Physical chemistry of aerosols

Aerosols influence many areas of our daily life. As suspensions of solid or liquid particles in a gas they appear in many different forms such as clouds, smoke, fog, dust, or smog. They affect the global climate and as air pollutants visibility as well as our health, but they are also used in high-technology material processing and for the administration of therapeutic drugs. This creates a strong demand not only for techniques to monitor and control aerosol particles, but also to produce them in a controlled fashion. Since aerosols are highly complex systems with very diverse properties, this is a demanding task. There are almost no limits on the variety of their chemical composition, size, shape, internal structure, and phase. Let us single out the particle size as an example. It varies over orders of magnitude from the subnanometer range to approximately a few hundred microns. The upper limit is defined by the fact that larger particles cannot remain suspended in the gas phase for prolonged periods. The lower limit is not well defined and often includes what is called clusters of molecules. The particle size influences many other properties as for instance the surface to volume ratio, which increases dramatically with decreasing size. The surface to volume ratio in turn determines the reactivity and the internal structure of the particles, for example.

Scientists have long been aware of the need for a better basic understanding of the complex behavior and properties of aerosols. The field of aerosol science has expanded greatly in recent years and physical chemistry has become central in tackling the diverse challenges. This themed issue contains a collection of contributions on the latest developments in physical chemistry of aerosols devoted to the unraveling of the properties of aerosols. The focus lies on fundamental studies that mainly deal with the characterization of physical and chemical processes of aerosol particles, ranging from transformations of their size, morphology, and phase to chemical reactions. This often includes the development of specialized instruments as a first step.

Atmospheric aerosols represent an important field within the broad area of aerosol science. It is thus not surprising that the vast majority of contributions in this special issue are devoted to aerosols with relevance in atmospheric processes. The impact of atmospheric aerosols is still poorly understood although they are supposed to play a major role in many atmospheric processes. They affect the Earth’s radiation balance and climate through diverse physical and chemical processes including direct absorption and scattering of light by particles. Atmospheric aerosols also act as sites of condensation, chemical reactions, and cloud formation, and hence have an impact on atmospheric removal processes, atmospheric transformations and pollutant scavenging. The magnitude of these effects, however, is highly uncertain, mostly because of the as yet limited understanding of the physical chemistry of these tiny systems.

The Perspective article of Finlayson-Pitts and coworkers comprises an excellent introductory chapter into atmospheric aerosols. They discuss reactive processes at surfaces and the importance of integrating experiment and theory for better understanding. Reactive processes involving aerosol particles or reactive processes which lead to the formation of aerosols are studied in several other contributions (Donahue, Nizkorodov, Wilson, Donaldson, Al-Abadleh, Sullivan, Ammann, Bedjanian, Herrmann, Ziemann, Bertram, Smith and coworkers). A variety of different experimental methods are used to unravel various reactive processes with atmospheric relevance. These contributions demonstrate that we can already obtain an astonishingly detailed picture of these highly complex reactive systems.

Several other contributions provide detailed insight into the hygroscopic properties of aerosol particles including deliquescence, efflorescence, ice nucleation properties, cloud condensation properties and rates of water evaporation (Cziczo, Reid, Zellner, Knopf, Gysel, DeMott, Booth and coworkers). The contributions from Zhang, Devlin, Robertson, Niedziela and coworkers use spectroscopy to probe the physical and chemical properties of aerosol particles. One of the focuses of these studies is optical constants and optical properties, which are important for a range of issues including climate forcing and remote sensing. The contributions of Wilson, Rudich, Bertram, Reid, and coworkers involve the development and characterization of new instrumentation for probing the physical and chemical properties of aerosols particles. The contribution from Fernandez de la Mora and coworkers focused on ion evaporation from nanodroplets, which has implications for several areas including electrospray mass spectrometry. Lastly, the contributions of Caleman, Signorell and coworkers illustrate that aerosols are also important as a means to understand biological processes and in material processing.

We would like to thank all authors for their contributions to this issue. A special thank you goes to Nicola Nugent and the editorial and production offices of PCCP for their competent and very kind support.

We hope that the papers assembled here provide an enjoyable overview on the physical chemistry of aerosols not only for aerosol scientists but also for readers outside the field.

Ruth Signorell and Allan Bertram, University of British Columbia, Vancouver, Canada

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