Photodegradation of nitro-PAHs in viscous organic media used as models of organic aerosols.

Many types of organic aerosol material have been found to behave as viscous liquids. In the current work, photodegradation of selected nitrated polycyclic aromatic compounds, 1-nitropyrene and 3-nitrofluoranthene, in viscous organic phases used as model systems for organic aerosols has been investigated. Experiments have been conducted in the presence and absence of anthraquinone, an atmospherically relevant photochemical radical sensitizer. The photodegradation of nitro-PAH is strongly accelerated by the presence of anthraquinone, and a viscous medium does not inhibit the photoinduced decay of nitro-PAH. In fact, the effect of anthraquinone is more pronounced in a viscous solvent containing tertiary hydrogen, DOP, compared to cyclohexane, a nonviscous solvent. This is related to a faster decay of anthraquinone in DOP and hence, presumably, a higher radical yield in DOP. The nitro-PAH degradation in DOP is faster at higher temperature; an observation primarily ascribed to faster diffusion at higher temperature giving rise to more efficient radical formation. In glycerine, a polar viscous solvent, significant formation of amino-PAH occurs due to photoreduction of the nitro-PAH. This is also the case in N2-purged guaiacol, a model substance for wood smoke organic material. Nitro-PAH photoreduction is enabled by the presence of hydrogen-atom donors and low levels of dissolved O2. A simple approach to estimate the O2 concentration in atmospheric organic aerosol material is presented. Low levels of O2 are predicted for wood smoke and secondary organic aerosol making amino-PAH formation in this aerosol material an interesting possibility.