Significant alteration in the hygroscopic properties of urban aerosol particles by the secondary formation of organics

The temporal variation in the hygroscopicity of urban aerosol particles was investigated in Tokyo based on the hygroscopic growth factor (g) measurement in the summer of 2004. For 100 nm particles, sporadic appearance of intermediately hygroscopic (1.11 <= g < 1.29) particles was observed when the oxygenated organic mass increased (r = 0.76), suggesting that the formation of secondary organics governs the hygroscopicity of these particles. During a photochemically active period in the daytime, the mode hygroscopicity of 100 nm particles above g = 1.11 rapidly decreased in a few hours. This is explained by the condensation of secondary organics on pre-existing particles that are highly hygroscopic (g >= 1.29) and are transported from outside the Tokyo metropolitan area. The observed rapid change in particle hygroscopicity suggests that the condensation of secondary organics increase the dry particle diameter at rates of 8 - 17 nm h(-1).