<p><strong>Abstract.</strong> Anthropogenic emissions such as NO_<i>x</i> and SO₂ influence the biogenic secondary organic aerosol (SOA) formation, but detailed mechanisms and effects are still elusive. We studied the effects of NO_<i>x</i> and SO₂ on the SOA formation from the photooxidation of <i>α</i>-pinene and limonene at ambient relevant NO_<i>x</i> and SO₂ concentrations (NO_<i>x</i>: &amp;lt;<span

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... </span>1to 20<span class="thinspace"></span>ppb, SO₂: &amp;lt;<span class="thinspace"></span>0.05 to 15<span class="thinspace"></span>ppb). In these experiments, monoterpene oxidation was dominated by OH oxidation. We found that SO₂ induced nucleation and enhanced SOA mass formation. NO_<i>x</i> strongly suppressed not only new particle formation but also SOA mass yield. However, in the presence of SO₂ which induced a high number concentration of particles after oxidation to H₂SO₄, the suppression of the mass yield of SOA by NO_<i>x</i> was completely or partly compensated for. This indicates that the suppression of SOA yield by NO_<i>x</i> was largely due to the suppressed new particle formation, leading to a lack of particle surface for the organics to condense on and thus a significant influence of vapor wall loss on SOA mass yield. By compensating for the suppressing effect on nucleation of NO_<i>x</i>, SO₂ also compensated for the suppressing effect on SOA yield. Aerosol mass spectrometer data show that increasing NO_<i>x</i> enhanced nitrate formation. The majority of the nitrate was organic nitrate (57–77<span class="thinspace"></span>%), even in low-NO_<i>x</i> conditions (&amp;lt;<span class="thinspace"></span> ∼ <span class="thinspace"></span>1<span class="thinspace"></span>ppb). Organic nitrate contributed 7–26<span class="thinspace"></span>% of total organics assuming a molecular weight of 200<span class="thinspace"></span>g<span class="thinspace"></span>mol⁻¹. SOA from <i>α</i>-pinene photooxidation at high NO_<i>x</i> had a generally lower hydrogen to carbon ratio (H<span class="thinspace"></span>∕<span class="thinspace"></span>C), compared to low NO_<i>x</i>. The NO_<i>x</i> dependence of the chemical composition can be attributed to the NO_<i>x</i> dependence of the branching ratio of the RO₂ loss reactions, leading to a lower fraction of organic hydroperoxides and higher fractions of organic nitrates at high NO_<i>x</i>. While NO_<i>x</i> suppressed new particle formation and SOA mass formation, SO₂ can compensate for such effects, and the combining effect of SO₂ and NO_<i>x</i> may have an important influence on SOA formation affected by interactions of biogenic volatile organic compounds (VOCs) with anthropogenic emissions.</p>