Aircraft measurements of sub micrometer aerosol particles ( > 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 lower stratosphere over Western Europe in summer 1994. A statistical analysis of the occurrence and variability of condensation nuclei larger than 18 rm diameter (N18) and nuclei mode particles (UCN, 7 nm < Dp < 18 nm) showed the following results: The median Na18 concentrations were found to be around 1500 cm−3 throughout the middle and upper free troposphere and dropped down to a few hundred per cm 3 1–2 km inside the stratosphere. Nuclei mode particles were always present above about 5 km altitude and showed the highest median concentrations in the 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 troposphere and decreased to about 150 cm−3 above the tropopause. At the same time the number ratio of UCN to the rest of the sub-micron spectrum steadily increased with altitude from 0.05 to almost 0.4. During the passage of a cold front tropospheric UCN concentrations doubled and occasionally ‘burst’ events with several thousand UCN cm−3 were observed. Above the tropopause nucleation bursts occurred more frequently and were typically extended over 10–30 km horizontal distance. These burst regions were also characterised by high mean wind speeds and horizontal wind shears. Our measurements suggest frequently occurring particle production processes in the free troposphere that do not necessary require low values of pre-existing aerosol surface area. Nucleation events may primarily be attributed to dynamically- induced mixing processes and their strength might depend on the amplitude of temperature- and RH-fluctuations.