# Evolution of particle composition in CLOUD nucleation experiments

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Atmospheric Chemistry and Physics ()

#### Abstract

Sulphuric acid, ammonia, amines, and oxidised organics play a crucial\nrole in nanoparticle formation in the atmosphere. In this study, we\ninvestigate the composition of nucleated nanoparticles formed from these\ncompounds in the CLOUD (Cosmics Leaving Outdoor Droplets) chamber\nexperiments at CERN (Centre europeen pour la recherche nucleaire). The\ninvestigation was carried out via analysis of the particle\nhygroscopicity, ethanol affinity, oxidation state, and ion composition.\nHygroscopicity was studied by a hygroscopic tandem differential mobility\nanalyser and a cloud condensation nuclei counter, ethanol affinity by an\norganic differential mobility analyser and particle oxidation level by a\nhigh-resolution time-of-flight aerosol mass spectrometer. The ion\ncomposition was studied by an atmospheric pressure interface\ntime-of-flight mass spectrometer. The volume fraction of the organics in\nthe particles during their growth from sizes of a few nanometers to tens\nof nanometers was derived from measured hygroscopicity assuming the\nZdanovskii-Stokes-Robinson relationship, and compared to values gained\nfrom the spectrometers. The ZSR-relationship was also applied to obtain\nthe measured ethanol affinities during the particle growth, which were\nused to derive the volume fractions of sulphuric acid and the other\ninorganics (e. g. ammonium salts). In the presence of sulphuric acid and\nammonia, particles with a mobility diameter of 150 nm were chemically\nneutralised to ammonium sulphate. In the presence of oxidation products\nof pinanediol, the organic volume fraction of freshly nucleated\nparticles increased from 0.4 to similar to 0.9, with an increase in\ndiameter from 2 to 63 nm. Conversely, the sulphuric acid volume fraction\ndecreased from 0.6 to 0.1 when the particle diameter increased from 2 to\n50 nm. The results provide information on the composition of nucleated\naerosol particles during their growth in the presence of various\ncombinations of sulphuric acid, ammonia, dimethylamine and organic\noxidation products.

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