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<p>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 <span class="inline-formula">SO₂</span>, wet and wet with <span class="inline-formula">SO₂</span>) to investigate the effect of <span class="inline-formula">SO₂</span> under different relative humidities on the composition and optical properties of SOA at wavelengths of 375 and 532&thinsp;<span class="inline-formula">nm</span>. 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 <span class="inline-formula">SO₂</span> slightly lowers the real part of the complex refractive index, RI(<span class="inline-formula"><i>n</i></span>), of toluene-derived SOA (RI(<span class="inline-formula"><i>n</i></span>)<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi/><mrow><mi mathvariant="normal">dry</mi><mo>,</mo><mspace linebreak="nobreak" width="0.125em"/><msub><mi mathvariant="normal">SO</mi><mn mathvariant="normal">2</mn></msub></mrow></msub><mo>&lt;</mo><mtext>RI</mtext><mo>(</mo><mi>n</mi><msub><mo>)</mo><mi mathvariant="normal">dry</mi></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="81pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="3f549125f962304b53ec80913e32d618"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-4477-2020-ie00001.svg" width="81pt" height="15pt" src="acp-20-4477-2020-ie00001.png"/></svg:svg></span></span>, RI(<span class="inline-formula"><i>n</i></span>)<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M12" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi/><mrow><mi mathvariant="normal">wet</mi><mo>,</mo><mspace width="0.125em" linebreak="nobreak"/><msub><mi mathvariant="normal">SO</mi><mn mathvariant="normal">2</mn></msub></mrow></msub><mo>&lt;</mo><mtext>RI</mtext><mo>(</mo><mi>n</mi><msub><mo>)</mo><mi mathvariant="normal">wet</mi></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="83pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="899330242d643de8fd6914049145569f"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-4477-2020-ie00002.svg" width="83pt" height="15pt" src="acp-20-4477-2020-ie00002.png"/></svg:svg></span></span>), which might be a result of the partitioning of low-oxidation-state products. The imaginary part of the complex refractive index, RI(<span class="inline-formula"><i>k</i></span>), is enhanced under dry conditions with <span class="inline-formula">SO₂</span> compared to that of only dry conditions, which might be due to acid-catalyzed aldol condensation reactions. Wet conditions with <span class="inline-formula">SO₂</span> shows the combined effect of <span class="inline-formula">SO₂</span> and humidity. The extinction properties of toluene-derived SOA under wet conditions with <span class="inline-formula">SO₂</span> increased by approximately 30&thinsp;% 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.</p>