In situ observations of new particle formation in the tropical upper troposphere: the role of clouds and the nucleation mechanism

Abstract

Abstract. New particle formation which generates ultrafine aerosol was observed in the continental tropical Upper Troposphere (UT) and Tropical Tropopause Layer (TTL), particularly at the bottom of the TTL, by in situ airborne measurements over South America (January–March, 2005) and West Africa (August, 2006). Measurements with a set of condensation particle counters with different dp50 (50% detection efficiency cut-off particle diameter) were conducted in the altitude range of 12.0–20.5 km on board the high altitude research aircraft M-55 "Geophysica" and at up to 11.5 km altitude on board the research aircraft DLR Falcon-20. Concentrations of ultrafine particles in the size range of 6 to 15 nm were derived from these measurements and several events of new particle formation (NPF) were identified. For two flight segments (24 February 2005 and 7 August 2006, at 12.5 km altitude) when recent lifting had influenced the probed air mass, the concentration of ultrafine particles reached up to 16 000 particles cm−3 (ambient concentration). A sensitivity study by using an aerosol model which includes neutral and ion induced nucleation processes revealed predicted concentrations of ultrafine particles in reasonable agreement with the in situ observations. NPF over South America was observed in cloud free air, above thin cirrus, while over West Africa, in the outflow of a Mesoscale Convective System (MCS), newly formed particles in the range of several hundred per cm3 were found to coexist with ice cloud particles as long as the concentration of cloud particles (dp>2 μm) remained below 2 cm−3. The occurrence of NPF within the upper troposphere and the TTL was generally confined within an altitude band extending from 340 K to 380 K potential temperature, of particular strength between 350 K and 370 K. By means of a heated aerosol inlet line (at 250 °C) measurements of particle volatility were performed which show that within the TTL over South America and West Africa, on average 10–25% of the particles contained non-volatile cores. In background UT/TTL conditions the fractions of non-volatile particles typically ranged up to 50%. Our measurements provide further evidence for the hypothesis that the tropical UT and the TTL are aerosol source regions supplying freshly nucleated particles which, if lifted, may contribute to maintain the stratospheric background aerosol. These particles can become important for cloud formation in the tropical upper troposphere, if they further grow such that they can act as cloud condensation nuclei.