Potential climatic effects of meteoric smoke in the Earth's paleo-atmosphere

Modelling of the growth of meteoric smoke in the Earth's atmosphere, by assuming the formation of either simple spherical ( compact) particles or, more realistically, fractal ( porous) aggregates, highlights important differences in the predicted atmospheric size distributions as a function of altitude. The calculated UV extinction and direct radiative forcing ( DRF) of these types of particles is also quite different. It is shown that, with regard to ( a), forming a UV barrier before the presence of significant ozone levels in the atmosphere and ( b), triggering 'snowball Earth' glaciations by negative DRF, fractal smoke particles are unlikely to have been important even if the flux of interplanetary dust into the atmosphere was 3 orders of magnitude higher than the present day. However, if these particles are effective ice nuclei, then subsequent indirect forcing through ice cloud formation could have made a more significant contribution to the onset of ancient glaciation episodes.