Microplastics samples underwent exposure to O3 in a rotating-wall flow-tube reactor (see Supplement Information, Figure ). A few milligrams of each microplastic sample was placed within a glass tube that was inserted inside the flow-tube reactor. The glass tube was then rotated Scientific, 49i) continuously monitored the O3 concentration. We set the exposure duration for the microplastic samples at 2 h and used an O3 concentrations of 2.8 ppmv. Background concentrations of O3 in the planetary boundary layer (PBL) are around 30 ppbv. Assuming that the aging of microplastic surfaces is proportional to both concentration and time, the exposures in our experiment correspond to 7.7 days at 30 ppbv.