Effects of NOx and SO2 on the Secondary Organic Aerosol Formation from Photooxidation of α-pinene and Limonene
Defeng Zhao1, Sebastian H. Schmitt1, Mingjin Wang1,2, Ismail-Hakki Acir1,a, Ralf Tillmann1, Zhaofeng Tan1,2, Anna Novelli1, Hendrik Fuchs1, Iida Pullinen1,b, Robert Wegener1, Franz Rohrer1, Jürgen Wildt1, Astrid Kiendler-Scharr1, Andreas Wahner1, and Thomas F. Mentel11Institute of Energy and Climate Research, IEK-8: Troposphere, Forschungszentrum Jülich, Jülich, 52425, Germany 2College of Environmental Science and Engineering, Peking University, Beijing, 100871, China anow at: Institute of Nutrition and Food Sciences, University of Bonn, Bonn, 53115, Germany bnow at: Department of Applied Physics, University of Eastern Finland, Kuopio, 7021, Finland
Received: 30 Mar 2017 – Accepted for review: 03 Apr 2017 – Discussion started: 04 Apr 2017
Abstract. Anthropogenic emissions such as NOx and SO2 influence the biogenic secondary organic aerosol (SOA) formation, but detailed mechanisms and effects are still elusive. We studied the effects of NOx and SO2 on the SOA formation from photooxidation of α-pinene and limonene at ambient relevant NOx and SO2 concentrations (NOx: < 1 ppb to 20 ppb, SO2: < 0.05 ppb to 15 ppb). In these experiments, monoterpene oxidation was dominated by OH oxidation. We found that SO2 induced nucleation and enhanced SOA mass formation. NOx strongly suppressed not only new particle formation but also SOA mass yield. However, in the presence of SO2 which induced high number concentration of particles after oxidation to H2SO4, the mass yield of SOA at high NOx was comparable to that at low NOx. This indicates that the suppression of SOA yield by NOx was mainly due to the suppressed new particle formation, leading to a lack of particle surface for the organics to condense on. By compensating the suppressing effect on nucleation of NOx, SO2 also compensated the suppressing effect on SOA yield. Aerosol mass spectrometer data show that increasing NOx enhanced 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 a molecular weight of 200 g/mol. SOA from α-pinene photooxidation at high NOx had generally lower hydrogen to carbon ratio (H / C), compared with at low NOx. The NOx dependence of the chemical composition can be attributed to the NOx dependence of the branching ratio of the RO2 loss reactions, leading to lower fraction of organic hydroperoxide and higher fractions of organic nitrate at high NOx. While NOx suppressed new particle formation and SOA mass formation, SO2 can compensate such effects, and the combining effect of SO2 and NOx may have important influence on SOA formation affected by interactions of biogenic volatile organic compounds (VOC) with anthropogenic emissions.
Zhao, D., Schmitt, S. H., Wang, M., Acir, I.-H., Tillmann, R., Tan, Z., Novelli, A., Fuchs, H., Pullinen, I., Wegener, R., Rohrer, F., Wildt, J., Kiendler-Scharr, A., Wahner, A., and Mentel, T. F.: Effects of NOx and SO2 on the Secondary Organic Aerosol Formation from Photooxidation of α-pinene and Limonene, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2017-294, in review, 2017.