Aerosol atmospheric particles refer to solid or liquid phase material suspended into air. Aerosol particles (AP) are primary products of combustion and of industrial or natural processes, and secondary products of gas to particle conversion in the atmosphere. They have been extensively studied because of the adverse health effects and poor visibility they cause. However, APs also have an effect on climate. Estimates have shown that anthropogenic APs can exert a negative radiative forcing of the same magnitude with greenhouse gases (GHG) but of opposite sign. In other words, it was found that APs can have a cooling effect.

The aerosol radiative effect can be direct or indirect. The direct effect is due to scattering and absorption of solar radiation by APs. The indirect effect comes from the action of APs as cloud condensation nuclei (CNC) and from their participation in heterogeneous chemical reactions (HCR). Their action as CNCs can increase the occurrence of clouds and change their properties (for example modify the size and number concentration of cloud droplets - smaller and more numerous droplets increase the cloud albedo). The participation of aerosol particles in HCR can affect the concentrations of trace species in the atmosphere, such as GHGs and stratospheric ozone depleting compounds (stratospheric ozone loss has been estimated to have a cooling effect).

Of these, the direct effect is the best understood and studied and estimates of the direct aerosol forcing range between –0.5 and –2.5 W m^-2. Estimates of the indirect effect are very uncertain and significant research efforts are directed toward its quantification. A discussion of the scientific issues in need of exploration for the accurate quantification of the direct and indirect effect is beyond the scope of this introduction.

Aerosol radiative forcing is also a very important issue for policy drawing. The latest El Nino event has proven beyond doubt the value of climate prediction and in advance planning for reduction and mitigation of the adverse effects. An extensive scientific effort has been under way in order to forecast future climate change due to increased concentrations of GHGs. AP cooling cannot be simply subtracted from GHG warming to estimate the net anthropogenic climate forcing. APs have small residence times in the atmosphere (days to weeks), so their concentrations are not uniform over the planet like those of GHGs that have residence times of years to hundreds of years. Anthropogenic APs mostly occur over the industrialized and urbanized regions of the Northern Hemisphere. Therefore, one has to account for a spatially varying cooling effect.

It becomes apparent that aerosol forcing on climate is one more area that requires continuous feedback between scientists and policy makers, who work on climate change. Therefore, I suggest that Global Nest becomes a forum for presenting the latest scientific findings and discussing their potential policy implications.

Sotiria Koloutsou-Vakakis

Postdoctoral Research Associate, University of Illinois at Urbana-Champaign

Department of Civil Engineering, Environmental Engineering and Science