Multiphase reactivity of polycyclic aromatic hydrocarbons is driven by phase separation and diffusion limitations

Area of Science:

• Environmental Chemistry
• Atmospheric Chemistry
• Chemical Kinetics

Background:

• Benzo[a]pyrene (BaP) is a carcinogenic polycyclic aromatic hydrocarbon (PAH) found in soot, often coated by organic compounds.
• The chemical transformation kinetics and mechanisms of BaP with ozone, especially within organic mixtures, remain incompletely understood.
• Understanding BaP's environmental fate is crucial due to its health risks and atmospheric persistence.

Purpose of the Study:

• To elucidate the kinetics and mechanisms of benzo[a]pyrene (BaP) ozonolysis in thin organic films.
• To investigate the reasons behind the slow and incomplete decay of BaP upon prolonged ozone exposure.
• To model the diffusion and phase separation effects on BaP's chemical lifetime.

Main Methods:

• Kinetics studies of BaP ozonolysis in thin films using direct analysis in real-time mass spectrometry (DART-MS).
• Kinetic multilayer modeling to simulate BaP decay over time.
• Thermodynamic modeling to predict phase separation behavior in multicomponent systems.

Main Results:

• Ozonolysis of BaP in thin films showed rapid initial decay followed by a slower decay phase.
• Slow BaP decay was attributed to slow diffusion from the film interior, simulated by kinetic multilayer modeling.
• Thermodynamic modeling predicted BaP phase separation from secondary organic aerosol, creating a shielding effect.
• BaP oxidation products formed immiscible surface crusts with oils, hindering diffusion.

Conclusions:

• Phase separation and slow diffusion significantly extend the chemical lifetime of PAHs like BaP.
• These processes impact the long-range atmospheric transport and environmental fate of PAHs.
• Findings highlight the importance of considering diffusion limitations and phase behavior in atmospheric chemistry models.