<p>A numerical model is developed to simulate the photocatalytic degradation of trichloramine (NCl₃) in an annular reactor exposed to artificial or solar irradiation. The approach combines two complementary aspects: 1) a Monte Carlo-based radiative transfer simulation to calculate the angular distribution of irradiance absorbed by the catalyst, and 2) a kinetic framework taking into account mass transport and surface reaction mechanisms, with the reaction rate following Langmuir-Hinshelwood kinetics. The model is first compared with experimental results, obtained with artificial or natural light, and shows good agreement despite local deviations. It is then used to evaluate the spatial distribution of trichloramine decomposition in the reactor, highlighting the complex interaction between heterogeneity of illumination on the reactor and chemical kinetics. The simulation reveals that the boundary regime—controlled by either diffusion or kinetics—varies both radially and angularly, depending on the light available locally. This modeling also enables predictive analysis of operational parameters, including the effects of initial concentration, incident irradiance, and reflector configuration. It provides a basis for analyzing the behavior of photocatalytic reactors and for future optimization of operating conditions and reactor configurations. Beyond trichloramine, the methodology demonstrates the relevance of coupled light–reaction models to guide the development of efficient photocatalytic reactors for air treatment applications.</p>