Acidic sodium nitrate (NaNO₃) is the fine particles commonly found in urban atmosphere, which is beneficial to the formation of nitro-aromatic compounds (NACs). The NACs and optics of secondary organic aerosol (SOA) generated from the photo-oxidation of ethylbenzene with acid NaNO₃ fine particles were characterized in the current study. The results demonstrated that acidic NaNO₃ can significantly promote the formation of SOA. Compared with the carbonyls detected in SOA without fine particles, nitrophenols, nitrocatechols and their derivatives are measured as the major components of SOA with acid NaNO₃. The N=O chromosphore in NACs enhanced the light absorption capacity of SOA. The mass absorption coefficient averaged over 200-600 nm () of SOA gradually increased with the increasing of acidic NaNO₃. While, the of SOA with 100 μg/m³ acidic NaNO₃ was comparable to that of biomass burning organic aerosols. These would provide the experimental basis for studying the formation and optics of anthropogenic NACs in the background of high concentration of acid NaNO₃ fine particles.