From Glossary of Meteorology
A condensation nucleus formed as a result of industrial, transport, or natural combustion processes.
The chemical nature of such nuclei may vary almost as much as can the nature of reactants in combustion processes, but because of the prevalence of sulfur impurities in many fuels, the process transforming sulfur dioxide (SO2) to sulfur trioxide (SO3) and thence sulfuric acid is perhaps the most important in producing combustion nuclei. Sulfuric acid is very hygroscopic and hence can serve to nucleate atmospheric condensation processes. The role of the ammonium ion (NH4+), which is formed in some combustion processes, is probably of almost equal importance to that of the sulfite (SO3-2) and sulfate (SO4-3) ions. Such nuclei may be completely or (in association with soot) partly soluble in water.