Measurements on Puy de Dome

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

ˆ In October 1996, prior to the CIME, the present CVI was tested at the ECN Ž . Netherlands Energy Research Foundation cloud chamber with a Forward Scattering Ž . Spectrometer Probe FSSP . Comparison of LWC derived from both devices as well as number concentrations of residuals and droplets were in good agreement within devia- tions less than 10. During the subsequent CIME pre-campaign in December 1996 and January 1997 on the Puy de Dome, the ambient air ahead of the CVI was accelerated up to 150 m s y1 , ˆ causing short-term peak supersaturations up to 150. The integral growth effect, Ž however, was negligible below 1 growth in diameter for cloud elements of 5 mm, . decreasing rapidly with size . The only device to check the CVI performance in the pre-campaign was the airborne PVM-A that was operated next to the samplers in the Ž large wind tunnel. In this way a total of six separate cloud events duration between 4 to . 14 h were sampled by CVI and PVM-A. The temperatures during events a2 to a6 were observed in the range from 08C to 48C, whereas event a1 was observed at y68C to y78C. Thus, the PVM-A is most likely influenced during event a1 by the ice phase Ž . fraction. Wendisch et al. 1998 pointed out the fact that measurements of LWC carried out with five PVMs in parallel deviated significantly. Thus, the good correlation Ž . Ž . between LWC and LWC cf. Fig. 6a for cloud event a1 a2, respectively PV M-A CVI through a6 demonstrates representative liquid mass collection by the CVI. Regression lines are added to the data, where each data point represents a 1-min average. Another point that may affect the comparison of CVI and PVM-A is a slightly different size range of detection. While the CVI collects 5 to 50 mm particles, the PVM-A collects particles between 2 and 70 mm. Lower and upper size limits are 50 collection points. Ž Besides measuring LWC, the PVM-A gives the effective diameter Gerber et al., . 1994 ` 3 N r d r H r rs0 D s 2 1 Ž . eff ` 2 N r d r H r rs0 In this study, D is related to the volume-averaged droplet diameter D eff VMD 1 6LWC 3 D s 2 Ž . VM D ž pr N W Drop derived from LWC and number concentration N , where N is assumed to equal CV I CVI CVI Ž . N . It is suggested in Gerber 1996 that a suitable parameterization for the effective Drop diameter is 1 1 3 D s D , 3 Ž . eff VMD ž k Ž Ž . 2 . where k s exp y3 ln b , r denotes the density of water and b the geometric W standard deviation of a lognormal distribution fitted to the droplet spectrum. Gerber Ž . reported k values of 0.75 and 0.95 for a light drizzle stratocumulus Sc cloud. Ž . Ž . Ž . Referring to Eq. 3 , Martin et al. 1994 distinguished maritime k s 0.8 from Ž . Ž . continental k s 0.67 cloud cases. Others e.g., Bower and Choularton, 1992 gave Ž . values of k s 1 Sc clouds . Due to the fact that D follows from measurements of VM D LWC and N , error propagation deteriorates the correlation of the calculated CV I CVI Ž . D vs. the measured D cf. Fig. 6b . Parameterization lines of the effective VM D eff Ž Ž . Ž . diameter cf. Eq. 3 for k s 0.67, 0.75, 0.8, 0.95, and 1 as well as a linear regression line are added to the correlation plot. In general, D slightly exceeds D . This can VM D eff be explained by a small N deficit due to sampling losses of cloud elements in the CV I horizontal section of the CVI that is calculated to reach a maximum of 10 depending A. Schwarzenboeck et al. r Atmospheric Research 52 2000 241 – 260 249 Ž . Ž . Fig. 6. Correlation of PVM-A- and CVI-derived LWC measurements a and PVM-A-derived D correlated to CVI related D b . Linear regression lines with eff VMD 2 Ž . coefficients of determination R as well as parameterization lines for D k s 0.67, . . . , 1 are given in the diagrams. eff on the cloud element spectrum. Assuming variations in b within cloud durations of 4 to 14 h, the variance in CVI derived D compared to the PVM-A measured D can be VM D eff partly explained even within a single cloud. Within the experimental uncertainties of the instruments and by the comparison of D to D , we can derive representative VM D eff values for total number and total mass of cloud elements from the CVI. A wide range of cloud conditions was observed during the time period from December 1996, to January 1997. For the two cloud events discussed below the Ž . variability of pre-, post-, as well as in-cloud