Exposure Sequence of Photoaging and Natural Organic Matters Governs the Complex Release Dynamics of Additives from Tire Tread Particles

The detrimental effects of tire particles are closely related to additive leaching, yet how the exposure sequence of sunlight and natural organic matter (NOM) regulates this process remains unclear. We investigated the release dynamics of 11 additives, dissolved organic carbon, and nontarget molecular features in tire tread particle (TTP) leachates over 21 d under two exposure scenarios: (i) dry-photoaging followed by incubation in different NOM, and (ii) wet-photoaging occurring directly in humic acid (HA). For dry-photoaged TTPs, NOM suppressed the release of most organic additives with increasing photoaging duration and NOM concentration through surface passivation and depletion of leachable reservoirs, whereas Zn and diphenylguanidine release were enhanced by up to 44.5% and 47.2%, respectively. By comparison, HA facilitated organic additive release but suppressed Zn during wet-photoaging, with additive levels remaining below those from dry-photoaged TTPs. Compared to dry-photoaging, wet-photoaging in HA also resulted in lower contributions of quantified additives to measured DOC (2–5%), greater reduction in environmentally persistent free radicals (22%), and enhanced formation of 3 HA* and 1 O 2, indicating the photosensitizing role of HA. Nontarget screening further revealed greater diversity of molecular features after prolonged HA incubation than in NOM-free or early stage HA treatments. These findings underscore the exposure-order-dependent divergence in reshaping chemical complexity and the fate of tire-derived mixtures.