Simulated atmospheric transformation of the tire additives in the aqueous phase: 6-PPD, 6-PPD quinone, diphenylguanidine, and dicyclohexylamine

Tire and road wear particles (TRWP) are emitted in large amounts into the environment; a fraction of them becomes part of atmospheric aerosol. The degradation of four tire-derived chemicals, N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6-PPD), its transformation product 6-PPD quinone (6-PPDQ), diphenylguanidine (DPG), and dicyclohexylamine (DCHA) was studied under simulated atmospheric conditions in the aqueous phase (direct photolysis, reactions with hydroxyl-radicals and sulfate-radical anions). A high reactivity of the four chemicals towards radicals was observed and 141 transformation products (TPs) were detected by liquid chromatography-high resolution-mass spectrometry, of which more than the half were newly reported. Key intermediates of DPG and DCHA were detected by suspect-screening in urban and rural aerosol, thus demonstrating their environmental relevance. TPs of 6-PPD and 6-PPDQ were not found, likely due to the 100 times lower concentration of these compounds in the aerosol compared to DPG and DCHA. As long as the chemicals are embedded into the tire-matrix, their reactivity is expected to be lower. Future monitoring studies in aerosol should consider both the tire-derived chemicals and the TPs of their reaction with hydroxyl-radicals and sulfate-radical anions during atmospheric transport. TPs of tire-derived chemicals may be of relevance for human exposure and considered in future risk assessment.