Abstract. Secondary organic aerosol (SOA) has great impacts on air quality, climate change and human health. The composition and physicochemical properties of SOA differ greatly because they form under different atmospheric conditions and from various precursors as well as differing oxidation. In this work, photooxidation experiments of toluene were performed under four conditions (dry, dry with SO2, wet and wet with SO2) to investigate the effect of SO2 under different relative humidities on the composition and optical properties of SOA at wavelengths of 375 and 532 nm. According to our results, the increase in humidity enhances not only light absorption but also the scattering property of the SOA. Oligomers formed through multiphase reactions might be the reason for this phenomenon. Adding SO2 slightly lowers the real part of the complex refractive index, RI(n), of toluene-derived SOA (RI(n)dry,SO2<RI(n)dry, RI(n)wet,SO2<RI(n)wet), which might be a result of the partitioning of low-oxidation-state products. The imaginary part of the complex refractive index, RI(k), is enhanced under dry conditions with SO2 compared to that of only dry conditions, which might be due to acid-catalyzed aldol condensation reactions. Wet conditions with SO2 shows the combined effect of SO2 and humidity. The extinction properties of toluene-derived SOA under wet conditions with SO2 increased by approximately 30 % compared to that of toluene-derived SOA formed under dry conditions. Our results suggest that various atmospheric conditions will affect the composition and optical proprieties of SOA, which has significant implications for evaluating the impacts of SOA on the rapid formation of regional haze, global radiative balance and climate change.