Particles in theatmosphere 49Figure 3.7Modelled geographic distribution of estimates of the annual mean
direct radiative forcing (in watts per square metre) from anthropogenic sulphate
aerosols in the troposphere. The radiative forcing, which is negative, is largest in
regions close to industrial activity.
So far for aerosol we have been describingdirectradiative forcing.
There is a further way by which particles in the atmosphere could in-
fluence the climate; that is through their effect on cloud formation that
is described asindirectradiative forcing. Two mechanisms of indirect
forcing have been proposed. The first is the influence of the number of
particles and their size on cloud radiative properties. It arises as follows.
If particles are present in large numbers when clouds are forming, the
resulting cloud consists of a large number of smaller drops – smaller
than would otherwise be the case – this is similar to what happens as
pollutedfogs form in cities. Such a cloud will be more highly reflecting
to sunlight than one consisting of larger particles, thus further increasing
the energy loss resulting from the presence of the particles. The second
mechanismarises because of the influence of droplet size and numberon
precipitation efficiency, the lifetime of clouds and hence the geographic
extent of cloudiness. There is observational evidence for both of these
mechanisms but the processes involved are difficult to model and will
vary a great deal with the particular situation. Estimates of their magni-
tude as shown in Figure 3.8 therefore remain very uncertain. To refine
these estimates, more studies need to be made especially by making
careful measurements on suitable clouds.
The estimates for the radiative effects of particles as in Figure 3.8
can be compared with the global average radiative forcing to date due