Field Observations Reveal Substantially Higher Scattering Refractive Index in Secondary Versus Primary Organic Aerosols

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 is predominantly determined by the real part of the refractive index (m_r). Prevailing models often assume a constant m_r for organic aerosols (e.g., 1.53 or 1.45) at different wavelengths or claim that m_r of primary organic aerosols (POA) is substantially higher than that of secondary organic aerosols (SOA) (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 m_r  at 1064 nm, closely associated with primary aerosol emissions and secondary aerosol formation. Source apportionment of aerosol size distributions reveals that the m_r 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 this finding. These results challenge existing model assumptions. In addition, further analysis reveals m_r of SOA increases with oxidation level, which is contrary to results of most laboratory studies on evolution of m_r of SOA, which is likely associated with multiphase SOA formation. Our analysis recommends m_r values at 525 nm of 1.37 for POA and 1.59 for SOA. These findings underscore that current modeling practices may introduce substantial inaccuracies in estimating the radiative effects of organic aerosols.