Physical and chemical properties of urban aerosols in São Paulo,Brazil: Links between composition and size distribution ofsubmicron particles

Abstract. In this work, the relationships between size and composition of submicron particles (PM1) were analyzed at an urban site in the Metropolitan Area of Sao Paulo (MASP), a megacity with about 21 million inhabitants. The measurements were carried out from 20th December 2016 to 15th 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. PM1 mass concentrations in the MASP had an average mass concentration of 11.4 μg m−3. Organic aerosol (OA) dominated the PM1 composition (56 %), followed by sulfate (15 %) and equivalent black carbon (eBC, 13 %). Four OA classes were identified using Positive Matrix Factorization: oxygenated organic aerosol (OOA, 40 % of OA), biomass burning organic aerosol (BBOA, 13 %), and two hydrocarbon-like OA components (a typical HOA related to vehicular emissions (16 %), and a second HOA (21 %) representing a mix of anthropogenic sources). Particle number concentrations averaged 12100 ± 6900 cm−3, dominated by the Aitken mode. Accumulation mode increased under relatively high PM1 concentrations, suggesting an enhancement of secondary organic aerosol (SOA) production. Conversely, the contribution of nucleation mode particles was less dependent on PM1 levels, coherent with vehicular emissions. The relationship between aerosol size modes and PM1 composition was assessed by multilinear regression models. Mass loading in the nucleation mode was associated mostly with eBC, HOA and OOA, suggesting the contribution of primary and secondary particles from traffic sources. Secondary inorganic aerosols were partitioned between Aitken and accumulation modes, related to condensation particle growth processes. Submicron mass loading in accumulation mode was mostly associated with highly oxidized OOA and also traffic-related emissions. To the author's knowledge, this is the first work that use the MLR methodology to estimate the chemical composition of the different aerosol size modes. The results emphasize the relevance of vehicular emissions to the air quality at MASP and highlight the key role of secondary processes on the PM1 ambient concentrations in the region since 56 % of PM1 mass loading was attributed to SOA and secondary inorganic aerosol.