The evolution of urban aerosol size distribution growing by heterogene
ous reactions has been observed in winter fog. Particle diameter growt
h rates from such data are presented. The functional dependence of the
growth rates on particle diameter infer clues about chemical mechanis
ms of heterogeneous interaction. The data are consistent with aerosol
formation in winter fog resulting from condensation of low vapor press
ure species formed by gas phase reactions and droplet phase reactions.