Existence of Crystalline Ammonium Sulfate Nuclei Affects Chemical Reactivity of Oleic Acid Particles Through Heterogeneous Nucleation

摘要

Organic aerosol particles are oxidized by atmospheric oxidants. These particles are occasionally internally mixed with solid materials such as soot and inorganic crystals. However, potential impacts of the particles' mixing states on chemical reactivity have rarely been investigated. This study investigated the influence of the existence of crystalline ammonium sulfate on chemical reactivity of oleic acid particles with ozone for the temperature range of -20 degrees C to +35 degrees C using an aerosol flow tube reactor. The chemical compositions of the resulting particles were monitored using online instruments for deriving the reactive uptake coefficients (gamma) of ozone by oleic acid. The values of gamma were not significantly influenced by the existence of ammonium sulfate when the temperature of the reactor was higher than the melting point of oleic acid (similar to 13 degrees C). The values of gamma were unmeasurably small for the lower temperature range when oleic acid particles were internally mixed with crystalline ammonium sulfate. No significant change in gamma was observed for the temperature range down to -13 degrees C when the inorganic salt was absent, likely due to the formation of supercooled liquid. The difference in chemical reactivity can be explained by the occurrence of heterogeneous nucleation induced by inorganic seed. Plain Language Summary Mixing states of particles in the atmosphere are complex and change with time. Part of fresh particles could consist of organic matter only after emission, while some particles are internally mixed with other materials, such as soot, mineral dust, and inorganic salts. Particles experience complicated atmospheric processes during transport, like oxidation, coagulation, and condensation. These processes could modify particle physical and chemical properties, including mixing state. However, the impact of mixing state of particles on oxidation process has rarely been investigated. We conducted a series of ozonolysis experiments of non-seeded and seeded oleic acid particles under a wide temperature range. This study demonstrated that internally mixed inorganic crystals suppressed chemical reactivity of oleic acid by promoting heterogeneous nucleation under lower temperature. Colder condition was needed for homogeneous nucleation of non-seeded particles, as they could form supercooling liquid. Our results indicate mixing state is also an important factor to regulate particle viscosity and phase state.