On the
role of locally produced ultrafine aerosol for regional climate modification
Wolfgang Junkermann
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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