Products and Mechanisms of the Reaction of Oleic Acid with Ozone and Nitrate Radical

Abstract

The heterogeneous reactions of deposited, millimeter-sized oleic acid droplets with ozone and nitrate radicals are studied. Attenuated total reflectance infrared spectroscopy (ATR-IR), gas chromatography-mass spectrometry (GC-MS), and liquid chromatography-mass spectrometry (LC-MS) are used for product identification and quantification. The condensed-phase products of the ozonolysis of oleic acid droplets are 1-nonanal (30 +/- 3% carbon yield), 9-oxononanoic acid (14 +/- 2%), nonanoic acid (7 +/- 1%), octanoic acid (1 +/- 0.2%), azelaic acid (6 +/- 3%), and unidentified products. The infrared spectra show that a major fraction of the unidentified products contain an ester group. Additionally, the mass spectra show that at least some of the unidentified products have molecular weights greater than 1000 amu, which implicates a polymerization reaction. The observed steps of 172 amu (9-oxononanoic acid) and 188 amu (azelaic acid Criegee intermediate) in the mass spectra suggest that these species are the monomers in the condensed-phase polymerization reactions. 9-Oxononanoic acid is proposed to lengthen the molecular chain via secondary ozonide formation; the azelaic acid Criegee intermediate links molecules units via ester formation (specifically, alpha-acyloxyalkyl hydroperoxides). For the reaction of oleic acid with nitrate radicals, functional groups including -ONO(2), -O(2)NO(2), and -NO(2) are observed in the infrared spectra, and high molecular weight molecules are formed. Environmental scanning electron microscopy (ESEM) is employed to examine the hygroscopic properties of the oleic acid droplets before and after exposure to ozone or nitrate radical. After reaction, the droplets take up water at lower relative humidities compared to the unreacted droplets. The increased hygroscopic response may indicate that the oxidative aging of atmospheric organic aerosol particles has significant impact on radiative forcing.