Rapid growth of new atmospheric particles by nitric acid and ammonia condensation
Authors: Wang Mingyi , Kong Weimeng , Marten Ruby , He Xu-Cheng , Chen Dexian , Pfeifer Joschka , Heitto Arto , Kontkanen Jenni , Dada Lubna , Kürten Andreas , Yli-Juuti Taina , Manninen Hanna E. , Amanatidis Stavros , Amorim António , Baalbaki Rima , Baccarini Andrea , Bell David M. , Bertozzi Barbara , Bräkling Steffen , Brilke Sophia , Murillo Lucía Caudillo , Chiu Randall , Chu Biwu , De Menezes Louis-Philippe , Duplissy Jonathan , Finkenzeller Henning , Carracedo Loic Gonzalez , Granzin Manuel , Guida Roberto , Hansel Armin , Hofbauer Victoria , Krechmer Jordan , Lehtipalo Katrianne , Lamkaddam Houssni , Lampimäki Markus , Lee Chuan Ping , Makhmutov Vladimir , Marie Guillaume , Mathot Serge , Mauldin Roy L. , Mentler Bernhard , Müller Tatjana , Onnela Antti , Partoll Eva , Petäjä Tuukka , Philippov Maxim , Pospisilova Veronika , Ranjithkumar Ananth , Rissanen Matti , Rörup Birte , Scholz Wiebke , Shen Jiali , Simon Mario , Sipilä Mikko , Steiner Gerhard , Stolzenburg Dominik , Tham Yee Jun , Tomé António , Wagner Andrea C. , Wang Dongyu S. , Wang Yonghong , Weber Stefan K. , Winkler Paul M. , Wlasits Peter J. , Wu Yusheng , Xiao Mao , Ye Qing , Zauner-Wieczorek Marcel , Zhou Xueqin , Volkamer Rainer , Riipinen Ilona , Dommen Josef , Curtius Joachim , Baltensperger Urs , Kulmala Markku , Worsnop Douglas R. , Kirkby Jasper , Seinfeld John H. , El-Haddad Imad , Flagan Richard C. , Donahue Neil M. 
Published: May 2020

A list of authors and their affiliations appears at the end of the paper New-particle formation is a major contributor to urban smog1,2, but how it occurs in cities is often puzzling3. 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 storms4,5.