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<p>Sulfur dioxide (SO<span class="inline-formula">₂</span>) can affect aerosol formation in the atmosphere, but the underlying mechanisms remain unclear. Here, we investigate aerosol formation and composition from the ozonolysis of cyclooctene with and without SO<span class="inline-formula">₂</span> addition in a smog chamber. Liquid chromatography equipped with high-resolution tandem mass spectrometry measurements indicates that monomer carboxylic acids and corresponding dimers with acid anhydride and aldol structures are important components in particles formed in the absence of SO<span class="inline-formula">₂</span>. A 9.4–12.6-times increase in particle maximum number concentration is observed in the presence of 14–192 ppb SO<span class="inline-formula">₂</span>. This increase is largely attributed to sulfuric acid (H<span class="inline-formula">₂</span>SO<span class="inline-formula">₄</span>) formation from the reactions of stabilized Criegee intermediates with SO<span class="inline-formula">₂</span>. In addition, a number of organosulfates (OSs) are detected in the presence of SO<span class="inline-formula">₂</span>, which are likely products formed from the heterogeneous reactions of oxygenated species with H<span class="inline-formula">₂</span>SO<span class="inline-formula">₄</span>. The molecular structures of OSs are also identified based on tandem mass spectrometry analysis. It should be noted that some of these OSs have been found in previous field studies but were classified as compounds from unknown sources or of unknown structures. The observed OSs are less volatile than their precursors and are therefore more effective contributors to particle formation and growth, partially leading to the increase in particle volume concentration under SO<span class="inline-formula">₂</span>-presence conditions. Our results provide an in-depth molecular-level insight into how SO<span class="inline-formula">₂</span> alters particle formation and composition.</p>