ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-10-27227-2010 Concurrent observations of atomic iodine, molecular iodine and ultrafine particles in a coastal environment Mahajan A. S. 1 Sorribas M. 2 Gómez Martín J. C. 1 MacDonald S. M. 3 Gil M. 2 Plane J. M. C. 3 Saiz-Lopez A. 1 Laboratory for Atmospheric and Climate Science (CIAC), CSIC, Toledo, Spain Atmospheric Research and Instrumentation Branch, National Institute for Aerospace and Technology (INTA), Huelva, Spain School of Chemistry, University of Leeds, Leeds, UK 10 11 2010 10 11 27227 27253 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys-discuss.net/10/27227/2010/acpd-10-27227-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/10/27227/2010/acpd-10-27227-2010.pdf

Simultaneous measurements of atomic iodine (I), molecular iodine (I<sub>2</sub>) and ultrafine particles were made at O Grove, Galicia (42.50° N, 8.87° W), on the northwest coast of Spain. The observations show a strong tidal signature, and indicate that the most probable sources of reactive iodine species are the exposed macroalgae during low tide. For the first time, I<sub>2</sub> and I were concurrently measured revealing a high average I<sub>2</sub>/I ratio of ~32, which is higher than previously inferred by modelling studies. A 1-dimensional photochemical model is employed to simulate the observations showing that the high I<sub>2</sub>/I ratio can be reproduced in the presence of fast vertical mixing close to the surface, or using an extra chemical loss for I atoms with an unknown species. There is a lack of strong correlation between the I<sub>2</sub>/I and ultrafine particles, indicating that although they both have macroalgal sources, these were not at the same location. The model simulations also suggest that the source of the observed ultrafine particles is likely not very close to the measurement site, in order for the particles to form and grow, but the source for I and I<sub>2</sub> must be local. Finally, the effect of NO<sub>x</sub> levels on iodine oxides, and the conditions under which iodine particle bursts will be suppressed, are explored.

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