Abstract The heterogeneous reactions of NO 2 and OH radicals with polycyclic aromatic hydrocarbons (PAHs) naturally present in diesel particulate exhaust (NIST SRM 1650a) have been investigated. Using a fast flow reactor, relative degradation rate constants have been determined by following the decays of particulate-PAH concentrations vs. the reaction time. Quantitative analyses have been performed by gas chromatography coupled to mass spectrometry detection using internal standards. Relative rate constants show a significant structure reactivity effect for the reaction of PAHs with NO 2. Pyrene and benzo( a)pyrene are the most reactive with NO 2 whereas all PAHs studied present similar reactivities with OH within uncertainties. All PAHs appeared to be approximately four orders of magnitude more reactive with OH than with NO 2. These rate constants confirm that the OH heterogeneous reaction will be the dominant atmospheric loss process of PAHs compared to that with NO 2. Our data are compared to previous literature studies, concerning both the particulate and the gas phases. This work demonstrates that the reactivity of PAHs in the gas phase would be significantly larger than when associated with carbonaceous particulate substrates.