ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-11-9983-2011 In situ observations of new particle formation in the tropical upper troposphere: the role of clouds and the nucleation mechanism Weigel R. 1 Borrmann S. 1 2 Kazil J. 3 4 Minikin A. 5 Stohl A. 6 Wilson J. C. 7 Reeves J. M. 7 Kunkel D. 2 de Reus M. 1 Frey W. 2 Lovejoy E. R. 3 Volk C. M. 8 Viciani S. 9 D'Amato F. 9 Schiller C. 10 Peter T. 11 Schlager H. 5 Cairo F. 12 Law K. S. 13 Shur G. N. 14 Belyaev G. V. 15 Curtius J. 1 16 Institut für Physik der Atmosphäre, Johannes Gutenberg-Universität, Mainz, Germany Abteilung Partikelchemie, Max-Planck-Institut für Chemie, Mainz, Germany Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, Colorado, USA NOAA Earth System Research Laboratory, Boulder, Colorado, USA Institut für Physik der Atmosphäre, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Oberpfaffenhofen, Germany Norwegian Institute for Air Research (NILU), Kjeller, Norway Department of Engineering, University of Denver, Denver, Colorado, USA Institut für Atmosphärenphysik, Bergische Universität Wuppertal, Germany Istituto Nazionale di Ottica, Consiglio Nazionale delle Ricerche, Firenze, Italy Institut für Energie- und Klimaforschung – Stratosphäre, Forschungszentrum Jülich, Germany Institute for Atmospheric and Climate Science, Eidgenössische Technische Hochschule (ETH), Zürich, Switzerland Istituto di Scienze dell'Atmosfera e del Clima ISAC-CNR, Consiglio Nazionale delle Ricerche, Italy UPMC Univ. Paris 06; Université Versailles St-Quentin; CNRS/INSU, UMR 8190, LATMOS-IPSL, Paris, France Central Aerological Observatory, Dolgoprudny, Moskow Region, Russia MDB-Myasishchev Design Bureau, Zhukovsky-5, Moscow Region, Russia now at: Institut für Atmosphäre und Umwelt, Goethe-Universität, Frankfurt am Main, Germany 29 09 2011 11 18 9983 10010 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/11/9983/2011/acp-11-9983-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/11/9983/2011/acp-11-9983-2011.pdf

New particle formation (NPF), which generates nucleation mode aerosol, was observed in the tropical Upper Troposphere (UT) and Tropical Tropopause Layer (TTL) by in situ airborne measurements over South America (January–March 2005), Australia (November–December 2005), West Africa (August 2006) and Central America (2004–2007). Particularly intense NPF was found at the bottom of the TTL. Measurements with a set of condensation particle counters (CPCs) with different <i>d</i><sub>p50</sub> (50% lower size detection efficiency diameter or "cut-off diameter") were conducted on board the M-55 <i>Geophysica</i> in the altitude range of 12.0–20.5 km and on board the DLR <i>Falcon</i>-20 at up to 11.5 km altitude. On board the NASA WB-57F size distributions were measured over Central America in the 4 to 1000 nm diameter range with a system of nucleation mode aerosol spectrometers. Nucleation mode particle concentrations (<i>N</i><sub>NM</sub>) were derived from these measurements which allow for identifying many NPF events with <i>N</i><sub>NM</sub> in the range of thousands of particles per cm<sup>3</sup>. Over Australia and West Africa, we identified NPF in the outflow of tropical convection, in particular of a Mesoscale Convective System (MCS). Newly formed particles with <i>N</i><sub>NM</sub> > 1000 cm<sup>−3</sup> were found to coexist with ice cloud particles (<i>d</i><sub>p</sub> > 2 μm) as long as cloud particle concentrations remained below 2 cm<sup>−3</sup>. The occurrence of NPF within the upper troposphere and the TTL was generally confined within 340 K to 380 K potential temperature, but NPF was of particular strength between 350 K and 370 K (i.e. ~1–4 km below the cold point tropopause). Analyses of the aerosol volatility (at 250 °C) show that in the TTL on average 75–90% of the particles were volatile, compared to typically only 50% in the extra-tropical UT, indicative for the particles to mainly consist of H<sub>2</sub>SO<sub>4</sub>-H<sub>2</sub>O and possibly organic compounds. Along two flight segments over Central and South America (24 February 2005 and 7 August 2006, at 12.5 km altitude) in cloud free air, above thin cirrus, particularly high <i>N</i><sub>NM</sub> were observed. Recent lifting had influenced the probed air masses, and <i>N</i><sub>NM</sub> reached up to 16 000 particles cm<sup>−3</sup> (ambient concentration). A sensitivity study using an aerosol model, which includes neutral and ion induced nucleation processes, simulates <i>N</i><sub>NM</sub> in reasonable agreement with the in situ observations of clear-air NPF. Based on new, stringent multi-CPC criteria, our measurements corroborate the hypothesis that the tropical UT and the TTL are regions supplying freshly nucleated particles. Our findings narrow the altitude of the main source region to the bottom TTL, i.e. to the level of main tropical convection outflow, and, by means of measurements of carbon monoxide, they indicate the importance of anthropogenic emissions in NPF. After growth and/or coalescence the nucleation mode particles may act as cloud condensation nuclei in the tropical UT, or, upon ascent into the stratosphere, contribute to maintain the stratospheric background aerosol.

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