Nature and climate effects of individual tropospheric aerosol particles

Aerosol particles in the atmosphere exert a strong influence on climate by interacting with sunlight and by initiating cloud formation. Because the tropospheric aerosol is a heterogeneous mixture of various particle types, its climate effects can only be fully understood through detailed knowledge of the physical and chemical properties of individual particles. Here we review the results of individual-particle studies that use microscopy-based techniques, emphasizing transmission electron microscopy and focusing on achievements of the past ten years. We discuss the techniques that are best suited for studying distinct particle properties and provide a brief overview of major particle types, their identification, and their sources. The majority of this review is concerned with the optical properties and hygroscopic behavior of aerosol particles; we discuss recent results and highlight the potential of emerging microscopy techniques for analyzing the particle properties that contribute most to climate effects.