AAAR 34th Annual Conference
October 12 - October 16, 2015
Hyatt Regency
Minneapolis, Minnesota, USA
Abstract View
Effects of Sulfate Seed Particles on Secondary Organic Aerosol Formation from α-pinene Photooxidation
JIMING HAO, Biwu Chu, Hideto Takekawa, John Liggio, Shao-Meng Li, School of Environment, Tsinghua University
Abstract Number: 309 Working Group: Haze in China: Sources, Formation Mechanisms, and Current Challenges
Abstract Understanding of the effects of inorganic seed particles on secondary organic aerosol (SOA) formation is important for revealing SOA formation mechanisms. In this work, α-pinene/NOx and α-pinene/HONO photooxidation as well as α-pinene ozonolysis experiments were conducted in smog chambers in the presence or absence of different sulfate seed particles. SOA yields were significantly influenced by the presence of sulfate seed aerosols in both photooxidation and ozonolysis of α-pinene. Under similar conditions, less SOA with a higher oxidation degree as indicated by its O/C ratio was generated from both ozonolysis and OH-oxidation experiments in the presence of ferrous or ferric sulfate (FeSO4 or Fe2(SO4)3) seed aerosols compare to that in the presence of zinc sulfate or ammonia sulfate (ZnSO4 or (NH4)2SO4).
This decreasing effect of FeSO4 seed particles on SOA mass varied with humidity in the chamber experiments. FeSO4 seed particles decrease SOA mass as long as water was present on the seed particle surface, but have no decreasing effect on SOA under dryer conditions at 12% relative humidity (RH). The decreasing effect of FeSO4 seed particles on SOA mass is proposed to be related to metal-mediated oxidation of organics in the surface layer of water on the seed particles. Free radicals, including OH, can be formed from catalytic cycling of Fe2+ and Fe3+ in the aqueous phase. These radicals can react further with the organic products of α-pinene oxidation on the seed particles. It is hypothesized that both functionalization and fragmentation of organics may happen during the metal-mediated aerosol-phase oxidation, resulting in the increase of oxidation level and the decrease of mass concentration of the SOA.
The gap in the SOA yields between experiments with different seed aerosols was more significant in OH-oxidation experiments compare to ozonolysis experiments. The different degrees of the mass decreasing and oxidation level increasing between the ozonolysis and OH-oxidation experiments may be resulted from the different oxidation levels of the generated SOA from gas-phase reaction in the two systems.