Atmospheric HO2/RO2 Ratios Weaken NOₓ Suppression of α-Pinene SOA

Secondary organic aerosol (SOA) form from the atmospheric oxidation of volatile organic compounds (VOC), and impacts both climate and human health. Chamber studies typically show significant decrease of SOA by nitrogen oxides (NO_x), yet these experiments often neglect atmospherically relevant hydroperoxy radical (HO₂) levels. Here we investigate α-pinene photooxidation under low and high hydroperoxy-to-organic peroxy radical (RO₂) ratios with added NO_x. While NO_x reduces aerosol formation under both conditions, suppression is substantially weaker under high HO₂ conditions (33–55%) than under low HO₂ conditions (60–70%). Under high HO₂ conditions, enhanced formation of low-volatility monomers offsets enhanced fragmentation, yielding a more condensable product mixture despite higher bulk volatility. These results demonstrate that laboratory studies conducted under low under high HO₂ conditions likely underestimate secondary organic aerosol formation in NO_x-influenced atmospheres.