The surface of sedimentary aerosol particles of polyacrolein (polyAC) was chemically modified by 1,3,5,7-tetramethylcyclotetrasiloxane (ITS) vapor under light irradiation with a low pressure mercury lamp and with N2 laser. FT-IR spectra measured with the surfacemodified polyAC particles strongly suggested that Si-0 bond of electronically excited TTS molecule was cleaved and the resulting active end groups produced silacyclobutane structure at one end or Si0-C bond at the other end on the surface of polyAC particles. Prolonged light irradiation with a low pressure mercury lamp produced poly(methylsiloxane) as the major product. From a gaseous mixture of TI'S and AC, composite aerosol particles were produced under irradiation with a medium pressure mercury lamp through a UV31 and a UVD33S filters. Aerosol particle formation process of the gaseous mixture was monitored by measuring monitor (He-Ne laser) light intensity scattered by the aerosol particles which were produced under light irradiation. With increasing partial pressure of TTS, induction period to detect scattered light became longer and the scattered light intensity became weaker. These results strongly suggested that electronically excitedAC molecules were consumed by the chemical reaction with TTS to induce polymerization reaction of US.