22The production of condensate compounds from the degradation of benzene by OH radical 23 chemistry was studied. Secondary organic aerosol (SOA) formation was investigated in the 24 EUPHORE (European Photoreactor) simulation chambers. Experiments were performed under 25 different OH-production conditions -addition of H 2 O 2 , NO or HONO -, in a high-volume 26 reactor, with natural light and in the absence of seed aerosols. The consumption of 27 precursor/reagents, the formation of gas-phase and particulate-phase products and the temporal 28 evolution of aerosol were monitored. Several aerosol physical properties -mass concentration, 29 overall aerosol yield, particle size distribution and density -were determined and found to be 30 clearly dependent on OH radical production and NO x concentrations. Furthermore, the use of 31 one and/or two products gas-particle partitioning absorption models allowed us to determine the 32 aerosol yield curves. The SOA yield ranged from 1.6 to 9.7 %, with higher SOA formation 33 under low-NO x conditions. Chemical characterization of the SOA was carried out, determining 34 multi-oxygenated condensed organic compounds by a method based on the gas 35 chromatography-mass spectrometry technique. Several ring-retaining and ring-cleavage 36 products were identified and quantified. The compounds with the highest percentage 37 contribution to the total aerosol mass were 4-nitrobenzene-1,2-diol, butenedioic acid, succinic 38 acid and trans-trans-muconic. In addition, a multigenerational study was performed comparing 39 with the photo-oxidations of phenol and catechol. The results showed that although the mass 40 concentration of SOA produced was different, the physical and chemical properties were quite 41 similar. Finally, we suggest a general mechanism to describe how changes in benzene 42 degradation pathways -rate of OH generation and concentration of NO x -could justify the 43 variation in SOA production and properties. 44 , and it plays a critical role in atmospheric chemistry. Atmospheric studies on benzene 50 acquired greater relevance when this volatile organic compound (VOC) became established as a 51 petrol additive, thus increasing its direct emission. Other benzene sources are the chemical 52 industry, biomass burning, cracking of aromatic hydrocarbons, solvent usage and industries 53 related to vegetable oil processing. The increasing levels of this pollutant -up to 5 g m -3 54 higher than European Directive 2000/69/EC stipulates -are of great concern not only because 55 benzene can promote carcinogenic effects and lung disease in humans but also because it is an 56 important precursor of ground-level ozone (Martin-Reviejo and Wirtz, 2005). 57The atmospheric transformations of VOCs, including aromatic hydrocarbons, have been 58 widely examined using simulation photoreactors which reproduce reactions isolated from 59 meteorological variations or dispersion (Volkamer et al., 2002 the atmospheric degradation is mainly initiated by adding the OH radical to the aromati...

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