Cloud condensation nuclei in atmospheric and lab aerosols

The role of the atmospheric aerosol in the climate system has become an important topic because of anthropogenic climate change. A large uncertainty in the models of radiative forcing by aerosols is the indirect effect, i. e. the effect of changes in cloud microphysics due to anthropogenic aerosols on the radiative balance. The proposed project is focussed on a sub-set of the aerosol, the cloud condensation nuclei (CCN), which can grow to cloud droplets in water vapour super-saturated conditions. Numerous field and laboratory studies have been performed, but due to the highly complex chemical composition of CCN, the actual role of especially the organic material in the activation process is still not fully understood. In these field studies, the chemical composition of the CCN usually is not measured, as chemical analyses are performed mainly on total aerosol. In laboratory studies, the CCN activation process is investigated using laboratory generated aerosols consisting of only a limited set of specific substances. The proposed project is designed to bridge this gap by collecting for one year large samples of the size fraction of the aerosol (<100nm) where most of the CCN are found and using the samples to create "synthetic atmospheric aerosols" from aqueous extracts of these samples, which are also analyzed for chemical composition and surface tension. The CCN activation process of the water soluble fraction (WSF) of the ultrafine aerosol (size < 100 nm) can then be investigated in the laboratory in the usual way. A pilot study was already performed to find a proper way of generating and handling the extracts. We expect new insights on the activation of ambient CCN from this study by comparing the results of the laboratory study (WSF) to the results we will obtain during the collection phase for the samples, when a one-year data base of CCN concentrations and activation ratios (i. e. the fraction of CCN number concentration in the total aerosol) will be collected. Together with measurements of the number size distribution (measured with a Differential Mobility Analyzer) and measurements of the mass size distribution (measured with impactors, which will also be used to obtain the size-selected chemical composition of the aerosol) a complete picture of the physical and chemical properties of the CCN aerosol will be obtained. The data from the collection phase will also be used to investigate the conclusions drawn by a recent study (Dusek et al., 2006a) which state that information on particle size is more important than information on chemical composition once the activation behaviour of a certain aerosol type is known.

Cloud condensation nuclei (CCN) are an important fraction of atmospheric aerosols because cloud droplets formed on these nuclei determine the radiative properties of clouds which in turn determine the radiative balance of the earth-atmosphere system and global climate. CCN play an important but as yet not fully understood role in the aerosol indirect effect (i. e. the effect of anthropogenic aerosols on global climate change through their influence on cloud properties). Knowledge on CCN has multiplied in recent years, but because of their highly complex chemical composition their role in cloud droplet activation is still not sufficiently included in cloud or climate models. The abundance of CCN and their relation to the size distribution of the atmospheric aerosol was investigated in P 195 15 - N20 in a one year intensive field study in the urban aerosol of Vienna. Unexpectedly, no seasonal variation of CCN was found despite the seasonally different aerosol sources and chemical composition. Significant differences were found between the activation behaviour of atmospheric CCN and CCN in the "synthetic atmospheric aerosols" produced from nebulized extracts of atmospheric aerosols samples containing most of atmospheric CCN. Together with the results from the field study we conclude that CCN cannot be modelled with sufficient accuracy from data on aerosol size distribution and/or bulk chemistry and that actual measurements of CCN in certain regions and their activation behaviour are needed for modelling of climatic effects of anthropogenic aerosols. Another important fraction of atmospheric aerosols are the ultra fine particles (i. e. particles with diameters <100nm) because of their effects on human health. The chemical composition of the ultra fine aerosol was found to be significantly different from the total aerosol (usually expressed in terms of PM10). Carbonaceous material (total carbon, TC) contributed twice as much to the ultra fine fraction than to PM10, and elemental carbon ("soot") contributed again twice as much to TC in the ultra fine range compared to its contribution to TC in PM10. As ultra fine soot is suspected to cause most of the health effects associated with PM10, this result is highly relevant for public health concerns. The seasonal variation of ultra fine particles was also studied and compared to the behaviour of the ultra fine aerosol in other Central European urban areas (Prague and Budapest).