Hygroscopic properties and water-soluble volume fraction of atmospheric particles in the diameter range from 50 nm to 3.8 μm during LACE 98 -: art. no. 8119

[1] Hygroscopic properties of atmospheric aerosol particles in the Aitken, large, and giant particle range were studied during the Lindenberg Aerosol Characterization Experiment (LACE 98) in a rural area 80 km southeast of Berlin. The hygroscopic behavior of Aitken particles were determined in situ in four size classes (50, 100, 150, 250 nm) with a Hygroscopic Tandem Differential Mobility Analyzer for relative humidities (RH) of 60% and 90%. Measurements at 60% RH served as reference data used by other LACE 98 investigators for mass closure and radiative transfer calculations. In most cases, at 90% RH, the atmospheric particles could be classified into two groups ("more" and "less" hygroscopic fraction) with different hygroscopic properties, whereas this classification was not possible for the measurements at 60% RH. The measured average growth factors of the "more" hygroscopic fraction were found to be at 1.43, 1.49, 1.56, and 1.63 for particles with a size of 50, 100, 150, and 250 nm, respectively. Growth factors of the "less" hygroscopic mode were about 1.1 for all particle sizes. The water-soluble volume fraction of the particles was estimated using a hygroscopic model, assuming that the particles consist of ammonium sulfate with an insoluble core. Chemical analysis of size-segregated impactor samples confirmed this assumption. The corresponding estimated water-soluble volume fractions for the "more" hygroscopic particles were about 0.47, 0.52, 0.59, and 0.68, whereas the estimated water-soluble volume fractions of the "less" hygroscopic particles were in the order of 0.1. At 60% RH, the measured average growth factors were in the range of 1.15-1.22, the estimated water-soluble volume fractions were in the range of 0.41-0.59. For large and giant aerosol particles, the determination of the soluble volume fraction was extended by direct measurements with the water-soluble fraction of large and giant aerosol particles system. Quasi-monodisperse samples of particles 0.4-3.8 mum in diameter were investigated for seven specific particle sizes. The particles measured within this size range show up to three classes of water-soluble volume fractions: In some cases, a class of nearly insoluble particles corresponding to the less hygroscopic fraction in the Aitken range can be identified. A second class corresponding to the more hygroscopic fraction is found with a water-soluble volume fraction of about 0.5-0.7. In addition to the Aitken range, a third class of particles with a soluble volume fraction of about 0.85 is usually found. Below 0.7 mum particle size, the third class of nearly complete water-soluble particles is strongly dominant; above 0.7 mum, there is an equal distribution of all classes. No significant variation can be seen during the LACE 98 campaign in the large and giant particle size range.