residual plus interstitial particle aerosol number concentrations within 10 h of measurements is in the range of 900–1300 cm y3 Ž y3 . y3 700–1050 cm , respectively . Short time fluctuations of up to 150-250 cm occur Ž on time scales of 2–20 min and are superimposed by the large-scale trend time scale of . several hours . The variability of residual particle sizes incorporated in cloud droplets Ž . will be demonstrated in terms of the size-dependent partitioning fractions F D of N residual particles over the sum of residual plus interstitial particles. The chosen experimental set-up continuously monitors the partitioning from simultaneous interstitial and residual data, thus becoming largely independent of temporal variations in the aerosol population. At first, the cloud event from December 6 will be discussed, thereafter, the event on December 4. The order in time was reversed in order to begin with the less complex cloud event. 3.1. Cloud eÕent of Dec. 6, 1996 The cloud event on December 6 was of orographic nature. Winds were moderate with air masses approaching from southern directions. A short microphysical summary of the cloud is given in Table 1. Ice crystals were not present due to the shallow vertical extent of the clouds and temperatures in the range of 274.5 to 275.5 K at the sampling site Ž . 1460 m a.s.l. . Number concentrations N and N in both samplers were anti-corre- RJI CVI lated. The low value of LWC is typical for an orographic cloud on Puy de Dome. ˆ CV I The integral partitioning number fraction F of CVI-collected residual particles is small N while the respective partitioning volume fraction F is high due to the largely V incorporated accumulation mode. Both fractions were derived from DMPS measure- ments in the range of 25 to 850 nm: N V CV I CVI F s and F s 4 Ž . N V N q N V q V CV I RJI CVI RJI Ž . Size distributions over the whole lifetime of the cloud cf. Fig. 7 are shown in 5-min intervals. During pre- and in-cloud conditions aerosol spectra including Aitken mode Table 1 Microphysical parameters for the orographic cloud event on 961206 y3 y3 y3 Ž . Ž . Ž . Ž . N cm N cm F F LWC mg m D mm RJ I CVI N V CVI VMD a a 500–1000 F 300 0.17 –0.45 0.40 –0.88 F150 9–11 a LWC s 50 mg m y3 . CV I A. Schwarzenboeck et al. r Atmospheric Research 52 2000 241 – 260 251 Fig. 7. Overview of particle size distributions recorded with RJI and CVI for the orographic cloud event on 961206. Ž . Ž . D - 100 nm and accumulation mode D 100 nm were observed. Non-zero residual Ž size distributions indicate the presence of the cloud with cloud elements larger 5 mm in . diameter . The accumulation mode that was incorporated to a large fraction into cloud droplets returned to the reservoir of the interstitial sampler after cloud dissipation. Fig. Ž . Ž . 8a–d presents four time periods of pre-cloud 18:30 , in-cloud 19:45, 23:00 , and Ž . post-cloud 01:30 size scans. In order to reduce statistical uncertainties in the DMPS Ž . data the distributions were averaged over 20 min equivalent to four DMPS scans . The pronounced bimodal shape of the pre-cloud aerosol largely disappeared after cloud dissolution. This change is based on the temporal development of the air mass. Aerosol residence time in the orographic cloud was typically in the order of a few minutes. The two in-cloud averages predominantly show accumulation mode particles incorporated in Ž the cloud elements. The very small fraction of incorporated Aitken particles D - 100 . nm is not neglected but will be discussed in connection with the following cloud event where the effect is more dominant. From maximum in-cloud lifetimes of cloud droplets in the range of few minutes and from complete lack of smallest residual particles it can be concluded that diffusion collection, possibly creating additional post-nucleation residual Aitken particles, is negligible and nucleation scavenging was the predominant aerosol controlling process. Ž . Fig. 8. DMPS number size distributions averaged over 20 min for the cloud event on 961206 for a pre-cloud Ž . Ž . Ž . Ž . Ž . 18:30 , b, c in-cloud 19:45, 23:00 , and d post-cloud 01:30 . In Fig. 9a,b the incorporation of aerosol particles in cloud elements is illustrated in Ž . terms of the size-dependent partitioning number fraction F D . Both in-cloud averages N Ž . 