  Aerosols under chaotic advection often approach a strange attractor. They
move chaotically on this fractal set but, in the presence of gravity, they have
a net vertical motion downwards. In practical situations, observational data
may be available only at a given level, for example at the ground level. We
uncover two fractal signatures of chaotic advection of aerosols under the
action of gravity. Each one enables the computation of the fractal dimension
$D_{0}$ of the strange attractor governing the advection dynamics from data
obtained solely at a given level. We illustrate our theoretical findings with a
numerical experiment and discuss their possible relevance to meteorology.
