Secondary organic aerosol (SOA) particles are formed in the atmosphere from condensable oxidation products of anthropogenic and biogenic volatile organic compounds (VOCs)1,2,3,4,5,6,7. On a global scale, biogenic VOCs account for about 90% of VOC emissions1,8 and of SOA formation (90 billion kilograms of carbon per year)1,2,3,4. SOA particles can scatter radiation and act as cloud condensation or ice nuclei, and thereby influence the Earth’s radiation balance and climate1,2,5,9,10. They consist of a myriad of different compounds with varying physicochemical properties, and little information is available on the phase state of SOA particles. Gas–particle partitioning models usually assume that SOA particles are liquid1,5,11, but here we present experimental evidence that they can be solid under ambient conditions. We investigated biogenic SOA particles formed from oxidation products of VOCs in plant chamber experiments and in boreal forests within a few hours after atmospheric nucleation events. On the basis of observed particle bouncing in an aerosol impactor and of electron microscopy we conclude that biogenic SOA particles can adopt an amorphous solid—most probably glassy—state. This amorphous solid state should provoke a rethinking of SOA processes because it may influence the partitioning of semi-volatile compounds, reduce the rate of heterogeneous chemical reactions, affect the particles’ ability to accommodate water and act as cloud condensation or ice nuclei, and change the atmospheric lifetime of the particles12,13,14,15. Thus, the results of this study challenge traditional views of the kinetics and thermodynamics of SOA formation and transformation in the atmosphere and their implications for air quality and climate.
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We acknowledge support by the Academy of Finland (decision numbers 110763, 111543, 131019, 218115 and Centre of Excellence Programme) and the Maj and Tor Nessling foundation. We also acknowledge J. Keskinen and A. Arffman for fruitful discussions concerning impactor performance, H. Kuuluvainen for his help in bounce factor measurements, K. Rissa for SEM imaging, L. Hao, P. Tiitta, A. Kortelainen and P. Miettinen for aerosol mass spectrometer analyses and their help during experiments, J. Jokiniemi, U. Tapper and J. Lyyränen (VTT Technical Research Centre of Finland) for the development of SEM sample collection and image analysis methods, A. Diekmann for the differential scanning calorimeter measurements and T. Wagner for providing polystyrene samples.
The authors declare no competing financial interests.
This file contains a Supplementary Discussion, Supplementary Figures 1-8 with legends, Supplementary Methods, Supplementary Tables 1-2 and additional references. (PDF 1819 kb)
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Virtanen, A., Joutsensaari, J., Koop, T. et al. An amorphous solid state of biogenic secondary organic aerosol particles. Nature 467, 824–827 (2010). https://doi.org/10.1038/nature09455
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