Abstract: Secondary organic aerosol, or SOA, forms in the atmosphere through the oxidation of volatile biogenic compounds. The majority of the oxidation mechanisms involve small molecules, such as ozone, in the initial stages of the chemistry. Our group studies the formation of SOA through photochemical mechanisms, where electronically excited molecules play the role of the oxidizing agent. We use a laser-based, ambient pressure photoelectric charging method to monitor the decay of aerosol phase triplet photosensitizer molecules. The results bridge the gap between aerosol phase measurements, which are normally steady-state, and bulk-phase, transient absorption measurements. They also demonstrate how the morphology of particle phase systems can control the chemistry.