Physical and chemical processes related to aerosols are influenced by the microstructure of the individual particles, and in most cases, the region near the surface of a particle dictates its interactions with surrounding gas phase molecules. For example, surface properties dictate condensation of water vapour on cloud nuclei, and the water cycle depends on whether the surface of the particles is hydrophilic or hydrophobic. In addition, the presence of a layer or coating on a particle can significantly influence its physical and chemical transformations. For instance, although black carbon (BC) is hydrophobic, the presence of layers of hygroscopic materials, such as OC, sulphates, and sea salts, on BC particles can significantly increase the water absorbing capacity of BC-containing aerosol. One important factor that dictates cloud coverage is the water content of aerosols. The water content depends on the relative humidity, and deliquescence and crystallization points. The physical state of aerosols (i.e., solid particles or liquid droplets) is dictated by the deliquescence and crystallization humidities and the hysteresis loop. We have very limited understanding of the hygroscopic properties of microstructured aerosols, such as BC-containing aerosols.

In view of the situation we have examined hygroscopic properties of microstructured aerosols. In this study we present results on deliquescence, growth and recrystallization characteristics of inorganic salt particles coated with water-insoluble layers, as well as soot particles coated with inorganic salts. An electrodynamic balance was used to suspend single charged particles, and a suspended particle was then exposed to an air stream whose relative humidity was adjusted in a controlled manner. The mass of the particle was determined from the d.c. voltage required to maintain the particle at the centre of the balance. The deliquescence and recrystallization points were measured from the jump in the balancing d.c. voltage as well as from the light scattering signals. For examining salt particles coated with water insoluble layers, a droplet generated from a homogeneous solution, for example, of NaCl, DOP, water and ethanol, containing known amounts of NaCl and DOP, was suspended in the balance. The droplet was exposed to a dry air stream for about 20 minutes, and during this period the water and ethanol evaporated leaving a particle containing known proportion of solid NaCl and liquid DOP. After drying, the relative humidity of the air stream was altered in steps. For generating soot particles coated with inorganic salts, we dispersed a known amount of soot particles in a given amount of an aqueous solution that was saturated with a salt. We obtained a particle of soot aggregates coated with salt when a droplet generated from this mixture was dried.

The results obtained from a pure NaCl particle with the results from a NaCl particle coated with a DOP layer. For the pure NaCl particle the observed deliquescence relative humidity of 75.5% and recrystallization humidity of 45% agree well with the values reported in the literature. The results from the coated particle show that the particle gradually absorbs water as the relative humidity is increased from 0 to 24%, where the water content reaches about 36.5 wt% on a DOP-free basis. Then the water content remains constant up to 75% humidity, where the water content rises sharply similar to the deliquescence of a pure NaCl particle. During the period of lowering of humidity, NaCl remains in the solution at very low humidities. For soot particles coated with inorganic salts we observed that deliquescence relative humidities increase slightly, but the water content after the deliquescence is similar to pure salt particles.