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This article is protected by German copyright law. You may copy and distribute this article for your personal use only. Other use is only allowed with written permission by the copyright holder. Z. Phys. Chem. 224 (2010) 1095–1117 . DOI 10.1524.zpch.2010.6143 © by Oldenbourg Wissenschaftsverlag, München Studies of the Formation and Growth of Aerosol from Molecular Iodine Precursor By R. W. Saunders 1 , R. Kumar 1 , J. C. Gómez Martín 2 , A. S. Mahajan 2 , B. J. Murray 1 , and J. M. C. Plane 1, * 1 School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, United Kingdom 2 Laboratorio de Ciencias de la Atmósfera y el Clima (CIAC) - CSIC, Polígono de Sta. María de Benquerencia, 45007 Toledo, Spain Dedicated to Prof. Dr. Reinhard Zellner on the occasion of his 65 th birthday (Received December 10, 2009; accepted March 21, 2010) Iodine Oxides . Nucleation . Humidity . Acid Uptake . Temperature The formation and growth of iodine oxide particles (IOPs), originating from molecular iodine precursor, has been studied at room temperature as a function of water vapour, and sulphuric and oxalic acid vapours. A linear variation in total IOP mass was observed over a wide range of iodine atom production rates under both dry and humid formation conditions. Particle forma- tion was also observed in the absence of ozone, and was found to be temperature sensitive, with elevated temperatures resulting in reduced particle number and mass. Electronic structure calculations are used to show that particle formation is initiated by polymerization of I 2 O 4 with I 2 O 3 , or with itself. Formation of IOPs in humid conditions results in lower numbers and smaller particles than formed in the absence of water vapour, because H 2 O forms relatively stable complexes with molecules such as I 2 O 3 and I 2 O 4 , inhibiting their polymerization. Addition of H 2 O to particles formed under dry conditions shows the collapse of fractal-like, aggregate particle structures. The uptake of sulphuric acid vapour onto humidified particles was studied over a wide range of relative humidity (RH) at room temperature, with the calculated accommo- dation coefficient (α) for this process increasing with RH to a value of 0.75±0.05 at RH = 90%. In contrast, growth of particles exposed to oxalic acid vapour was not observed on the experi- mental timescales employed, indicating an upper limit for α of 10 K3 . 1. Introduction The earliest kinetic and spectroscopic studies of the iodine monoxide molecule (IO) produced by the photo-oxidation of gas-phase precursors such as CH 3 I and * Corresponding author. E-mail: [email protected]
