The chemical composition and water content of atmospheric PM2.5 aerosols in a Tainan urban and coastal area were evaluated in January (winter) and April (spring) of 2002. Controlling relative humidity (RH) at 40±5 %, it was found that NH4+, SO4-2 and NO3- were the dominant water-soluble ionic species in both winter and spring. These accounted for an average 44.6% and 36.1% of PM2.5 mass at the urban site in winter and spring, respectively, and 44.6% and 38% at the coastal site, demonstrating that secondary aerosols were a larger part of the PM2.5 mass in winter at both sites. Utilizing the property of free-water methanol to extract aerosol water, water content was measured by GC-TCD and Karl Fischer methods. Detection limit by GC-TCD and Karl Fischer methods were 11µg and 10µg respectively. The water content (mass concentration) of atmospheric PM2.5 aerosols controlled at RH 602% and measured by Karl Fischer method was 31.3±8.4% and 38.0±7.2% at the coastal site in the daytime and nighttime, respectively, and 25.76±16.9% and 24.3±21.5% at the urban site. The water content was clearly higher in the coastal area in both winter and spring. Apart from urban winter, when daytime water content was slightly higher than nighttime, nighttime values were higher than daytime. Controlling RH at 60±2%, the measured water content was clearly higher than and different from values calculated using the water ascending model (Lee and Hsu, 1998). It may be that the water absorptive properties of soluble organic carbons in aerosols are not taken into consideration in this model.
