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The objective of this work is to evaluate the impact of vehicularemissions on the formation of fine particles (PM2.5;  ≤  2.5 µmin diameter) in the Sao Paulo Metropolitan Area (SPMA) in Brazil, whereethanol is used intensively as a fuel in road vehicles. The Weather Researchand Forecasting with Chemistry (WRF-Chem) model, which simulates feedbacksbetween meteorological variables and chemical species, is used as aphotochemical modelling tool to describe the physico-chemical processesleading to the evolution of number and mass size distribution of particlesthrough gas-to-particle conversion. A vehicular emission model based onstatistical information of vehicular activity is applied to simulatevehicular emissions over the studied area. The simulation has been performedfor a 1-month period (7 August–6 September 2012) to cover theavailability of experimental data from the NUANCE-SPS (Narrowing theUncertainties on Aerosol and Climate Changes in Sao Paulo State) project thataims to characterize emissions of atmospheric aerosols in the SPMA. Theavailability of experimental measurements of atmospheric aerosols and theapplication of the WRF-Chem model made it possible to represent some of themost important properties of fine particles in the SPMA such as the mass sizedistribution and chemical composition, besides allowing us to evaluate itsformation potential through the gas-to-particle conversion processes. Resultsshow that the emission of primary gases, mostly from vehicles, led to aproduction of secondary particles between 20 and 30 % in relation to thetotal mass concentration of PM2.5 in the downtown SPMA. Each ofPM2.5 and primary natural aerosol (dust and sea salt) contributed with 40–50 % of the total PM10 (i.e. those  ≤  10 µm in diameter)concentration. Over 40 % of the formation of fine particles, by mass, wasdue to the emission of hydrocarbons, mainly aromatics. Furthermore, anincrease in the number of small particles impaired the ultraviolet radiationand induced a decrease in ozone formation. The ground-level O3concentration decreased by about 2 % when the aerosol-radiation feedback istaken into account

Impact of vehicular emissions on the formation of fine particles in the Sao Paulo Metropolitan Area: a numerical study with the WRF-Chem model