Comparative analysis of new particle formation events in less and severely polluted urban atmosphere

In this paper, we conducted a comparative study of new particle formation (NPF) events occurring between Qingdao and Toronto during spring. The extent of air pollution in Qingdao was much severer than that in Toronto, but the occurrence frequency of NPF events in Qingdao (41%) was almost same as that (42%) in Toronto. The geometric median diameter of new particles (Dpg,1) increased up to >40 nm in 15 days out of the total 16 NPF days in Qingdao, the Dpg,1 at least in eight days increased up to >60 nm and even reached >80 nm in two days. Two-stage growth was generally observed in these eight NPF events. The first-stage growth occurred in daytime and it was likely associated with formation of secondary organic aerosol (SOA) on basis of the modeling results. The second-stage growth was generally observed at nighttime when the modeling results showed increases of NH4+ and NO3− in concentration together with SOA, implying that NH4NO3 possibly played a role in the growth. In Toronto, the maximum Dpg,1 of the observed new particles in all 13 NPF events was less than 50 nm. A slight second-stage growth of new particles was observed only in four days when either the increase of NH4+ and NO3− in concentration or the increase of relative humidity occurred. The NPF events in Toronto less likely had a significant contribution to cloud condensation nuclei due to the small size of the observed new particles.