Abstract. Secondary organic aerosol (SOA) formation from photooxidation of α-pinene has been investigated in a photochemical reaction chamber under varied inorganic seed particle acidity levels at moderate relative humidity. The effect of particle acidity on SOA yield and chemical composition was examined under high- and low-NO_x conditions. The SOA yield (4.2–7.6 %) increased nearly linearly with the increase in particle acidity under high-NO_x conditions. In contrast, the SOA yield (28.6–36.3 %) was substantially higher under low-NO_x conditions, but its dependency on particle acidity was insignificant. A relatively strong increase in SOA yield (up to 220 %) was observed in the first hour of α-pinene photooxidation under high-NO_x conditions, suggesting that SOA formation was more effective for early α-pinene oxidation products in the presence of fresh acidic particles. The SOA yield decreased gradually with the increase in organic mass in the initial stage (approximately 0–1 h) under high-NO_x conditions, which is likely due to the inaccessibility to the acidity over time with the coating of α-pinene SOA, assuming a slow particle-phase diffusion of organic molecules into the inorganic seeds. The formation of later-generation SOA was enhanced by particle acidity even under low-NO_x conditions when introducing acidic seed particles after α-pinene photooxidation, suggesting a different acidity effect exists for α-pinene SOA derived from later oxidation stages. This effect could be important in the atmosphere under conditions where α-pinene oxidation products in the gas-phase originating in forested areas (with low NO_x and SO_x) are transported to regions abundant in acidic aerosols such as power plant plumes or urban regions. The fraction of oxygen-containing organic fragments (C_xH_yO₁⁺ 33–35 % and C_xH_yO₂⁺ 16–17 %) in the total organics and the O ∕ C ratio (0.52–0.56) of α-pinene SOA were lower under high-NO_x conditions than those under low-NO_x conditions (39–40, 17–19, and 0.61–0.64 %), suggesting that α-pinene SOA was less oxygenated in the studied high-NO_x conditions. The fraction of nitrogen-containing organic fragments (C_xH_yN_z⁺ and C_xH_yO_zN_p⁺) in the total organics was enhanced with the increases in particle acidity under high-NO_x conditions, indicating that organic nitrates may be formed heterogeneously through a mechanism catalyzed by particle acidity or that acidic conditions facilitate the partitioning of gas-phase organic nitrates into particle phase. The results of this study suggest that inorganic acidity has a significant role to play in determining various organic aerosol chemical properties such as mass yields, oxidation state, and organic nitrate content. The acidity effect being further dependent on the timescale of SOA formation is also an important parameter in the modeling of SOA.