Unveiling the organic contribution to the initial particle growth in 3–10 nm size range

Organic compounds play an important role in driving atmospheric particle formation and initial growth along with sulfuric acid. However, the detailed chemical composition of newly formed particles remains limited due to analytical challenges. In this study, we conducted the laboratory experiments using a flow tube reactor to investigate the roles of sulfuric acid (SA) and oxygenated organic molecules (OOMs, from α-pinene oxidation) in nanoparticle growth. For the first time, the size-resolved hygroscopicity parameter (κ) and organic mass fraction (forg) of 3–10 nm particles were measured using a custom-built scanning flow condensation particle counter (SFCPC). The hygroscopicity of SA decreased 49 % as particle size increased (from 0.413 ± 0.011 at 3 nm to 0.209 ± 0.004 at 10 nm) and declined by up to 18 % with increasing RH, primarily due to hydration effects. In contrast, the κ values of OOMs increased with RH by as much as 57 %, attributable to changes in oxidation product. Size-resolved forg revealed that larger particles contained a greater proportion of organics, indicating that SA dominates the growth of small particles, whereas OOMs contribute more significantly to growth at larger sizes. Moreover, elevated humidity enhanced the organic contribution to particle growth by up to 81 %. This enhancement was more pronounced for 5–10 nm particles due to the incorporation of increased yields of more volatile oxidation products and Kelvin effect. These valuable information on hygroscopicity and chemical composition of 3–10 nm particles during new particle formation and subsequent growth could further the understanding of related atmospheric mechanisms.