Abstract: The near-field evolution of chemically reactive pollutants in the exhaust plume of a 270 m crude oil tanker is investigated using a high-fidelity Computational Fluid Dynamics (CFD) framework coupled with a Conditional Moment Closure (CMC) turbulence-chemistry interaction model and the CBM-IV atmospheric chemistry scheme. Two complementary approaches, a canonical zero-dimensional CMC (0D-CMC) and a spatially resolved Imperfectly Stirred Reactor Network (ISRN), quantify micromixing effects using information from the CFD flow field. Micromixing strongly modulates the evolution of NO, NO2, O3, and PAN, explaining spatial discrepancies between detailed CFD-CMC results and Gaussian plume estimates. While conventional Gaussian formulations overpredict near-field NO by up to 45% within the first 300 m, they are used here as a contextual reference to highlight where turbulence-chemistry coupling matters. The CFD-CMC-ISRN framework therefore provides a physically consistent basis for integrating micromixing effects into next-generation dispersion and air-quality models. This framework bridges the gap between combustion-derived turbulence-chemistry models and environmental dispersion simulations, enabling improved parameterization of subgrid mixing in regulatory contexts.