VAHE BABOOMIAN, Xinke Wang, Rachel Gemayel, Yiran Gu, Lisa M. Wingen, Chrisitan George, Dmitry Fishman, Sergey Nizkorodov, University of California, Irvine
Abstract Number: 92 Working Group: Aerosol Chemistry
Abstract The sunlight driven transformations of atmospheric organic aerosols are important for understanding and controlling the climate and health-relevant properties of particulate matter, but these photochemical processes are not well understood. Molecules within aerosol particles are often considered shielded from reactions with oxidants but this view does not include condensed phase photochemical reactions. Our report presents experimental studies identifying the long-term physical and compositional changes occurred due to photochemical aging as well as the mechanisms responsible for them. The physical changes were investigated by utilizing a quartz crystal microbalance to quantify the mass loss rate from various lab generated particles irradiated at various wavelengths. We observed that 254 nm irradiation degraded 86 % of toluene secondary organic aerosol (SOA) after 24 hours, but only 74 % of α-pinene SOA suggesting different levels of resilience. The compositional changes due to long term irradiation were also investigated using High Resolution Mass Spectrometry techniques. The chemical mechanisms responsible for this aging include direct photochemical processes, such as Norrish type splitting of carbonyls, as well as indirect photosensitized reactions. Photosensitized reactions involve energy transfer from an initial chromophore (photosensitizer/3C*) to a neighboring molecule which creates a cascade of chemical reactions that may drastically alter the chemical composition of the particle. These reactions were investigated by probing the triplet states of naphthalene 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. 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.