Hygroscopic behavior of atmospheric aerosol in Taipei

Hygroscopic growth of particles of different sizes and the resultant size distribution changes were observed as a function of the relative humidity (RH). A particle generation device, RH module, and a Tandem Differential Mobility Analyzer system were set up to measure the particle size distributions under different RH conditions. Adopting Nafion as an RH adjusting module, the aerosol hygroscopic observations were successfully performed without the interference caused by blending sample stream with humidified air. The measured deliquescence humidity of model compounds, NaCl and (NH4)2SO4, agree with the theoretical values reported by other investigators. The particle growth factor is enhanced around the RH of 70%. In addition, particle size distribution behaves as two split groups of particles with the RH >76%. The average growth factors of hygroscopic ambient particles in Taiwan are similar to those reported elsewhere. There are several hygroscopic salt compositions in ambient aerosols, (NH4)2SO4 is the most abundant one. Observed particle deliquescence behaviors showed limited alternation of organics on particle growth at higher RH.