Open Access

Mingyi Wang, Weimeng Kong, Ruby Marten, Xu-Cheng He, Dexian Chen, Joschka Pfeifer, Arto Heitto, Jenni Kontkanen, Lubna Dada, Christoph Andreas Kürten, Taina Yli-Juuti, Hanna Elina Manninen, Stavros Amanatidis, Antonio Amorim, Rima Baalbaki, Andrea Baccarini, David M. Bell, Barbara Bertozzi, Steffen Bräkling, Sophia Brilke, Lucía Caudillo Murillo, Randall Chiu, Biwu Chu, Louis-Philippe De Menezes, Jonathan Duplissy, Henning Finkenzeller, Loic Gonzalez Carracedo, Manuel Granzin, Roberto Guida, Armin Hansel, Victoria Hofbauer, Jordan Krechmer, Katrianne Lehtipalo, Houssni Lamkaddam, Marku Lampimäki, Chuan Ping Lee, Vladimir Makhmutov, Guillaume Marie, Serge Mathot, Roy Lee Mauldin, Bernhard Mentler, Tatjana Müller, Antti Onnela, Eva Partoll, Tuukka Petäjä, Maxim Philippov, Veronika Pospisilova, Ananth Ranjithkumar, Matti P. Rissanen, Birte Rörup, Wiebke Scholz, Jiali Shen, Mario Simon, Mikko Sipilä, Gerhard Steiner, Dominik Stolzenburg, Yee Jun Tham, António Tomé, Andrea Christine Wagner, Dongyu S. Wang, Yonghong Wang, Stefan K. Weber, Paul M. Winkler, Peter J. Wlasits, Yusheng Wu, Mao Xiao, Qing Ye, Marcel Zauner-Wieczorek, Xueqin Zhou, Rainer Volkamer, Ilona Riipinen, Josef Dommen, Joachim Curtius, Urs Baltensperger, Markku Kulmala, Douglas R. Worsnop, Jasper Kirkby, John H. Seinfeld, Imad El Haddad, Richard C. Flagan, Neil McPherson Donahue

• A list of authors and their affiliations appears at the end of the paper New-particle formation is a major contributor to urban smog, but how it occurs in cities is often puzzling. If the growth rates of urban particles are similar to those found in cleaner environments (1–10 nanometres per hour), then existing understanding suggests that new urban particles should be rapidly scavenged by the high concentration of pre-existing particles. Here we show, through experiments performed under atmospheric conditions in the CLOUD chamber at CERN, that below about +5 degrees Celsius, nitric acid and ammonia vapours can condense onto freshly nucleated particles as small as a few nanometres in diameter. Moreover, when it is cold enough (below −15 degrees Celsius), nitric acid and ammonia can nucleate directly through an acid–base stabilization mechanism to form ammonium nitrate particles. Given that these vapours are often one thousand times more abundant than sulfuric acid, the resulting particle growth rates can be extremely high, reaching well above 100 nanometres per hour. However, these high growth rates require the gas-particle ammonium nitrate system to be out of equilibrium in order to sustain gas-phase supersaturations. In view of the strong temperature dependence that we measure for the gas-phase supersaturations, we expect such transient conditions to occur in inhomogeneous urban settings, especially in wintertime, driven by vertical mixing and by strong local sources such as traffic. Even though rapid growth from nitric acid and ammonia condensation may last for only a few minutes, it is nonetheless fast enough to shepherd freshly nucleated particles through the smallest size range where they are most vulnerable to scavenging loss, thus greatly increasing their survival probability. We also expect nitric acid and ammonia nucleation and rapid growth to be important in the relatively clean and cold upper free troposphere, where ammonia can be convected from the continental boundary layer and nitric acid is abundant from electrical storms.