Physical, Chemical and Optical Properties of Fine Aerosol as a Function of Relative Humidity at Gosan, Korea during ABC-EAREX 2005

In order to understand the effect of air pollution in Asia to the formation of the brownish haze which is widely spread over the Asian continents, the water uptake by fine aerosol in the atmosphere has been investigated at Gosan background site, Korea during ABC-EAREX 2005. The concentrations of inorganic ions and carbon components, size distributions, and light scattering coefficients in normal and dry conditions were simultaneously measured by using a parallel integrated monitoring system. The ambient fine aerosols collected at Gosan was dominated by water-soluble ionic species (35%) and carbonaceous materials (18%). Size distribution of particulate surface area showed that the elevation of RH make the ambient fine aerosol grow to be the droplet mode, around 0.6 um, and coarse mode, larger than 2.5 um. The hydroscopic growth was mainly observed in larger size range than 10 um. However, when RH is lower than 80%, the increase of droplet mode area was apparently observed in fine size range (D^p < 2.5 μm). Growth factor calculated from scattering coefficients of PM2.5 in dry and normal conditions revealed that the water uptake in fine size range began at intermediate RH range, from 40% to 60%, with the average growth factor 1.10 for 40% RH, 1.11 for 50% RH, and 1.17 for 60% RH, respectively. Finally, average chemical composition of PM_2.5 and the corresponding growth curves were analyzed in order to investigate the relationship between carbonaceous material fraction and hygroscopicity of fine aerosol. As a result, the positive correlation of them was observed suggesting that carbonaceous components also make increase the aerosol water uptake in fine size range as much as water soluble ionic species. In conclusion, the hygroscopic growth of fine aerosol was dominated by the growth of droplet mode aerosol. Especially, the hygroscopic behavior of the droplet mode particles was definitely different with the coarse mode ones. This study suggests that the hygroscopic growth of fine aerosol can be mainly impacted by the organic carbon fraction. In addition, the large fraction of particulate organic materials in the fine size range could accelerate the formation of haze which contains large amount of carbonaceous aerosol, especially in the moderately humidified environment (RH < 80%).

Keywords Hygroscopic aerosol, growth factor, scattering coefficient