Molecular-scale evidence of aerosol particle formation via sequential addition of HIO3


Homogeneous nucleation and subsequent cluster growth leads to the formation of new aerosol particles in the atmosphere1. The nucleation of sulfuric acid and organic vapours is thought to be responsible for the formation of new particles over continents1,2, whereas iodine oxide vapours have been implicated in particle formation over coastal regions3,4,5,6,7. The molecular clustering pathways that are involved in atmospheric particle formation have been elucidated in controlled laboratory studies of chemically simple systems2,8,9,10, but direct molecular-level observations of nucleation in atmospheric field conditions that involve sulfuric acid, organic or iodine oxide vapours have yet to be reported11. Here we present field data from Mace Head, Ireland, and supporting data from northern Greenland and Queen Maud Land, Antarctica, that enable us to identify the molecular steps involved in new particle formation in an iodine-rich, coastal atmospheric environment. We find that the formation and initial growth process is almost exclusively driven by iodine oxoacids and iodine oxide vapours, with average oxygen-to-iodine ratios of 2.4 found in the clusters. On the basis of this high ratio, together with the high concentrations of iodic acid (HIO3) observed, we suggest that cluster formation primarily proceeds by sequential addition of HIO3, followed by intracluster restructuring to I2O5 and recycling of water either in the atmosphere or on dehydration. Our study provides ambient atmospheric molecular-level observations of nucleation, supporting the previously suggested role of iodine-containing species in the formation of new aerosol particles3,4,5,6,7,12,13,14,15,16,17,18, and identifies the key nucleating compound.

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Figure 1: A typical particle formation event recorded at Mace Head.
Figure 2: Plot of mass defect versus cluster mass depicting the abundance and atomic composition of nucleating neutral clusters during the event.
Figure 3: Cluster concentration versus HIO3 concentration.


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This work was partly funded by the Academy of Finland (Centre of Excellence Project 1118615 and Projects 251427 and 266388), the PEGASOS project (funded by the European Commission under the Framework Program 7 (FP7-ENV-2010-265148)), ACTRIS, the EPA-Ireland, the Nordic Centre of Excellence (CRAICC), the Finnish Antarctic Research Program and the European Research Council (ATMNUCLE grant 227463, MOCAPAF grant 257360 and COALA grant 638703).

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M.S., N.S., T.J., S.R., J.Ka., A.F. and O.P. performed the field measurements. M.S., N.S., T.J., J.Ko. and H.J. analysed the data. H.H. suggested the chemical mechanism and performed quantum chemical calculations. S.R., T.B. and M.S. performed the laboratory experiments. M.S., V.M.K. and C.O. wrote the manuscript. M.S., M.K., H.J. and C.O. designed the measurements. D.C. and C.O. organized the field study at Mace Head and contributed to the data analysis. All authors contributed to data interpretation and to the development of the final manuscript.

Correspondence to Mikko Sipilä.

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TofTools for Matlab used for processing the mass spectrometer data is available upon request from the corresponding author.

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Sipilä, M., Sarnela, N., Jokinen, T. et al. Molecular-scale evidence of aerosol particle formation via sequential addition of HIO3. Nature 537, 532–534 (2016).

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