19:45 and 23:00 show an S-shaped size dependency comparable to observations in Ž . earlier studies Noone et al., 1992b; Hallberg et al., 1994 . The 50 partitioning Ž . diameter fulfilling F D s 0.5 is about 140 nm for both fractions. Error bars in the N Ž . curves for F D show the standard deviation below and above the 20-min average N values. 3.2. Cloud eÕent of Dec. 4, 1996 The meteorological situation on December 04, 1996 showed a broad cloud passage over France in connection with an occluding frontal system of a cyclone over the North Sea. The observed non-precipitating cloud event on December 4 was part of the warm front undulating from Southwest to Northeast of France, consisting of moderately polluted air masses. The cloud was characterized by strong winds, again from southern directions, such that sampling from crosswinds with sampling losses of hydrometeors Ž . Ž cf. Noone et al., 1992a did not affect the sampling due to north–south orientation of . the rigid PDD wind tunnel . The temperature was between 276 and 278 K at the site. The presence of the ice phase throughout the stratocumulus layer can most likely be excluded. Cloud microphys- Ž . ical properties cf. Table 2 were characterized by higher N and maximum LWC CV I CVI Žthree times the maximum LWC compared to the orographic cloud on December 06, CV I . 1996 . The total number and volume partitioning fractions F and F were similar to N V those presented for December 6. Fig. 10 shows the individual size distributions during the evolution of the cloud. Before and after cloud transition, distinct bimodal aerosol spectra including Aitken mode Ž Ž . D - 100 nm and accumulation mode D 100 nm were observed. For four time Ž . Ž . Ž . Fig. 9. Averaged in-cloud partitioning fractions F D at 19:45 a and 23:00 b on 961206. Error bars N denote one standard deviation above and below the average partitioning fraction. Table 2 Microphysical parameters for the frontal non-precipitating cloud event on 961204 y3 y3 y3 Ž . Ž . Ž . Ž . N cm N cm F F LWC mg m D mm RJ I CVI N V CVI VMD a a 500–1000 F 500 0.16 –0.55 0.40 –0.92 F 450 11–13 a LWC s 50 mg m y3 . CV I Ž . Ž . periods, Fig. 11a–d gives 20-min averages of pre-cloud 13:00 , in-cloud 15:00, 18:00 , Ž . Ž . Ž and post-cloud 20:30 size distributions of the interstitial RJI data and residual CVI . data particles. In addition, for the in-cloud size scans the sum of CVI plus RJI sampled size distributions is shown. The pronounced bimodal shape was retained after cloud dissipation but shows a slightly broader spectrum before cloud formation, particularly in the accumulation mode. This change is due to the possible temporal development of the aerosol size distribution outside this frontal cloud. The possibility of precipitation further upwind of the site may also have lead to the slight depletion of accumulation mode particles. In the cloud at 15:00 a large fraction of the accumulation mode appeared as residual particles while the Aitken mode largely remained in the interstitial phase. At 18:00, however, a number fraction of 37 of the Aitken mode was found in the residual particles of the evaporated cloud elements. To answer the question why a fraction of Aitken mode aerosol is incorporated into cloud elements while the accumulation mode did not completely become activated subsequently few simple scenarios based on equilibrium considerations are presented. In a scenario of a size-dependent chemical composition of the aerosol population the Aitken mode with predominantly water-soluble particulate matter could grow beyond 5 mm, while a less hygroscopic fraction with less soluble matter in the accumulation mode may not activate. One may propose a scenario with an externally mixed aerosol Ž . population comprised of NH SO and insoluble soot particles. Necessary peak 4 2 4 Ž . Ž . supersaturations Kohler, 1926 to activate NH SO particles of 30 or 40 nm in ¨ 4 2 4 diameter are 1 and 0.7, respectively. These high supersaturations on the Puy de Ž . Dome may be realistic, according to recent modeling results Flossmann, 1998 with the ˆ Ž DEtailed SCAvenging and Microphysical Model DESCAM Flossmann et al., 1985, . 