Indirect aerosol particle effects

The existence of aerosol particles even in the most pristine polar air or in free tropospheric air guarantee cloud droplet formation at very low super-saturation in water clouds. Even in strong updrafts within convective clouds supersaturation will not surmount a few percent. However, the stronger the updrafts and the lower the aerosol particle number density (particles per unit volume) the higher is the percentage of the particles activated as cloud condensation nuclei, if they are not hydrophobic or too small for activation at the supersaturation reached. A typical minimum radius of activation is 0.02 ^m. Cloud droplet concentration and size distribution is therefore not only a function of updraft but also of aerosol particle size distribution, their chemical composition and - mainly for stratus clouds - of the radiative cooling rate in the atmosphere and after cloud formation of the top layers of a cloud.

Many different influences on cloud properties are thus due to aerosol particle characteristics. In this section only the semi-direct, the first and second indirect effect of aerosol particles and the lifting of clouds by air pollution will be discussed. At the end a potential physical mechanism of cloud bursts driven by air pollution will be presented.

The semi-direct effect of aerosol particles

Cloud layers sometimes disappear during the day, because absorption of solar radiation by organic aerosol particles including soot within and above the clouds warm the cloud layer. This effect has been observed in the large continental-scale air pollution plume over India and the adjacent Indian Ocean by Ramanathan et al. (2005) during the Indian Ocean Experiment (INDOEX). It has been termed semi-direct aerosol effect as the result is a direct radiative forcing by aerosol particles which is driven by the absorption potential of particles. This adds to the tendency for warming in contrast to non-absorbing or only weakly absorbing aerosol particles.

The first indirect aerosol effect

Already Twomey (1974) and Grassl (1975) published numerical studies of optical cloud property changes caused by either more aerosol particles or high soot content of an aerosol particle population. In the first case cloud albedo increases with aerosol particle number for thin and thick clouds in the second it decreases. Depending on the optical depth or geometrical thickness of clouds, the addition of particles that also contain some soot will make them look either brighter (thin clouds) or darker (thick clouds), if looked from above. The overall effect for a region is thus depending on the distribution of cloud optical depth. In other words: thin low level water clouds will be brighter in a polluted environment while thick water clouds will become darker, but only if the relative soot content is comparably high.

Has the effect been confirmed by observations? Yes, in local studies (Raes et al., 2000) and in long satellite time series (Krueger and Grassl, 2002, 2004) for Europe and later for China, because drastic changes in air pollution after the collapse of the East Block and strong pollution increase from the 1980s to the late 1990s in China allowed a differentiation.

The second indirect aerosol effect

When air pollution by higher aerosol particle density leads to more, but smaller cloud droplets at nearly the same liquid water content the probability for coalescence of the larger cloud droplets with the smaller ones, initiating drizzle formation, is lowered. Therefore, water clouds forming in polluted air will have higher liquid water content because drizzle formation is inhibited, and thus will exist longer. This aerosol particle effect has also been observed (Albrecht, 1989) and is often called cloud lifetime effect. Its relevance on global scale is not yet assessed.

Lifting of clouds by air pollution

When water clouds form in polluted air their liquid water content stays comparably high because water removal from the clouds by drizzle is inhibited. If such an air parcel is lifted to a level where some of the insoluble aerosol particles within droplets or outside act as freezing nuclei the cloud gets glaciated and more heat is released during this phase change in polluted areas pushing the clouds higher up and thus lowering their top temperature. The drop in cloud top temperature at higher pollution levels has for the first time been observed in satellite data over Europe by Devasthale (2005) and Devasthale et al. (2005) and has recently also been observed over high pollution along shipping channels in the North Sea and the Mediterranean Sea by Devasthale et al. (2006). Lifted cloud tops radiate less to space; hence they would increase the greenhouse effect. With this new finding the overall influence of pollution by aerosol particles has to be assessed anew.

Cloud bursts caused by air pollution

Clouds freezing at higher liquid water content create larger updrafts, thus higher cloud tops and hence more intense precipitation. As cloud formation is inhibited because of lower solar radiation flux densities at the surface, reducing the height of the convective planetary boundary layer, these precipitation events are less frequent under air pollution conditions, but when they occur are more intense. This hypothesis has still to be substantiated. It is mentioned here in order to show that atmospheric physics still has many problems to solve.

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