The chemical mixing characteristics of atmospheric aerosols and the degree to which they take up water have large impacts upon the size distribution and subsequent optical properties used to estimate visibility and climate impacts. Although it is understood that particulate matter is, in general, a rather complex mixture of different species, there is evidence that submicron aerosols consist of a mixture of several distinct types of particles, with each type itself consisting of a mixture of species ( Zhang et al. [1993b]). The radiative properties computed for such a mixture can be significantly different than those computed assuming all particles have the bulk composition.
Zhang et al. [1993b] demonstrated the combined use of size-resolved chemical composition determinations, electron microscopy, and tandem differential mobility analysis techniques to investigate the hygroscopicity and degree of internal mixing of particulate matter. Results reported for Los Angeles and Grand Canyon particulate matter showed the existence of two particle types, distinguished by differences in hygroscopicity which were related to differences in composition. As noted by Zhang et al. [1993b], such observations can also provide information about atmospheric aerosol processing, and are needed to in the development of models.
The deliquescence and growth behavior of single salts and of some simple mixtures of salts is well-established. However, the hygroscopic behavior of complex inorganic mixtures must be described using theoretical and empirical equations. Tang and Munkelwitz [1993] derived expressions for the deliquescence humidity for inorganic single-salt and multicomponent aerosol as a function of composition and temperature and tested the theoretical predictions against measurements made on particle suspended in an electrodynamic particle balance. Good agreement was found between predictions and experiments. The levitated-particle technique is also used to study thermodynamic behavior of solutions of mixed ionic composition, such as those found at low humidities in atmospheric aerosols (e.g., the study by Kim et al. [1994]). Models for the hygroscopic behavior of mixed inorganic aerosols are discussed by Kim et al. [1993] and Wexler and Seinfeld [1991].
The partitioning of semivolatile organic compounds between the gas and particulate phases is of concern in estimating removal rates and atmospheric lifetimes of these species; in establishing sampling protocols; and in estimating the role of organics in direct and indirect aerosol climate effects. A model of gas/particle partitioning of organic compounds in the atmosphere is presented by Pankow [1994]. The hygroscopicity of organic-containing aerosols is an important unresolved issue. Hygroscopic properties of organic-coated laboratory salt aerosols were examined by Hameri et al. [1992]; although hygroscopicity was affected by the presence of the coating, the effect was not large. In contrast, the laboratory studies of Andrews and Larson [1993] determined that nonhygroscopic soot particles were rendered hygroscopic when coated with organic surfactants. Further work studying the role of organics in particle hygroscopicity is needed.