1987 . Cloud dynamic processes produce regions of significantly different supersaturations. Then different aerosol particles statistically experience different maximum supersatura- tions S inside the cloud. This will lead to a fractional activation of Aitken sizes while max Ž a fraction of the accumulation mode sizes will not become activated Kaufman and . Tanre, 1994 . ´ Furthermore, a size-dependent chemical composition may create differences in the non-equilibrium kinetic effects leading to limited growth before droplet activation Ž . occurs. Chuang et al. 1997 showed that, e.g., a decrease in accommodation coefficients due to an organic coating in the small accumulation mode possibly allows a fraction of Ž . the Aitken mode particle to activate. Similarly, Bigg 1986 reported droplet formation on aerosol particles in different supersaturated vapors to detect minor surface-active organic compounds, thus delaying or suppressing cloud formation. A. Schwarzenboeck et al. r Atmospheric Research 52 2000 241 – 260 255 Fig. 10. Overview of particle number size distributions recorded with RJI and CVI for the frontal cloud event on 961204. Ž . Fig. 11. DMPS number size distributions averaged over 20 min for the cloud event on 961204 for a Ž . Ž . Ž . Ž . Ž . pre-cloud 13:00 , b, c in-cloud 15:00, 18:00 , and d post-cloud 20:30 . Finally, entrainment from airmasses above the level of maximum supersaturation can never be ruled out when the orography is an operative process in cloud formation as on Puy de Dome. In particular, entrainment might be the most significant contribution to ˆ explain the flattening of scavenging efficiencies towards larger particle sizes. Lifetimes of cloud droplets in the stratocumulus cloud layer are estimated to be in the order of an hour. The main evidence that diffusion collection did not contribute to the residual number concentration came from the CVI data itself. Whereas in a first time Ž . window 15:00–15:45 small residuals were more or less absent, considerable numbers Ž . of Aitken mode residuals appear in a subsequent time window 15:45–16:30 . While on average CWC increased from 0.27 to 0.39 g m y3 and residual number concentration from 310 to 410 a cm y3 , the mean volume droplet radius stayed constant around 5.5 0.5 mm during both time intervals. Calculations of the diffusive collection scavenging were carried out whereby the droplet spectrum was represented by its derived mean volume diameter. For a 1-h in-cloud lifetime 9 of 60 nm interstitial Fig. 12. Hourly wind data of December 4th, 1996 from the Meteo France station Orcines le Puy de Dome. The ˆ vertical line at 15:45 denotes the transition between the two time windows discussed in the text. particles, 17 of 40 nm particles, and 26 of 30 nm particles are collected due to diffusion. Since such small particles are not seen during the first time window presumably in-cloud lifetimes were shorter, or more likely, Aitken mode particles after diffusion collection do not appear as secondary residual particles due to their solubility. Assuming similar in-cloud lifetimes and Aitken mode solubility for both time windows, we can exclude diffusion collection as the process leading to the observed small residual particles. Meteorological data of Dec. 4th were supplied by Meteo France on an hourly base for the station Orcines le Puy de Dome. This mountain station is located on the summit very ˆ Ž . Ž . Ž . Fig. 13. Averaged in-cloud partitioning fractions F D at 15:00 a and 18:00 b on 961204. Error bars N denote one standard deviation above and below the 20-min average partitioning fraction. close to our measurement site. From time window one to time window two the Ž . horizontal wind speed cf. Fig. 12 of air arriving at the meteorological station significantly increased by a factor of 2 to 2.5. As the airmass approaches the Puy de Dome the steep orography produces an updraft that linearly corresponds with the ˆ horizontal velocity. Since the updraft velocity scales with maximum supersaturations Ž . reached in the cloud Rogers and Yau, 1989; Pruppacher and Klett, 1997 the averaged activation diameter significantly dropped by several tens of nanometers as we proceed from time window one to time window two. Thus, we can claim nucleation scavenging as the factual process to explain Aitken mode residual particles within this cloud. Ž . Again, both in-cloud averages 15:00, 18:00 show an S-shaped partitioning fraction Ž . cf. Fig. 13a,b . At 15:00, mostly accumulation mode particles were incorporated in cloud elements and the 50 partitioning diameter is at 115 nm. Around 18:00 it dropped to approximately 70 nm due to the additional activation of a significant fraction of the Aitken mode aerosol particles, a value which is lower than in previous reports for the investigated type of clouds.

4. Summary and conclusions