| Summary: | Aromatic secondary organic aerosol (SOA) particles are known to contribute to radiative forcing and light absorption of atmosphere. However, the complex refractive index (CRI), single-scattering albedo (SSA) and other optical parameters of aromatic SOA are not well understood. SOA generated from photooxidation of toluene with a variety concentration of ammonium sulfate ((NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub>) seed particles in a smog chamber were investigated in the current study. The real part CRI of toluene SOA without seeds derived and based on aerosol albedometer measurements is 1.486 ± 0.002 at λ = 470 nm, showing a good agreement with available experimental data, and its SSA was measured to be 0.92 ± 0.02 at λ = 470 nm, indicating that the SOA particles without seeds have strong scattering ability. The SSA of SOA formed in the presence of 300 μg/m<sup>3</sup> (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> seed was 0.81 ± 0.02 at λ = 470 nm, less than the SSA of SOA without seed. SSA of SOA decreased, while the imaginary part of CRI (k) of SOA increased with increasing concentration of (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> seed, demonstrating that the adsorption capacity of SOA formed in the presence of (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> seed is enhanced. Different from the carboxyl compounds measured in the SOA without seed, imidazoles with strong chromophores of C=N that are responsible for the light absorption were detected as the principal constituents of SOA formed in the presence of (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> seed. These would provide valuable information for discussing the optics and components of aromatic SOA in the urban atmosphere containing a high concentration of (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> fine particles.
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