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Emerging contaminants (EC) distributed on surfaces in the environment can be oxidized by gas phase species (top–down) or by oxidants generated by the underlying substrate (bottom–up). One class of EC is the neonicotinoid (NN) pesticides that are widely distributed in air, water, and on plant and soil surfaces as well as on airborne dust and building materials. This study investigates the OH oxidation of the systemic NN pesticide acetamiprid (ACM) at room temperature. ACM on particles and as thin films on solid substrates were oxidized by OH radicals either from the gas phase or from an underlying TiO₂or NaNO₂substrate, and for comparison, in the aqueous phase. The site of OH attack is both the secondary >CH₂group as well as the primary –CH₃group attached to the tertiary amine nitrogen, with the latter dominating. In the case of top–down oxidation of ACM by gas phase OH radicals, addition to the –CN group also occurs. Major products are carbonyls and alcohols, but in the presence of sufficient water, their hydrolyzed products dominate. Kinetics measurements show ACM is more reactive toward gas phase OH radicals than other NN nitroguanidines, with an atmospheric lifetime of a few days. Bottom–up oxidation of ACM on TiO₂exposed to sunlight outdoors (temperatures were above 30 °C) was also shown to occur and is likely to be competitive with top–down oxidation. These findings highlight the different potential oxidation processes for EC and provide key data for assessing their environmental fates and toxicologies.

The neonicotinoid nitenpyram (NPM) is a multifunctional nitroenamine [(R₁N)(R₂N)C=CHNO₂] pesticide. As a nitroalkene, it is structurally similar to other emerging contaminants such as the pharmaceuticals ranitidine and nizatidine. Because ozone is a common atmospheric oxidant, such compounds may be oxidized on contact with air to form new products that have different toxicity compared to the parent compounds. Here we show that oxidation of thin solid films of NPM by gas-phase ozone produces unexpected products, the majority of which do not contain oxygen, despite the highly oxidizing reactant. A further surprising finding is the formation of gas-phase nitrous acid (HONO), a species known to be a major photolytic source of the highly reactive hydroxyl radical in air. The results of application of a kinetic multilayer model show that reaction was not restricted to the surface layers but, at sufficiently high ozone concentrations, occurred throughout the film. The rate constant derived for the O₃−NPM reaction is 1 × 10⁻¹⁸cm³⋅s⁻¹, and the diffusion coefficient of ozone in the thin film is 9 × 10⁻¹⁰cm²⋅s⁻¹. These findings highlight the unique chemistry of multifunctional nitroenamines and demonstrate that known chemical mechanisms for individual moieties in such compounds cannot be extrapolated from simple alkenes. This is critical for guiding assessments of the environmental fates and impacts of pesticides and pharmaceuticals, and for providing guidance in designing better future alternatives.