Abstract: More frequent global wildfire events intensify the exposure risks of nitrophenols (NPs) through brown carbon (BrC) emissions. Radical reactions dominate the fates of NPs in atmospheric and surface waters, yet the involved reaction kinetics and mechanisms remain poorly understood. In this work, the previously ill-quantified radical reactions of NPs are elucidated by combining transient and steady-state multispectral analyses. The second-order reaction rate constants (k) of NPs and real biomass-burning BrC with HO as well as common secondary radicals (e.g., Cl2−, CO3− and NO3) were determined, most of which are reported for the first time. We also experimentally demonstrated that the reactions of NPs and BrC with various radicals could produce a substantial amount of HONO/NO2− (a key precursor of atmospheric HO), with molar yields reaching ~50%. Mechanistic investigations revealed that the de-nitrated phenoxy radical (PhO), generated from either proton-coupled electron transfer or addition-elimination reactions during the radical reactions of NPs, acted as a universal intermediate. Hence, the new kinetic and mechanistic insights suggest that radical-induced de-nitration process of BrC’s NPs in acidic waters tends to serve as a previously overlooked HONO source, but such de-nitration does not guarantee detoxification due to the simultaneous generation of high-risk bisphenols. This work highlights the critical role of secondary radicals in BrC’s NP transformation, shedding light on unexpected chemical processes driven by intensified global wildfires.