For logistical reasons, we exposed the microplastics to these aging conditions for 2 h. To ensure uniform treatment of light and O3 throughout the entire sample, we opened the chamber every filtered out by the ozone layer in the stratosphere. The OH concentrations in these experiments were not measured. However, using the same wavelengths and a similar configuration, OH concentrations of two or three orders of magnitude higher than those observed in the atmosphere can be produced. Assuming OH concentrations two orders of magnitude higher than ambient, the OH exposures correspond to roughly 200 hours or 8.3 days under ambient conditions. Until additional data is available, these aging experiments with UVC and O3, should be considered as upper limits to aging in the troposphere. Additional studies are needed with a combination of UV and O3 exposures typical of the troposphere.