Ambient Measurements of Heterogeneous Ozone Oxidation Rates of Oleic, Elaidic, and Linoleic Acid Using a Relative Rate Constant Approach in an Urban Environment

Long‐chain unsaturated fatty acids (uFAs), such as oleic acid, undergo rapid degradation via heterogeneous reactions with atmospheric oxidants upon emission. The oxidation mechanism and kinetics have been extensively studied in laboratory experiments. However, quantitative knowledge of degradation rates under real‐world atmospheric conditions is scarce. We obtained the nighttime decay rates of three cooking‐related uFAs using a relative rate approach applied to bihourly measured data in urban Shanghai. The estimated lifetime of oleic acid was 6 h under conditions of ∼12 ppb ozone and 60%–100% relative humidity encountered at our urban location or an inferred ∼2 h at a higher ozone level of ∼40 ppb. The decay rates of elaidic and linoleic acid are determined to be 0.62 and 1.37 that of oleic acid, respectively. This work provides the first kinetic data pertaining to real‐world conditions. They are valuable for constraining the modeling of heterogeneous aging of ambient organic aerosols. Plain Language Summary Cooking emissions can contribute significantly to ambient aerosols in urban areas. Oleic and linoleic acid are among the major components in cooking‐emitted particles. They can be rapidly degraded in the atmosphere. This process modifies the aerosol compositions and properties, altering the aerosols' climate impact. Previous studies on the oxidation mechanism and reaction rate of the cooking molecules are mainly based on laboratory experiments, with simulated aerosol matrices limited to simple mixtures of a few components. Ambient aerosols, however, are far more complicated. The laboratory settings can only partially simulate atmospheric conditions. In this work, we determined the decay rate constants of three cooking molecules (i.e., oleic, elaidic, and linoleic acid) by taking advantage of their bihourly measured concentrations at an urban location. Between our ambient data‐derived lifetime for oleic acid against ozone oxidation and the previous laboratory data‐derived estimates, the agreement tends to be haphazard. Such comparison results indicate the deficiency of the laboratory systems in mimicking the real‐world conditions and highlight the necessity in producing more ambient measurements‐derived kinetic data. Key Points Bihourly measured oleic, elaidic, and linoleic acid allow first‐time determination of their decay rates under real‐world conditions The relative decay rates of elaidic and linoleic acid versus oleic acid from the ambient data are