Distinct but Likely Interdependent Roles of Secondary Organic and Inorganic Aerosol Formation in Aerosol Scattering

Aerosol scattering strongly influences the Earth’s atmosphere energy balance and actinic flux, yet its efficiency remains uncertain due to limited understanding of chemical effects. Scattering efficiency primarily depends on aerosol size, scattering refractive index, and hygroscopicity, which are determined by emissions and chemical processes; however, their covariation characteristics are rarely explored. Here, we use long-term measurements of submicron aerosol size distributions, chemical composition, scattering properties, and hygroscopicity in Guangzhou to investigate their covariations and links to secondary aerosol formation. The results indicate that dry-state volume scattering efficiency (VSE) was mainly driven by variations in aerosol size (R2 = 0.74), despite substantial refractive index variability (1.4–1.6), which showed overall independent variations with size. Source apportionment and case analyses suggest distinct size ranges for secondary organic (SOA) and inorganic aerosols (SIA). Accordingly, a new lognormal fitting methodology is proposed to retrieve particle volume size distribution (PVSD)-associated aerosol components by combining PVSD and composition data. Retrieved geometric mean diameters of SOA (Dg,SOA, 175–400 nm; 246 ± 44 nm) and SIA (Dg,SIA, 200–600 nm; 382 ± 68 nm) are significantly correlated (R2 = 0.43), indicating coupled formation of SOA and SIA and their interdependent roles in aerosol scattering. In addition, pronounced joint increases in dry-state VSE and aerosol hygroscopicity driven by the co-enhancement of aerosol size and hygroscopicity are further revealed. These results demonstrate the interconnected roles of secondary aerosol formation in controlling scattering efficiency and underscore the need to better represent SOA–SIA interactions in simulating aerosol radiative effects and address the covariations of aerosol hygroscopicity and dry-state scattering efficiency in aerosol remote sensing.