 1988; Anderson et al., 1993 . Along with the CVI, an interstitial phase RJI was designed for an upper cut size that closely complements the sampling characteristics of the CVI. This conventional im- pactor consists of a single aerosol jet impinging upon a flat porous impaction plate. Particles larger than the cut size of the impactor slip across the streamlines and impact on the plate while smaller particles follow the streamlines curving around the impaction plate. Thus, within this study, the only criterion of phase specification in terms of interstitial aerosol and cloud droplet plus ice crystals is the operational definition of the aerody- namic size of a particle or cloud element that is collected with the respective sampler. Ž . Identical sensors cf. Fig. 3 were connected to CVI and RJI, as there are DMPS Ž scanning between 25 and 850 nm, Condensation Particle Counters CPC 3760A; . Thermosystems, St. Paul, MN, USA for fast concentration measurements, and Lyman-a Ž . hygrometers detecting liquid water content LWC . In addition, an airborne-type Particle Ž . Volume Monitor PVM-A; Gerber Scientific, Reston, VA, USA was installed in the Fig. 3. Instrumental set-up during the Puy de Dome pre-campaign. ˆ Puy de Dome wind tunnel next to the CVI to check the CVI measurements of LWC and ˆ volume mean droplet diameter D . The latter quantity is related to the effective VM D diameter D of the PVM-A. eff 2.2. Calibration of the samplers Ž . The CVI calibration method presented in Anderson et al. 1993 , using hollow glass Ž . beads Polysciences, Warrington, PA; catalogue NR. 19823 to avoid evaporation losses, Ž . was utilized in this study. A polydispersion nominal sizes between 2–20 mm of glass Ž . beads was either drawn through the plumbing of the passive flows switched off CVI to create a reference size distribution, or was size-selected by the operational CVI. In either case the resulting size distribution was measured with an Aerodynamic Particle Sizer Ž . APS-3310, Thermosystems, St. Paul, MN, USA . Division of the operational by the reference size distribution then yielded the collection efficiency. The same calibration principle holds for the RJI, where the reference size distribution was measured by a dummy version of the RJI without impaction plate. The operational runs of the RJI were performed with an oil-coated impaction plate. The RJI transmits small sizes while the larger particles pass through the CVI. Although calibrated cut sizes for RJIs agree well with the theory, the RJI was subjected to the dry calibration method as a cross-check of its performance. Ž During the CVI calibrations, the air velocity in the high speed wind tunnel 225, 200, y1 . Ž y1 . 175, and 150 m s as well as the CVI counterflow 0.5, 1, and 3 l min were varied. Ž In Fig. 4, the collection efficiency curves for three different settings denoted in the . legend are presented. The settings were selected such that cut sizes of 4 mm, 5 mm and 6 mm were met. Over the wide ranges of velocity and counterflow settings D could 50 not be varied extensively. However, one has to keep in mind that the overall design goal in the CVI construction was the minimization of the lower cut size and not the cut size variation over a broad range. The excellent cut sharpness is demonstrated by the fact that Fig. 4. Collection efficiency curves of the improved CVI. Error bars are one standard deviation above and below the average. Fig. 5. Experimental transmission efficiencies and 50 cut-off diameters of the RJI for 4, 5, 6 mm cut size nozzles. Error bars are one standard deviation above and below the average. for each setting the collection efficiency increased from 10 to 90 within a size range in the cloud element diameter of less than 2 mm. Ž Inertial impactors have been studied extensively using numerical methods e.g., . Marple and Willeke, 1976; Rader and Marple, 1985; Willeke and Baron, 1993 . Disregarding bounce-off of particles, an impactor efficiency curve can be determined with as high an accuracy from theoretical analysis as from experimental calibration. Rao Ž . 1975 carried out a detailed experimental study to determine the degree of impaction plate bounce-off from several types of impaction surfaces. It was concluded that an oil-coated plate yields almost 100 efficiency of the theoretical collection curve. The three calibrated collection efficiency curves for the 4 mm, 5 mm and 6 mm impactor nozzles are presented in Fig. 5. The modeled 50 cut-off diameters were clearly met, whereas bounce-off effects of particles larger than the cut-off could not be totally avoided. The latter fact is seen in the flattening of the transmission efficiency curves towards larger sizes.

3. Measurements on Puy de Dome