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. Anthropogenic emissions such as NO x and SO 2 influence the biogenicsecondary organic aerosol (SOA) formation, but detailed mechanisms andeffects are still elusive. We studied the effects of NO x and SO 2 onthe SOA formation from the photooxidation of α -pinene and limonene atambient relevant NO x and SO 2 concentrations (NO x : 2 : < 0.05 to 15 ppb). In these experiments,monoterpene oxidation was dominated by OH oxidation. We found that SO 2 induced nucleation and enhanced SOA mass formation. NO x stronglysuppressed not only new particle formation but also SOA mass yield. However,in the presence of SO 2 which induced a high number concentration ofparticles after oxidation to H 2 SO 4 , the suppression of the massyield of SOA by NO x was completely or partly compensated for. This indicatesthat the suppression of SOA yield by NO x was largely due to thesuppressed new particle formation, leading to a lack of particle surface forthe organics to condense on and thus a significant influence of vapor wallloss on SOA mass yield. By compensating for the suppressing effect onnucleation of NO x , SO 2 also compensated for the suppressing effecton SOA yield. Aerosol mass spectrometer data show that increasing NO x enhanced nitrate formation. The majority of the nitrate was organic nitrate(57–77 %), even in low-NO x conditions ( ∼  1 ppb). Organic nitrate contributed 7–26 % of total organics assuming amolecular weight of 200 g mol −1 . SOA from α -pinene photooxidation athigh NO x had a generally lower hydrogen to carbon ratio (H  ∕  C), compared tolow NO x . The NO x dependence of the chemical composition can beattributed to the NO x dependence of the branching ratio of the RO 2 loss reactions, leading to a lower fraction of organic hydroperoxides andhigher fractions of organic nitrates at high NO x . While NO x suppressed new particle formation and SOA mass formation, SO 2 cancompensate for such effects, and the combining effect of SO 2 andNO x may have an important influence on SOA formation affected byinteractions of biogenic volatile organic compounds (VOCs) with anthropogenicemissions.

atmospheric chemistry and physics

Effects of NO&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;x&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; and SO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; on the secondary organic aerosol formation from photooxidation of &amp;lt;i&amp;gt;α&amp;lt;/i&amp;gt;-pinene and limonene