In situ measurements of trace gases and aerosols were conducted at an urban site in Tokyo (35¿39'N, 139¿40'E). The data obtained in summer (July--August 2003), fall (September--October 2003), and winter (February 2003 and January--February 2004) are used for the present analysis. Size-resolved chemical composition of nonrefractory (vaporized at 600¿C under high vacuum) submicron aerosol was measured using an Aerodyne aerosol mass spectrometer (AMS). Organics are found to be the dominant component (40--60% of total nonrefractory aerosol mass) in all periods. Organic aerosol (OA) is classified by correlation with carbon monoxide (CO) and fragments of aliphatic and oxygenated organic compounds in the AMS mass spectra. Combustion-related organic aerosol (combustion OA) is defined as the primary organic aerosol (POA) fraction, as determined by a linear correlation with CO. Excess organic aerosol (excess OA) is defined by subtracting the combustion OA and the background OA from the total OA. The combustion OA and excess OA show good correlation (r2 = 0.65--0.85) with hydrocarbon-like organic aerosol (HOA) and oxygenated organic aerosol (OOA), respectively, which were derived from a custom principal component analysis. In the summer period the estimated excess OA concentrations show distinct diurnal variations and correlate with ozone (O3) during daytime. On average, the combustion OA does not exhibit a distinct diurnal variation for the summer, fall, and winter periods, while the excess OA shows a clear diurnal pattern (daytime peak at ~1300 LT). At the daytime peak the excess OA is found to be at nearly the same concentration as the combustion OA for all seasons, suggesting that significant formation of secondary organic aerosol (SOA) occurred in daytime throughout the measurement period.