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The Importance of Natural
Twomey (1974) first postulated that increases pollution results in greater CCN concentrations and numbers of cloud droplets, which, in turn, increase the reflectance of clouds. Twomey et al. (1984) argued that enhanced cloud albedo has a magnitude comparable to that of greenhouse warming and acts to cool the atmosphere, in opposition to greenhouse warming.
Subsequently Albrecht (1989) proposed that higher droplet concentrations in
polluted air will reduce the rate of drizzle formation, resulting in
wetter, more reflective clouds. Furthermore, Ackerman et al. (1993;
1994) proposed that heavily drizzling straticumulus clouds can reduce
cloud top cooling to such an extent that a stratus-topped boundary
layer can collapse. Pollution-caused increases in CCN can thereby
suppress the drizzle process leading to the formation of a
stratus-topped boundary layer in some boundary layer regimes that may
not otherwise be sustainable. Cotton
and Pielke (1995) noted, however, the susceptability of the drizzle
process in marine stratocumulus clouds to anthropogenic emissions of
CCN may depend on the presence or absence of large and ultra-giant
aerosol particles in the subcloud layer. In other words the drizzle
formation process is not solely regulated by the concentrations of CCN
and cloud liquid
water contents but possibly also by the details of the spectrum of the hygroscopic
aerosol population. This concept has been reinforced by the hygroscopic
seeding simulations by Cooper et al. (1997). Their model calculations
suggest that high concentrations of hygroscopic particles in the 0.1
to 1.0 size range can accelerate the drizzle
formation process. An implication is that even though pollution may
increase the total concentration of CCN particles, if the
concentration of particles greater than 0.1 is likewise
increased, the drizzle formation process may not be suppressed, but
instead could be actually enhanced. This could counter the tendency of clouds
in polluted air from being more reflective. Again, we emphasize that
knowledge of the total size-distribution of hygroscopic aerosol is
needed to assess the potential impacts of polution on global climate!
