Oxidation-driven acceleration of NPF-to-CCN conversion under polluted atmosphere: Evidence from mountain-top observations in Yangtze River Delta

Abstract. To what extent the new particle formation (NPF) contributed to the CCN remained unclear, especially at the boundary layer top (BLT) in polluted atmosphere. In this study, measurements at a mountain top background site in southeastern China during spring 2024 quantified NPF growth dynamics under different air masses, exploring the nucleation mechanism and its potential contribution to CCN. Eight NPF events were observed, and three of them occurred in the polluted conditions (NPF-P) which associated with regional transportation while the rest five events appeared in the clean conditions (NPF-C). The average formation rate (J_2.5: 2.4 vs. 0.7 cm⁻³ s⁻¹) and growth rate (GR: 6.8 vs. 5.5 nm h⁻¹) were significantly higher in NPF-P compared to NPF- C, accompany by higher concentrations of sulfuric acid and ammonia, suggesting the important role of ammonia that enhancing sulfuric acid nucleation. In addition, much higher CCN enhancement factor was observed in NPF-P (EF_CCN: 1.6 vs. 0.7 in NPF-C) due to the regional transported of anthropogenic pollutants from the urban cluster regions and their secondary transformation under enhanced atmospheric oxidation capacity. Furthermore, the duration of NPF-to-CCN conversion was quantified using “Time Window (τ)”, revealing polluted condition accelerated NPF-to-CCN conversion by 17.0 % (τ = 16.4 h vs. 19.8 h), with nitrate playing an important role in maintaining rapid GR and compressing τ, thereby enabling clouds to form more readily during NPF days. These findings reveal that polluted air masses enhance both the efficiency and speed of CCN production at the BLT through elevated atmospheric oxidation capacity.