Non-refractory submicron aerosol is characterized using an Aerosol ChemicalSpeciation Monitor(ACSM)in the fall and winter seasons of 2013 at the roadside in an Asianmegacity environment in Hong Kong.Organic aerosol(OA),characterized by application ofPositive Matrix Factorization(PMF),and sulfate are found dominant.Traffic-related organicaerosol shows good correlation with other vehicle-related species,and cooking aerosol displaysclear meal-time concentration maxima and association with surface winds from restaurant areas.Contributions of individual species and OA factors to high NR-PM1 are analyzed for hourly dataand daily data; while cooking emissions in OA contribute to high hourly concentrations,particularly during meal times,secondary organic aerosol components are responsible for episodicevents and high day-to-day PM concentrations.Clean periods are either associated withprecipitation,which reduces secondary OA with a lesser impact on primary organics,or cleanoceanic air masses with reduced long-range transport and better dilution of local pollution.Hazeevents are connected with increases in contribution of secondary organic aerosol,from 30%to 50%among total non-refractory organics,and influence of continental air masses.