Understanding new particle formation based on continuous nanoparticle ranking in boreal forest and urban megacity

Abstract. Understanding atmospheric new particle formation (NPF) across contrasting environments is essential for evaluating its global-scale impacts. Here, we applied a nanoparticle ranking framework to systematically compare four years (2018–2022) of continuous measurements at the boreal forest site of Hyytiälä and the megacity site of Beijing. Ranking values, as the indicator of the NPF strength, revealed contrasting seasonality: the highest-ranking days occurred mainly in spring in Hyytiälä but in winter in Beijing. The driving factors of NPF in these two distinct environments were then investigated. Unexpectedly, the concentrations of key gaseous precursors, i.e. sulfuric acid (SA) and highly oxygenated organic molecule (HOM) in Hyytiälä and SA in Beijing, did not increase monotonically with the ranking values. Together with the evolution of condensation sink (CS) as a function of ranking values, our results illustrated that NPF intensity is governed by source–sink competition rather than precursor availability alone. Further, we revealed quiet NPF signatures at low ranking values in Hyytiälä, whereas primary anthropogenic emissions obscured such signals in Beijing. The particle formation rate of 7 nm (J₇) and growth rates of 7–25 nm (GR_7–25) particles were calculated under different ranking levels. The results showed that J₇ increased with ranking value, whereas the GR_7–25 frequently peaked outside the highest ranking interval. In addition, we also compared the ranking-based J₇ and the J₇ calculated based on traditional methods, suggesting that conventional approaches may underestimate particle formation during weak or less clearly NPF conditions (low ranking days). By combining nanoparticle ranking with normalization method, our results continuously and quantitatively characterized NPF dynamics, thereby improving our understanding in the role of NPF in atmospheric aerosol populations in two distinct environments.