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3M Smog-reducing Roofing Granules are ceramic-coated minerals used as a surfacing material in asphalt roofing products that contain a photocatalytic layer as part of the ceramic coating matrix. These granules can be integrated into asphalt shingles as part of the shingle blend design or used in granule surfaced roofing membranes for low slope roofs. The photocatalytic layer generates oxygen (O2) and hydroxyl (·OH) radicals when activated by the sun. The radicals react with the nitrogen oxides (NO and NO2, or NOx) in smog to form NO3 which is a plant usable form of nitrogen. The NO3 ion deposits wash away with the rainwater over time. The catalyst activity is constant when exposed to UV, so the smog-reduction activity is a continuous process.