Observational and experimental evidence is presented here to verify a hypothesized recondensation-induced nucleation mechanism. While analyzing the diurnal variations in particle number concentration in an urban area, intense particle production was found to occur during the commuting hours even when there was no significant photochemical activity. Combustion-induced nucleation processes occurring in fresh automobile exhausts are believed to be responsible for the observed phenomenon. It is hypothesized that, besides the more familiar fuel-sulfur conversion mechanism, particle nucleation may also result from the recondensation of chemicals vaporized from preexisting particles in the intake air during combustion. An experimental verification was performed by heating the ambient air without the use of fuel, then cooling it down to allow for nucleation. A net particle production of over 700,000 cm-3 was achieved during this heating-then-cooling procedure. The results indicate that the recondensation-induced mechanism is potentially important in various types of combustion exhausts. ¿ 1999 American Geophysical Union