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4. Aerosol particles and clouds

How do aerosols affect the climate?

All atmospheric aerosols scatter incoming solar radiation, and a few aerosol types can also absorb solar radiation. BC is the most important of the latter, but mineral dust and some OC components are also sunlight absorbers. Aerosols that mainly scatter solar radiation have a cooling effect, by enhancing the total reflected solar radiation from the Earth. Strongly absorbing aerosols have a warming effect. In the atmosphere, there is a mixture of scattering and absorbing aerosols, and their net effect on Earth’s energy budget is dependent on surface and cloud characteristics. Scattering aerosols above a dark surface and absorbing aerosols above a bright surface are most efficient (see Figure 3a). Scattering (absorbing) aerosol above a bright (dark) surface are less efficient because the solar radiation is reflected (absorbed) anyway. Absorbing aerosols are particularly efficient when positioned above clouds, which are a main contributor to the total reflection of solar radiation back to space.The direct aerosol effect and the cloud albedo effect.
Figure 3: The direct aerosol effect and the cloud albedo effect.(a) The direct aerosol effect for low and high surface albedo, for scattering and absorbing aerosols. A dark surface (low albedo) will already absorb a large portion of the solar radiation, and absorbing aerosols will thus have a small effect. Scattering aerosols will instead amplify the total reflectance of solar radiation, since the solar radiation would otherwise be absorbed at the surface. Over a bright surface (high albedo) scattering aerosols have a reduced effect. Absorbing aerosols may, however, substantially reduce the outgoing radiation and thus have a warming effect. (b) The cloud albedo effect (first indirect aerosol effect), cloud lifetime effect (second indirect aerosol effect), and semi-direct effect.

Aerosols are vital for cloud formation because a subset of them may serve as cloud condensation nuclei (CCN) and ice nuclei (IN). An increased amount of aerosols may increase the CCN number concentration and lead to more, but smaller, cloud droplets for fixed liquid water content. This increases the albedo of the cloud, resulting in enhanced reflection and a cooling effect, termed the cloud albedo effect (Twomey 1977; Figure 3b). Smaller drops require longer growth times to reach sizes at which they easily fall as precipitation. This effect, called the cloud lifetime effect, may enhance the cloud cover (see illustration in Figure 3b) and thus impose an additional cooling effect (Albrecht 1989). However, the life cycles of clouds are controlled by an intimate interplay between meteorology and aerosol-and-cloud microphysics, including complex feedback processes, and it has proven difficult to identify the traditional lifetime effect put forth by Albrecht (1989) in observational data sets.

Absorbing aerosols also have the potential to modify clouds properties, without directly acting as CCN and IN, by: (1) heating the air surrounding them while reducing the amount of solar radiation reaching the ground, which stabilizes the atmosphere and diminishes the convection and thus the potential for cloud formation, (2) increasing the atmospheric temperature, which reduces the relative humidity, inhibits cloud formation, and enhances evaporation of existing clouds. This is collectively termed the semi-direct aerosol effect (Hansen et al. 1997). The net effect is uncertain (see Figure 3b) and highly depends on the vertical profile of BC (Koch & Del Genio 2010).

In addition, BC and other absorbing aerosols deposited on snow or ice surfaces may reduce the surface albedo, leading to reduced reflectance of solar radiation, and hence a heating effect (Hansen & Nazarenko 2004).

Radiative forcing (RF) is often used to quantify and compare the potential climate impact of the various aerosol effects. RF is defined as a change in the Earth’s radiation balance due to a perturbation of anthropogenic or natural origin.. The total aerosol forcing probability density function (PDF), in addition to individual aerosol components, indicating both the magnitudes and uncertainty of the effects, is shown in Figure 4a. The wider a PDF, the larger is the uncertainty. Combining all aerosol effects (blue dashed curve in Figure 4a) enhances the uncertainty compared to considering only the direct aerosol effect and cloud albedo effect.Aerosol functions.
Figure 4: Aerosol functions.(a) Probability density functions of aerosol effects (Isaksen et al. (2009), with small updates of cloud albedo and lifetime effects). The total aerosol radiative forcing (red and blue curves), with and without clouds are estimated by combining the individual effects in a Monte Carlo calculation (Boucher & Haywood 2001). Vertical lines show 90% confidence intervals. (b) Climate sensitivity for a doubling of CO2 as a function of the total aerosol RF. Radiative imbalances of 0.85 (solid line, Hansen et al. 2005), 0.7 and 1.0 Wm-2 (grey band) and 0.0 (radiative equilibrium, dashed line) are shown. Industrial era temperature change is taken as 0.8 Kelvin (K), and RF of non-aerosol components +2.9 Wm-2.

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