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

Organic compounds play an important role in atmospheric particle initial growth along with sulfuric acid (SA). However, the detailed composition of newly formed particles remains limited due to analytical challenges. In this study, we conducted flow tube experiments to investigate the nanoparticle growth processes of SA and oxygenated organic molecules (OOMs, from α -pinene oxidation) system. Utilizing a custom-built scanning flow condensation particle counter (SFCPC), we report, for the first time, size-resolved measurements of the hygroscopicity parameter ( κ ) and organic mass fraction ( f org ) for particles in the 3–10 nm size range within this atmospherically relevant system. 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, which may be explained by hydration effects. In contrast, the κ values of OOMs increased with RH by as much as 57 %, potentially involving changes in oxidation product. Size-resolved f org revealed that larger particles contained a greater proportion of organics, indicating OOMs contribute more significantly to growth at larger sizes. Moreover, elevated humidity enhanced the organic contribution to particle growth by up to 81 %. Compared to 3–5 nm, this enhancement was more pronounced for 5–10 nm particles associated with 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.