Refractive index enhancement by secondary organic aerosol formation in humid southern China challenges model assumptions

Abstract. Aerosol–radiation interactions play a crucial role in air pollution and climate change, with scattering being the dominant process. The complex refractive index of organic aerosols is essential for accurately simulating these interactions, with the scattering capability being predominantly determined by the real part of the refractive index (mr). Prevailing models often assume a constant mr for organic aerosols (e.g., 1.53 or 1.45) at different wavelengths or claim that the mr of primary organic aerosols (POAs) is substantially higher than that of secondary organic aerosols (SOAs) (e.g., 1.63 for POA and 1.43 for SOA), largely due to a lack of direct measurements. This study employs direct measurements from the DMA-SP2 system to demonstrate a strong diameter dependence of dry state mr at 1064 nm, closely associated with primary aerosol emissions and secondary aerosol formation. Source apportionment of aerosol size distributions reveals that the mr of SOA is substantially higher than that of POA. Optical closure calculations, based on extensive dry-state observations of aerosol scattering at 525 nm, size distributions, and chemical compositions, confirm that SOA formation can enhance aerosol mr substantially (from lower than 1.45 when POA dominates to higher than 1.55 when SOA dominates). These results challenge existing model assumptions. In addition, further analysis reveals the mr of SOA increases substantially with oxidation level, which is likely associated with multiphase SOA formation. Our analysis recommends mr values at 525 nm of 1.37 for POA and 1.59 for SOA in urban regions with emissions and meteorological conditions similar to those at the observation site in Guangzhou, a city located in humid southern China. These findings underscore that current modeling practices may introduce substantial inaccuracies in estimating the radiative effects of organic aerosols.