Pergamon J. Aend Sci. Vol. 29, Suppl. 1, pp. S71 I-S712. I!398 8 1998 Published by Elsevier Science Ltd. All rights nserved Printed in Gnat Britain 0021-8502/98 $19.00 + 0.00 CONDENSATION NUCLEI (CN) AND ULTRAFINE CN IN THE FREE TROPOSPHERE TO 12 KM ABOVE THE JUNGFRAUJOCH STATION S. Nyeki,“2 M. Kalberer,’ F. Schroder,3 A. Petzold,3 E. Weingartner,’ M. Lugauer,’ I. Colbeck,2 and U. Baltensperger’ ’ Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland. 2 University of Essex, Colchester, England, 3DLR, Oberpfaffenhofen, Germany. KEYWORDS Condensation Nuclei; Free Troposphere; Homogeneous Nucleation Whether the free troposphere (FT) acts as a source of condensation nuclei (CN), through homogeneous nucleation of new particles, and hence as a source of cloud CN after growth, is a theme of current interest (Clarke, 1993; Raes, 1995). At present, few measurements exist due to the large spatial extent of the FT and fewer still up to the tropopause. Of these studies, Clarke (1993) and, Schriider and Strom (1997) measured vertical profiles in which concentrations of ultrafine CN increased up to the tropopause in most cases. These studies and recent modelling (Raes, 1995) suggest that the FT may be a possible source of CN to the lower marine and planetary boundary layers. In order to gather data from the central European region, an airborne campaign to investigate aerosol parameters up to 12 km, was recently conducted on 30 July 1997 during a morning and afternoon descent over the Jungfraujoch (JFJ; 3454 m, CH). Stacked oval descents were flown in the German Aerospace (DLR) Falcon 20 jet aircraft. Ambient air was sampled using a shrouded inlet and passed to two CN counters (TSI 3760A). These were operated at different thermal gradients between saturator and condenser, with 50 % cut-off diameters at d - 5 and 15 nm (Schroder and Strom, 1997), giving CN (N>s, NZi5) and hence ultrafine CN concentrations (5 < d < 15 nm, N~.ls). The morning descent (0730 to 1000 hrs; LST = UTC + 1 hr) in Figure 1, illustrates NX5, N>i5 and the ratio N,s/NZ1s. Discussing the overall trend first, values of N>s and N*is decrease from 670 and 630 mg-’ at 4.5 km to 1090 and 570 mg-’ by 11.9 km, respectively. Median values during level flight periods, averaged over horizontal legs 25 - 35 km in length, appear as symbols. Analysis of the profiles suggests that an apparent linear behaviour of log(concentration) exists over discrete height intervals, as indicated by the linear regression models in Figures 1 and 2. The physical basis of these observations may possibly be related to synoptic features as well as to parameters included in homogeneous nucleation theory, such as pre-existing aerosol surface area concentration, but remains to be determined. The ratio N,s/N,,s also exhibits a surprisingly linear value and suggests a steady background increase in particle production with altitude up to a region at 10 - 11 km, as illustrated by the increase in Ns-15. An identical afternoon descent in Figure 2 (1350 to 1610 hrs) illustrates similar back- ground profiles to Figure 1 up to 8.9 km. Above this altitude N,s is similar to the morning profile but NZ15 decreases rapidly, resulting in N>5/Nkis - 5.7. As both N&Nris ratios exhibit similar changes in the regression models at 6200 and 8900 m, instrumental artifacts are not considered responsible for the observed linear behaviour. These results and those from slow ascents, suggest them to be representative of the regional background FT at that time. Simultaneous 1 minute values of N>io at the JFJ exhibit a median N>IO = 620 mg-’ during the FT influenced period of 0300 - 0900 hrs. A comparison of JFJ N>IOand N>is at the lowest altitude of 4.5 km during the same period (at 1000 hrs) gives medians of 677 and 625 mg-‘, s711