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Atmospheric aerosol particles have received attention recently due to their importance in climate change and cloud formation. The influence of these particles on Earth's radiative budget depends on a number of factors, including their size distribution and chemical composition. The size increase of aerosol particles due to water vapor uptake has important implications for the direct scattering of radiation and cloud droplets formation. The extent of the particles' affinity for water vapor depends on chemical composition and atmospheric parameters such as water vapor availability. We used a singlewavelength backscatter LIDAR (532 nm) and relative humidity profiles obtained with radiosondes to study particle growth over the Sao Paulo metropolitan region, Brazil, under different conditions of water vapor availability. On this day we had a breeze onset over the urban area, potentially bringing marine aerosols and humidity from the Atlantic Ocean. To simulate the breeze onset and path of the air parcels, we ran the BRAMS model. To infer the hygroscopic growth factor, we developed a fitting model algorithm, already proposed in the literature, calculating the backscatter coefficient at 532 nm for twentyminute periods after and during the breeze onset and comparing backscattering at various altitude levels with a reference backscattering at the lowest relative humidity below the breeze, i.e., below the base of a cloud cap. In addition, we compared the twenty minutes backscattering profiles inside the breeze to a reference twenty minutes backscattering profile before the breeze, expecting to infer some aerosol hygroscopic properties over the Sao Paulo urban area. [ABSTRACT FROM AUTHOR]