ACP Atmospheric Chemistry and Physics ACP Atmos. Chem. Phys. 1680-7324 Copernicus Publications Göttingen, Germany 10.5194/acp-4-701-2004 Direct evidence for coastal iodine particles from Laminaria macroalgae – linkage to emissions of molecular iodine McFiggans G. 1 Coe H. 1 Burgess R. 1 Allan J. 1 Cubison M. 1 Alfarra M. R. 1 Saunders R. 2 Saiz-Lopez A. 2 Plane J. M. C. 2 Wevill D. 3 Carpenter L. 3 Rickard A. R. 4 Monks P. S. 4 Atmospheric Physics Group, Physics Dept., University of Manchester Institute of Science & Technology, P.O. Box 88, Manchester, M60 1QD, UK School of Environmental Sciences, University of East Anglia, Norwich, NR4 7JT, UK Chemistry Department, University of York, York, YO10 5DD, UK Department of Chemistry, University of Leicester, Leicester, LE1 7RH, UK 07 05 2004 4 3 701 713 Copyright: © 2004 G. McFiggans et al. 2004 This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 2.5 Generic License. To view a copy of this licence, visit https://creativecommons.org/licenses/by-nc-sa/2.5/ This article is available from https://acp.copernicus.org/articles/4/701/2004/acp-4-701-2004.html The full text article is available as a PDF file from https://acp.copernicus.org/articles/4/701/2004/acp-4-701-2004.pdf

Renewal of ultrafine aerosols in the marine boundary layer may lead to repopulation of the marine distribution and ultimately determine the concentration of cloud condensation nuclei (CCN). Thus the formation of nanometre-scale particles can lead to enhanced scattering of incoming radiation and a net cooling of the atmosphere. The recent demonstration of the chamber formation of new particles from the photolytic production of condensable iodine-containing compounds from diiodomethane (CH<sub>2</sub>I<sub>2</sub>), (O'Dowd et al., 2002; Kolb, 2002; Jimenez et al., 2003a; Burkholder and Ravishankara, 2003), provides an additional mechanism to the gas-to-particle conversion of sulphuric acid formed in the photo-oxidation of dimethylsulphide for marine aerosol repopulation. CH<sub>2</sub>I<sub>2</sub> is emitted from seaweeds (Carpenter et al., 1999, 2000) and has been suggested as an initiator of particle formation. We demonstrate here for the first time that ultrafine iodine-containing particles are produced by intertidal macroalgae exposed to ambient levels of ozone. The particle composition is very similar both to those formed in the chamber photo-oxidation of diiodomethane and in the oxidation of molecular iodine by ozone. The particles formed in all three systems are similarly aspherical. When small, those formed in the molecular iodine system swell only moderately when exposed to increased humidity environments, and swell progressively less with increasing size; this behaviour occurs whether they are formed in dry or humid environments, in contrast to those in the CH<sub>2</sub>I<sub>2</sub> system. Direct coastal boundary layer observations of molecular iodine, ultrafine particle production and iodocarbons are reported. Using a newly measured molecular iodine photolysis rate, it is shown that, if atomic iodine is involved in the observed particle bursts, it is of the order of at least 1000 times more likely to result from molecular iodine photolysis than diiodomethane photolysis. A hypothesis for molecular iodine release from intertidal macroalgae is presented and the potential importance of macroalgal iodine particles in their contribution to CCN and global radiative forcing are discussed.