The sunlight driven transformations of atmospheric organic aerosols are important for controlling climate and health-relevant properties of particulate matter, but these photochemical processes are not well understood. The prevailing view in air pollution science is that molecules in aerosol particles are shielded from reactions with oxidants, but this view does not consider condensed-phase photochemical reactions occurring inside the particles, such as photosensitized reactions between particles’ components. Photosensitized reactions involve energy transfer from an initial chromophore (photosensitizer/<sup>3</sup>C<sup>*</sup>) to a neighboring molecule which creates a cascade of chemical reactions that may drastically alter the chemical composition of the particle. Our report presents experimental studies in which we probed the triplet states of naphthalene secondary organic aerosol (SOA) on ultrafast femtosecond and nanosecond timescales. Ultrafast experiments clearly demonstrate triplet states being populated within a few picoseconds and decay with a lifetime of a few microseconds. Furthermore, the decay is affected by the presence of molecules known to react with triplet excited states. In addition, experiments conducted by Dr. Christian George’s group show that size-selected naphthalene SOA grow in the presence of VOCs as a result of these photosensitized reactions. This unexplored aging mechanism shows that even photostable molecules within aerosols can undergo reactive energy transfer without directly absorbing sunlight, thus elucidating a new channel of photochemical aging.

Chemical reactions involving triplet excited states and other reactive species in secondary organic aerosol produced by photooxidation of naphthalene

2019 Fall National Meeting

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