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When new particles formed in the atmosphere grow over 50-80 nm,they can be activated as cloud condensation nuclei (CCN) and lead to an increase of cloud albedo.Knowledge gaps still existed,e.g.,1) in what type of new particle formation (NPF) events new particles can grow over 50 nm? 2) which chemicals determine the growth of new particles to be over 50 nm? In this study,NPF events were investigated at two urban sites,in Qingdao and in Toronto,using two identical Fast Mobility Particle Sizer (FMPS) in spring.The satellite column density of pollution gases and the particular chemical concentration in PM2.5 both showed much higher concentrations of anthropogenic air pollutants in Qingdao than in Toronto.NPF events were observed in 16 days out of 39 sampling days in Qingdao and 13 days out of 31 sampling days in Toronto.The occurrence frequency of NPF events between Qingdao (41%) and Toronto (43%) was comparable to each other.In Qingdao,the geometric mean diameter of grown nucleated particles (Dpg,i) in 15 days grew to larger than 40 nm except in one day when the growth of new particles terminated at ~20 nm.In addition,new particles in 8 days out of the 15 days partly or entirely grew over 50 nm and they could even reached 100 nm in two days.Two-phase growth was generally observed in these NPF events of Qingdao.The first-phase growth occurred in daytime and the CMAQ modeling results suggested that formation of secondary organics was likely the major cause for the growth.The second-phase growth was observed at night and was associated with the increased concentrations of NH4 + and NO3 -,implying that NH4NO3 condensation played an important role in the growth.In Toronto,NPF events in 4 days followed with the growth of new particles <~20 nm while new particles grew up to ~40 nm in the remaining NPF events.A slight growth of new particles at night was observed only in 3-days NPF events when the increased concentrations of NH4 +,NO3 - or the increased relative humidity were observed.However,the calculated Dpg,i was less than 45 nm for all NPF events in Toronto,implying a negligible contribution of new particles to the population of CCN.