This is the conclusion of Israeli scientists studying storm formation in the Amazon basin. While some particulates (ashes, aerosols) do promote rain formation, a well known fact for several decades now, their excess actually does the opposite: preventing rain and restricting the formation of clouds.
This may affect the ongoing process of global warming in several ways: restricting rain in wildfire and highly polluted areas and reducing the overall cloud cover of Earth and hence its albedo, allowing for more solar radiation to reach the surface.
More details at Science Daily.
Ref. O. Almaratz et al., Lightning response to smoke from Amazonian fires. Geophysical Research Letters, 2010. Pay per view.