Atmospheric aerosol particles that enable the condensation of water vapor and formation of cloud droplets are called cloud condensation nuclei. Elevated concentrations of CCN tend to increase the concentration and decrease the size of droplets in a cloud. In addition to changing the optical properties and radiative effects of clouds on climate, this could lead to the suppression of precipitation in shallow and short-lived clouds, but also to greater convective overturning and more precipitation in deep convective clouds. The response of cloud characteristics and precipitation processes to increasing anthropogenic aerosol concentrations represents one of the largest areas of uncertainty in the current understanding of climate change. One of the crucial challenges is to determine the ability of aerosol particles to act as CCN under relevant atmospheric conditions, an issue that has received increasing attention over the past years (McFiggans et al. 2006; IAPSAG 2007; IPCC 2007; Andreae and Rosenfeld 2008; and references therein).

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