Effects of NOsubix/i/sub and SOsub2/sub on the secondary organic aerosol formation from photooxidation of iα/i-pinene and limonene

摘要

Anthropogenic emissions such as NOx and SO2 influence the biogenicsecondary organic aerosol (SOA) formation, but detailed mechanisms andeffects are still elusive. We studied the effects of NOx and SO2 onthe SOA formation from the photooxidation of α-pinene and limonene atambient relevant NOx and SO2 concentrations (NOx: < 1to 20 ppb, SO2: < 0.05 to 15 ppb). In these experiments,monoterpene oxidation was dominated by OH oxidation. We found that SO2induced nucleation and enhanced SOA mass formation. NOx stronglysuppressed not only new particle formation but also SOA mass yield. However,in the presence of SO2 which induced a high number concentration ofparticles after oxidation to H2SO4, the suppression of the massyield of SOA by NOx was completely or partly compensated for. This indicatesthat the suppression of SOA yield by NOx 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 NOx, SO2 also compensated for the suppressing effecton SOA yield. Aerosol mass spectrometer data show that increasing NOxenhanced nitrate formation. The majority of the nitrate was organic nitrate(57–77 %), even in low-NOx 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 NOx had a generally lower hydrogen to carbon ratio (H ∕ C), compared tolow NOx. The NOx dependence of the chemical composition can beattributed to the NOx dependence of the branching ratio of the RO2loss reactions, leading to a lower fraction of organic hydroperoxides andhigher fractions of organic nitrates at high NOx. While NOxsuppressed new particle formation and SOA mass formation, SO2 cancompensate for such effects, and the combining effect of SO2 andNOx may have an important influence on SOA formation affected byinteractions of biogenic volatile organic compounds (VOCs) with anthropogenicemissions.