1 Contribution to the EXACT Project. Atmospheric Ozone Degradation Reaction of 1,2-dihydroxybenzene: Aerosol Formation Study R.I. Olariu, "Al.I.Cuza" University of Iasi, Department of Analytical Chemistry, Faculty of Chemistry, Romania A. Tomas, Ecole des Mines de Douai, Département Chimie et Environnement, France I. Barnes, I. Bejan, K.H. Becker, Bergische Universität Wuppertal, Germany K. Wirtz, Fundacion Centro de Estudios Ambientales del Mediterraneo (CEAM), Paterna (Valencia), Spain Objective The main objective of the present study was to quantify the particles formed by O 3 initiated oxidation of 1,2-dihydroxybenzene in purified air using EUPHORE chamber facilities. The scope of this is to improve the understanding of the mechanisms on secondary aerosol formation in aromatic system. Introduction The presence of the aromatic hydrocarbons, principally benzene, toluene and xylene isomers (BTX) in the urban area is directly correlated to anthropogenic activities. This class of hydrocarbons is assumed to have an important role into secondary organic aerosol in the urban area (Seinfeld and Pandis, 1998). Results from recent studies performed in EUPHORE chamber on the aerosol formation from the photo-oxidation of the phenol initiated by OH radicals under different conditions indicate that the formation of 1,2-dihydroxybenzene, as main primary gas-phase product (about 80% molar yield, Olariu et al., 2001a) is probably crucial in the initiation of the aerosol formation processes from such systems. With respect to the atmospheric chemistry of 1,2-dihydroxybenzene only little knowledge could be found in the literature. There are studies only on gas-phase kinetic of this aromatic compound class which shown that 1,2- dihydroxy(alkyl)benzenes react rapidly with OH radical (Olariu et al., 2000), with NO 3 radicals (Olariu et al., 2002b) and with O 3 as well (Tomas et al., 2003). Considering these results it can be suggested that 1,2-dihydroxybenzene will undergo a very fast secondary chemistry into complex aromatic system that could be probably involved in the secondary organic aerosol formation (Olariu et al., 2001b). To go far one-step in such subject we have conducted a series of experiments to measure aerosol formation yield during the oxidation of 1,2-dihydroxybenzene initiated by O 3 in EUPHORE outdoor smog chamber facility.