Source apportionment of SMPS-CPC measurement biases and its impact on aerosol hygroscopicity retrievals in an indoor environment

Accurate quantification of new particle formation (NPF) events relies on precise aerosol number concentration measurements, which are frequently hindered by systematic measurement biases between instruments. In this study, one-month synchronous observations were conducted in a stable indoor environment using a Scanning Mobility Particle Sizer (SMPS) and a Condensation Particle Counter (CPC). An average 23% systematic underestimation by the SMPS relative to the CPC was observed. By applying Positive Matrix Factorization (PMF) to resolve the modes of instrument disagreement, we identified that the variance in this negative bias was predominantly driven by low transmission efficiency and diffusion losses within the nucleation and Aitken modes. However, a closure analysis of aerosol hygroscopicity demonstrated that the chemistry-derived parameter (κchem, 0.31 ± 0.06) exhibited excellent agreement with the SMPS-CCN derived parameter at 0.28% supersaturation (κ0.28%SS, 0.30 ± 0.10). These findings suggest that while absolute numerical biases exist in ultrafine particle counting, the relative size distribution remains robust for mass-dominant larger particles, ensuring reliable physicochemical retrievals in this regime, though caution is strictly required when interpreting data at higher supersaturations.