Hygroscopic growth of aerosol particles in the Po Valley

A Tandem Differential Mobility Analyser (TDMA) was used to study the hygroscopic growth of individual ambient aerosol particles in the Po Valley, Italy. The measurements were made during the GCE fog experiment in November 1989. During fog, the interstitial aerosol (D_p (at ambient relative humidity) < 5 μm) was sampled. Two modes of particles with different hygroscopic growth were found for 0.030 μm < Dp(dry) < 0.20 μm. On average, the fraction of particles in the two modes were almost equal. The mean growth factor at 85% r.h. was 1.44 ± 0.14 for the more-hygroscopic mode and 1.1 ± 0.07 for the less-hygroscopic mode. The growth factors and the proportion of the particles that were less hygroscopic varied considerably from day to day, but no significant size dependence was seen. Comparison of growth factors for pure salt particles and the measured growth factors indicates that both hygroscopic modes contain a major insoluble part. The effect of the external mixing of hygroscopic properties on the activation of particles to fog droplets is discussed and the fraction of particles that were activated as a function of particle size is predicted. Comparison with the measured scavenging fraction as a function of particle size shows that the hygroscopic properties of the individual particle are as important as the particle size in determining if it will be activated in a fog.