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<p>In this work, the relationships between size and composition of submicron particles (<span class="inline-formula">PM₁</span>) were analyzed at an urban site in the Metropolitan Area of São Paulo (MASP), a megacity with about 21 million inhabitants. The measurements were carried out from 20 December 2016 to 15 March 2017. The chemical composition was measured with an Aerodyne Aerosol Chemical Speciation Monitor and size distribution with a TSI Scanning Mobility Particle Sizer 3082. <span class="inline-formula">PM₁</span> mass concentrations in the MASP had an average mass concentration of 11.4 <span class="inline-formula">µg m⁻³</span>. Organic aerosol (OA) dominated the <span class="inline-formula">PM₁</span> composition (56 <span class="inline-formula">%</span>), followed by sulfate (15 <span class="inline-formula">%</span>) and equivalent black carbon (eBC, 13 <span class="inline-formula">%</span>). Four OA classes were identified using positive matrix factorization: oxygenated organic aerosol (OOA, 40 <span class="inline-formula">%</span> of OA), biomass burning organic aerosol (BBOA, 13 <span class="inline-formula">%</span>), and two hydrocarbon-like OA components (a typical HOA related to vehicular emissions (16 <span class="inline-formula">%</span>) and a second HOA (21 <span class="inline-formula">%</span>) representing a mix of anthropogenic sources). Particle number concentrations averaged <span class="inline-formula">12 100±6900</span> <span class="inline-formula">cm⁻³</span>, dominated by the Aitken mode. The accumulation mode increased under relatively high-<span class="inline-formula">PM₁</span> conditions, suggesting an enhancement of secondary organic aerosol (SOA) production. Conversely, the contribution of nucleation-mode particles was less dependent on <span class="inline-formula">PM₁</span> levels, consistent with vehicular emissions. The relationship between aerosol size modes and <span class="inline-formula">PM₁</span> composition was assessed by multilinear regression (MLR) models. Secondary inorganic aerosols were partitioned between Aitken and accumulation modes, related to condensation particle growth processes. Submicron mass loading in the accumulation mode was mostly associated with highly oxidized OOA and also traffic-related emissions. To the authors' knowledge, this is the first work that uses the MLR methodology to estimate the chemical composition of the different aerosol size modes. The chemical composition with size-dependent PM provides innovative information on the properties of both primary and secondary organic aerosols, as well as inorganic aerosols in a complex urban environment. The results emphasize the relevance of vehicular emissions to the air quality at MASP and highlight the key role of secondary processes on the <span class="inline-formula">PM₁</span> ambient concentrations in the region since 56 <span class="inline-formula">%</span> of <span class="inline-formula">PM₁</span> mass loading was attributed to SOA and secondary inorganic aerosol.</p>