Atmospheric Physics and Chemistry

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Interaction between Aerosols and Clouds: Current Understanding

Jen-Ping Chen, Anupam Hazra, Chein-Jung Shiu, I-Chun Tsai and Hsiang-He Lee Department of Atmospheric Sciences, National Taiwan University, Taipei,, Taiwan

[email protected]. edu. tw

This article reviews our current understanding of the interactive processes between aerosols and clouds pertaining to the issues of climate change and the hydrological cycle. We first introduce the aerosol effects on clouds by the classification of hygroscopic aerosols, carbonaceous aerosols and mineral dust according to the aerosol chemical contents. Following that are discussions on how clouds influence aerosols via microphysical and chemical mechanisms, scavenging by precipitation, and vertical transport of cloud venting, as well as some indirect effects. The main topics covered here include cloud microphysics, cloud chemistry, photochemistry and gas-to-particle conversion, radiation and climate impact, and interactions with the biosphere. The examples given focus more on the work conducted by the Cloud and Aerosol Research Group in the Department of Atmospheric Sciences, National Taiwan University.

1. Introduction

Aerosols and clouds are both colloidal entities that contain suspended particles in the atmosphere. The two are mainly distinguished by whether water vapor may continuously condense/deposit on the particles, which also means that they may interchange by condensation and evaporation of water. Such transformations are frequent and have important consequences for their internal metamorphosis, as well as impacts on the environment.

Aerosol and cloud processes have strong influences on the chemical, hydrological, and radiative budgets of the atmosphere, and through these processes human activities may significantly alter the balances of our climate system. In particular, aerosols may produce climate forcing through their interaction with clouds, which are called the indirect effect of aerosols (Twomey, 1974; Albrecht, 1989; Haywood and Boucher, 2000). This forcing may proceed by enhanced cloud albedo due to more numerous cloud drops (commonly termed the first indirect effect or cloud albedo effect) or by a prolonged cloud lifetime due to reduced cloud drop size and precipitation efficiency (commonly termed the second indirect effect or cloud lifetime effect) if the aerosol number concentrations are increased (IPCC, 2001).

Many observational and modeling results support the first indirect effect (Charlson et al., 1992; Han et al., 1994; Ackerman et al., 2000; Rosenfeld, 2000), but rather limited evidence has been shown to verify the second indirect effect. For both effects a quantitative assessment is still difficult to achieve. The major issue is that there are still no convincing understandings of the aerosol-cloud interactions, including the interaction between aerosols and cloud condensation nuclei (CCNs), CCNs and cloud droplets, and cloud droplets and cloud albedo (IPCC, 2001). In fact, aerosols' effects, particularly the indirect effects, represent the largest source of uncertainty in the current estimates of global climate forcing.

One of the major complications stems from the diversified chemical compositions of aerosols. Different types (composition) of aerosols act differently in climate forcing. For instance, acidic (sulfate, nitrate) aerosols tend to cool the surface, while soot has the additional effect of warming the atmosphere, which may lead to changes in large-scale atmospheric circulation (Hansen et al., 1997; Hansen and Sato, 2001) and even "burning out" the clouds (Ackerman et al., 2000). Acidic particles and sea salt particles are both good condensation nuclei, but the increase of the former tends to reduce precipitation (Twomey, 1974; Rosenfeld, 2000), while the latter tend to enhance precipitation by acting as a rain embryo (Woodcock, 1971; Rosenfeld, 2002). Soot particles may also influence cloud formation, either acting as condensation nuclei or as ice nuclei (Motoi, 1951; Bigg, 1990). Mineral dust, on the other hand, may be the main source of ice nuclei (Motoi, 1951; Georgii and Kleinjung, 1967), which are of great importance in precipitation development, particularly in the mid-to-high latitudes. Moreover, different particulate chemicals may coexist in a specific air parcel by external or internal mixing, the numeral combination of which further muddles up their role in the direct and indirect forcing (Jacobson, 2001; Nenes et al., 2002). Therefore, the studies of aerosols, by either measurement or modeling, necessarily become utterly sophisticated.

The problem becomes even more challenging when aerosols interact with clouds. Aerosols may affect clouds directly through their activation into cloud drops or heterogeneous nucleation into ice crystals, and indirectly through the cloud burning effect by soot or dust particles. Clouds may remove aerosols by wet scavenging (collection and washout), or augment aerosols by aqueous chemical reactions and then deactivation of cloud drops. Clouds may also interact indirectly with aerosols through the alteration of actinic flux and photochemistry, which in turn produce condensable vapor of trace chemicals that may form new aerosols by nucleation or grow on existing ones. The interplay between aerosols and clouds also strongly influences the precipitation processes; therefore, much attention has also been paid to the impact of air pollution on the hydrological cycle, as well as the removal of pollutants by the cloud-cleansing mechanism.

Because of the crucial roles that aerosols and clouds play in the complex atmospheric system, coupled aerosol-cloud models with high accuracy and efficiency have become increasingly important, particularly in the absence of sufficient observational data. Here, we identify some of the most important mechanisms involved in the interaction of aerosols and clouds, as well as review current progress of the theoretical and technical development of the modeling aspect. The examples given are biased toward our research efforts.

2. Aerosol Effect on Clouds

Most atmospheric aerosols tend to have a mixed chemical composition, including a variety of inorganic and organic species. In the lower atmosphere, particulate matter is composed of highly water-soluble inorganic salts, insoluble mineral dust and carbonaceous material. This section discusses the effects of aerosols on clouds according to their main chemical compositions. Caution needs to be taken — natural aerosols are often not pure and may coexist by internal or external mixing.

2.1. Hygroscopic aerosols

Aerosol particles that are capable of initiating the formation of cloud drops and raindrops are called cloud condensation nuclei (CCNs). The ability of CCNs to activate into cloud drops is strongly related to the mass and composition of their water-soluble component. The most common of them include the gaseous and aqueous chemical conversions of their precursors

(such as the oxidation of SO2 and NOx into sulfate and nitrate), as well as a direct emission from the Earth's surface (such as ammonia or sea salt). Whether an aerosol particle may be activated into cloud drops can be decided from its Köhler curve, which describes the equilibrium saturation ratio Seq of a solution drop containing a fixed mass of dissolved salt as a function of the drop radius req. Figure 1 gives an example of the Köhler curve family for ammonium sulfate aerosols. For each droplet the solution (Raoult) effect reduces the surface vapor pressure while the curvature (Kelvin) effect does the opposite, and the combination gives a Köhler curve that has a peak in Seq (commonly called the critical saturation ratio). If the environmental saturation ratio never exceeds the critical Seq, this aerosol will not be acti-vateda into a cloud drop to grow further. From Fig. 1, it can be seen that larger aerosols

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