AIRCRAFT MEASUREMENTS OF SUB MICROMETER AEROSOL-PARTICLES (GREATER-THAN-7 NM) IN THE MIDLATITUDE FREE TROPOSPHERE AND TROPOPAUSE REGION

Submicron aerosol particles with detectable sizes of 7 nm diameter and larger were measured in the mid-latitude free troposphere up to the l ower stratosphere over Western Europe in summer 1994. A statistical an alysis of the occurrence and variability of condensation nuclei larger than 18 nm diameter (N-18) and nuclei mode particles (UCN, 7 nm < D-p < 18 nm) showed the following results: The median N-18 concentrations were found to be around 1500 cm(-3) throughout the middle and upper f ree troposphere and dropped down to a few hundred per cm(3) 1-2 km ins ide the stratosphere. Nuclei mode particles were always present above about 5 km altitude and showed the highest median concentrations in th e upper free troposphere. The median UCN concentrations increased from about 50 cm(-3) in the lower to about 300 cm(-3) in the upper free tr oposphere and decreased to about 150 cm(-3) above the tropopause. At t he same time the number ratio of UCN to the rest of the sub-micron spe ctrum steadily increased with altitude from 0.05 to almost 0.4. During the passage of a cold front tropospheric UCN concentrations doubled a nd occasionally 'burst' events with several thousand UCN cm(-3) were o bserved. Above the tropopause nucleation bursts occurred more frequent ly and were typically extended over 10-30 km horizontal distance. Thes e burst regions were also characterised by high mean wind speeds and h orizontal wind shears. Our measurements suggest frequently occurring p article production processes in the free troposphere that do not neces sary require low values of pre-existing aerosol surface area. Nucleati on events may primarily be attributed to dynamically-induced mixing pr ocesses and their strength might depend on the amplitude of temperatur e- and RH-fluctuations. (C) 1997 Elsevier Science B.V.