PerspectiveAtmospheric Science

The Aerosol Nucleation Puzzle

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Science  22 Feb 2013:
Vol. 339, Issue 6122, pp. 911-912
DOI: 10.1126/science.1233798

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Even if the air around us seems perfectly clear, there are hundreds or thousands of tiny particles in every cubic centimeter of it. Where do they come from? Some, like dust and sea spray, are lofted from Earth's surface, but most aerosol particles are formed by condensation from molecules present in the gas phase. In a process called nucleation, compounds of low volatility, such as sulfuric acid, condense to small particles when they have reached a high enough concentration in the atmosphere, much like water condenses into cloud drops (see the figure, panel A). How this actually happens at the molecular level has long been an enigma of atmospheric science. Like the shells of a Russian doll, scientists have peeled away layer after layer of this mystery by developing techniques to observe smaller and smaller particles (1). On page 943 of this issue, Kulmala et al. (2) report on their observations of aerosol nucleation in a remote forest in Finland (see the figure, panel B), using new analytical tools to reveal how gas molecules join together to form particles.