Diurnal, weekly, and seasonal variation of hygroscopic properties of submicrometer urban aerosol particles

3911-3922 Hygroscopic properties of aerosol particles at an urban background site in Leipzig, Germany, were measured using a Hygroscopicity-Tandem Differential Mobility Analyzer (H-TDMA) system. Hygroscopic properties of particles with dry diameter of 50 nm (Dd50) and 150 nm (Dd150) were determined at 92% relative humidity (RH) for a period of eight months in the year 2000. For both Dd, the results revealed two major hygroscopic groups (nearly hydrophobic group with an average growth factor of 1.06±0.04 for Dd50 and 1.09±0.04 for Dd150, and more hygroscopic with 1.44±0.08 for Dd50 and 1.65±0.08 for Dd150) and a less-pronounced third group (less hygroscopic with 1.25±0.08 for Dd50 and 1.33±0.07 for Dd150). A simple solubility model was applied to these growth factors to estimate soluble volume fractions. Based on these estimated fractions, for all three hygroscopic groups the Dd50 particles were composed of smaller amounts of soluble material than were the Dd150 particles; the soluble volume fraction for more hygroscopic particles was only 50% for the Dd50 particles and 70% for the Dd150 particles. Calculated number fractions revealed that 31% of Dd50 particles and 22% of Dd150 particles showed nearly hydrophobic behavior. For both particle sizes, the number fraction of nearly hydrophobic particles showed a characteristic diurnal variation dependent on the two daily rush-hour periods. Also, the number fraction of nearly hydrophobic particles on weekdays was about 10% higher than that on weekends. In winter (November–February), the number fraction of nearly hydrophobic particles showed two maxima that coincided with the two daily rush-hour periods. In summer (May–August), only one distinct maximum during the morning rush-hour period was observed.