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<p>This work aims at quantifying the relative contribution of secondary organic aerosol (SOA) precursors emitted by wildfires to organic aerosol (OA) formation during summer of 2007 over the Euro-Mediterranean region, where intense wildfires occurred. A new SOA formation mechanism, <span class="inline-formula">H²O_aro</span>, including recently identified aromatic volatile organic compounds (VOCs) emitted from wildfires, is developed based on smog chamber experiment measurements under low- and high-<span class="inline-formula">NO_<i>x</i></span> regimes. The aromatic VOCs included in the mechanism are toluene, xylene, benzene, phenol, cresol, catechol, furan, naphthalene, methylnaphthalene, syringol, guaiacol, and structurally assigned and unassigned compounds with at least six carbon atoms per molecule (USC<span class="inline-formula">&gt;</span>6). This mechanism <span class="inline-formula">H²O_aro</span> is an extension of the <span class="inline-formula">H²O</span> (hydrophilic–hydrophobic organic) aerosol mechanism: the oxidation of the precursor forms surrogate species with specific thermodynamic properties (volatility, oxidation degree and affinity to water). The SOA concentrations over the Euro-Mediterranean region in summer of 2007 are simulated using the chemistry transport model (CTM) Polair3D of the air-quality platform Polyphemus, where the mechanism <span class="inline-formula">H²O_aro</span> was implemented. To estimate the relative contribution of the aromatic VOCs, intermediate volatility, semi-volatile and low-volatility organic compounds (I/S/L-VOCs), to wildfires OA concentrations, different estimations of the gaseous I/S/L-VOC emissions (from primary organic aerosol – POA – using a factor of 1.5 or from non-methanic organic gas – NMOG – using a factor of 0.36) and their ageing (one-step oxidation vs. multi-generational oxidation) are also tested in the CTM.</p> <p>Most of the particle OA concentrations are formed from I/S/L-VOCs. On average during the summer of 2007 and over the Euro-Mediterranean domain, they are about 10 times higher than the OA concentrations formed from VOCs. However, locally, the OA concentrations formed from VOCs can represent up to 30&thinsp;% of the OA concentrations from biomass burning. Amongst the VOCs, the main contributors to SOA formation are phenol, benzene and catechol (CAT; 47&thinsp;%); USC<span class="inline-formula">&gt;</span>6 compounds (23&thinsp;%); and toluene and xylene (12&thinsp;%). Sensitivity studies of the influence of the VOCs and the I/S/L-VOC emissions and chemical ageing mechanisms on PM<span class="inline-formula">_2.5</span> concentrations show that surface PM<span class="inline-formula">_2.5</span> concentrations are more sensitive to the parameterization used for gaseous I/S/L-VOC emissions than for ageing.</p> <p>Estimating the gaseous I/S/L-VOC emissions from POA or from NMOG has a high impact on local surface PM<span class="inline-formula">_2.5</span> concentrations (reaching <span class="inline-formula">−30</span>&thinsp;% in the Balkans, <span class="inline-formula">−8</span>&thinsp;% to <span class="inline-formula">−16</span>&thinsp;% in the fire plume and <span class="inline-formula">+8</span>&thinsp;% to <span class="inline-formula">+16</span>&thinsp;% in Greece). Considering the VOC as SOA precursors results in a moderate increase in PM<span class="inline-formula">_2.5</span> concentrations mainly in the Balkans (up to <span class="inline-formula">24</span>&thinsp;%) and in the fire plume (<span class="inline-formula">+10</span>&thinsp;%).</p>