Direct observation of polymerization in the oleic acid-ozone heterogeneous reaction system by photoelectron resonance capture ionization aerosol mass spectrometry

摘要

High molecular weight products of the ozonolysis reaction of particle-phase 9-octadecenoic acid (oleic acid) have been studied by photoelectron resonance capture ionization (PERCI) mass spectrometry (MS). Oleic acid particles (d_g = 202 nm, σ_g = 1.97) were reacted with ozone (1.8 x 10~(-4) atm) in a flow reactor at reaction times of 8 and 23 s. Particles were sampled on-line with a differentially pumped particle inlet and chemically analyzed by PERCI-MS. PERCI is a soft ionization method that permits the direct measurement of relatively high molecular weight compounds, facilitating molecular identification. In addition to cyclic oxygenates, such as secondary ozonides and geminal diperoxides that were reported previously, we demonstrate the formation of polymers at the particle surface. Polymer formation is proposed to proceed via addition of a Criegee intermediate to oleic acid forming an α-acyloxyalkyl hydroperoxide, and subsequent additions of Criegee intermediates to this initial addition product and later generations. Ultimately, polyanhydrides are formed through dehydration of these α-acyloxyalkyl hydroperoxide intermediates. The highest molecular weight, unfragmented polyanhydride has been measured at 963 U, resulting from the successive addition of four Criegee intermediates to a parent oleic acid molecule. The low volatility of these products, coupled with their high polarity, may alter particle phase hygroscopicity that can enhance the cloud condensation properties of these particles. Further, the polymer products formed at the particles' surface may significantly alter diffusion of reactive gases, thereby affecting the reactive uptake of atmospheric trace gases and impacting the oxidizing capacity of the atmosphere.