Hygroscopic properties and water‐soluble volume fraction of atmospheric particles in the diameter range from 50 nm to 3.8 μm during LACE 98

Abstract

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 μm 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 μm particle size, the third class of nearly complete water‐soluble particles is strongly dominant; above 0.7 μm, 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.