residence times of SO 42 - and other particles in the atmosphere (typically a few
days) compared with those of the major greenhouse gases, which remain in the
atmosphere for periods of years. An example of this patchiness of the aerosol
radiative forcing is shown in Fig. 7.22, which gives the distribution across the
globe of the forcing due to anthropogenic SO 42 - aerosols. Not surprisingly in view
of where most of the precursor SO 2 is made, coupled with the short residence
time of SO 42 - particles, the effect is most pronounced over the continents and
especially in regions of high industrial activity.
Indirect effects of aerosols on climate arise from the fact that the particles act
as nuclei on which cloud droplets form. In regions distant from land, the number
density of SO 42 - particles is an important determinant of the extent and type of
clouds. By contrast, over land there are generally plenty of particles for cloud
formation from wind-blown soil dust and other sources. Since clouds reflect solar
radiation back to space, the potential link to climate is clear. The effect is likely
to be most sensitive over the oceans far from land and for snow-covered regions
like Antarctica, where land sources of particles have least effect. In such areas a
major source of aerosols is the DMS route to SO 42 - particles (Fig. 7.23). Thus,
marine phytoplankton are not only the major source of atmospheric acidity but
also the main source of cloud condensation nuclei (CCN) and so play an impor-
tant role in determining cloudiness and hence climate.272 Chapter Seven
–0.5 –1.0–1.0–0.5 –0.5–0.5–1.090 ∞W 180 ∞ 90 ∞E0 ∞45 ∞N45 ∞S180 ∞ 180 ∞Fig. 7.22Modelled geographical distribution of annual direct radiative forcing (W m-^2 ) from anthropogenic
sulphate aerosols in the troposphere. The negative forcing is largest over, or close to, regions of industrial
activity. After IPCC (1995). With permission of the Intergovernmental Panel on Climate Change.