The daytime atmospheric nitrous acid (HONO) source has attracted considerable attention due to its important role in determining the concentrations of hydroxyl radicals (OH˙), O₃, and secondary organic aerosols (SOAs). The widespread nitrophenols (NPs) are potential HONO sources; however, the relevant reaction mechanism has not been explored yet. We employ quantum mechanical calculations combined with the statistical RRKM rate theory to explore the photochemical reaction paths of two common NPs (2-NP and 4-NP) to form HONO in the gas and aqueous phases. It is revealed that both NPs have strong sunlight absorption, and photolysis occurs on the excited T₁ state. Both NPs can easily dissociate under light in both gas and aqueous phases, where OH˙ and NO are the major products rather than HONO. Due to the higher solubility, 4-NP is found to more easily generate HONO in aqueous solution, which experiences the intermolecular excited state hydrogen transfer with vicinal water molecules. Kinetics shows that the relative humidity (RH) hardly promotes the gas-phase photolysis, and the temperature slightly affects the reaction rates. However, as a minor product, HONO generations from gas-phase 2-NP and aqueous 4-NP have high photolysis frequencies of 5.73 × 10⁻⁵ and 5.25 × 10⁻⁶ s⁻¹, respectively, indicating that the hydrolysis of NPs can be important sources for atmospheric HONO.