On the role of locally produced ultrafine aerosol for regional climate modification
Forschungszentrum Karlsruhe, IMK-IFU, Kreuzeckbahnstr. 19, 82467 Garmisch-Partenkirchen, Germany, Wolfgang.Junkermann@imk.fzk.de
Aerosols and their interaction with clouds, respectively water vapour, and radiation are among the main uncertainties in the climate system. Main reasons for these uncertainties are the very variable temporal and spatial distributions and the additional variability of the size distributions (Chin et al., 2009). Hence an investigation of aerosols and their interactions is a multidimensional problem. Generally aerosol effects tend to cool the atmosphere as far as interaction or modification with the energy budget is concerned but depending on the vertical distribution of aerosols and clouds also heating is possible. Directly shortwave radiation interactive aerosols are fine and coarse particles in sizes above ~ 250 nm (direct effects). Smaller ones act through modification of cloud microphysics as cloud condensation nuclei (first indirect effect and second indirect effect) (Lohmannn and Feichter, 2005). Even the smallest ones with sizes of a few nm contribute through surface reactions to the production of light absorbing molecules. Considering the wide range if sizes transport pathways, and subsequently residence times and three dimensional distributions, local sources of aerosols become very important as elevated levels of aerosols may have a significant local to regional climate effect like dimming of shortwave radiation, suppression or redistribution of precipitation and affecting the environment also through feedback processes in the biosphere besides global positive or negative radiative forcings. Within regional climate the temporal and spatial distribution of precipitation can thus be a key factor.
Spatial distributions of fine and coarse aerosols and aerosol source appointment can be preformed using remote sensing techniques also from satellites (A-Train, MODIS etc.) for example for the large sources desert dust or biomass burning plumes. Smaller local sources are normally not distinguishable from satellites. It’s also possible to find areas of high ultrafine particle impact on clouds although the responsible aerosols are smaller than visible wavelengths and not detectable with optical techniques (Rosenfeld, et al, 2006). To identify these areas high concentrations are necessary in an otherwise clean environment as differences on a larger scale image are used. Studies of precipitation dependence linked to aerosols often require extended time series of precipitation measurements to obtain statistically significant data sets.
Detailed studies of the related processes from small and large particles still require in situ measurements, especially when clouds are involved, of particle size distributions, chemistry, optical properties and cloud microphysics. Ultrafine aerosols in the size range of a few nanometers, generated from gas to particle conversion (nucleation) are not detectable by optical techniques and always require sophisticated instrumentation for sizing and counting. Nucleation mode particles in the boundary layer were long time not considered to be significantly climate relevant although they are responsible for example for the blue haze present in many forested areas (Rasmussen and Went, 1965). Since about one decade now an increasing number of studies show that ultrafine particles originating from nucleation mode particles also have the chance to survive several hours and to grow at least into the accumulation mode not only in remote areas but also in moderate to heavily polluted air (Laaksoonen et al. 2005, Vaattovaara et al, 2006)) and that even in polluted environments their contribution to the total particle number in the accumulation mode can reach a significant percentage. These particles do not yet affect shortwave radiation but they act as precursors for cloud condensation nuclei (CCN). Though average global impacts on cloud the water budget might be low as whatever water vapor is evaporating has to return to the surface as precipitation somewhere, local to regional effects on the distribution and variability of rainfall may be significant. It should be noted that natural and anthropogenic sulphur dioxide emissions in global climate models are included as the source for sulphate aerosols, both as CCN and as scattering aerosol in direct radiation effects.
Experimental evidence is now growing that these processes in fact are detectable not only in heavily polluted (Quian et al, 2009) but also in remote agricultural areas. Results from Alkezweeny (1993) and from Rosenfeld et al, (2006, 2008) confirmed that urban and industrial pollution is able to reduce or redistribute precipitation on a regional scale, Junkermann et al, (2009) described an example of anthropogenic induced enhancement of ‘biogenic’ nucleation mode particles and CCN in an otherwise remote and ‘clean’ agricultural area. Clean remote areas, especially semiarid regions with already low annual rainfall would be more severely affected as the low number of ‘original’ CCN ( a few 100 / cm3) can easily be doubled already by comparably low numbers of freshly produced particles and cloud microphysics in these areas shows the highest sensitivity towards additional CCN.
Although long time considered to be not accessible for experiments due to the high variability of contributing meteorological parameters (Ayers, 2005) these recent experiments now show that in fact the model predicted indirect effects can be experimentally confirmed in selected areas of the world. Recent measurements under well characterized conditions in ‘natural laboratories’ together with the nowadays available long term meteorological records confirm a close link between ultrafine aerosol production and regional climate change and can be used to identify regions which are especially sensitive to changes in the aerosols source strength.
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