  We study a Smoluchowski equation describing a simple mean-field model of
particles moving in $d$ dimensions and aggregating with conservation of `mass'
$s=R^D$ ($R$ is the particle radius). In the scaling regime the scaled mass
distribution $P(s)\sim s^{-\tau}$, and $\tau$ can be computed by perturbative
and non perturbative expansions. A possible application to two-dimensional
decaying turbulence is briefly discussed.
