We use observations from the April 2008 NASA ARCTAS aircraft campaign to the North American Arctic, interpreted with a global 3-D chemical transport model (GEOS-Chem), to better understand the sources and cycling of hydrogen oxide radicals (HO<sub>x</sub>≡H+OH+peroxy radicals) and their reservoirs (HO<sub>y</sub>≡HO<sub>x</sub>+peroxides) in the springtime Arctic atmosphere. We find that a standard gas-phase chemical mechanism overestimates the observed HO<sub>2</sub> and H<sub>2</sub>O<sub>2</sub> concentrations. Computation of HO<sub>x</sub> and HO<sub>y</sub> gas-phase chemical budgets on the basis of the aircraft observations also indicates a large missing sink for both. We hypothesize that this could reflect HO<sub>2</sub> uptake by aerosols, favored by low temperatures and relatively high aerosol loadings, through a mechanism that does not produce H<sub>2</sub>O<sub>2</sub>. We implemented such an uptake of HO<sub>2</sub> by aerosol in the model using a standard reactive uptake coefficient parameterization with γ(HO<sub>2</sub>) values ranging from 0.02 at 275 K to 0.5 at 220 K. This successfully reproduces the concentrations and vertical distributions of the different HO<sub>x</sub> species and HO<sub>y</sub> reservoirs. HO<sub>2</sub> uptake by aerosol is then a major HO<sub>x</sub> and HO<sub>y</sub> sink, decreasing mean OH and HO<sub>2</sub> concentrations in the Arctic troposphere by 32% and 31% respectively. Better rate and product data for HO<sub>2</sub> uptake by aerosol are needed to understand this role of aerosols in limiting the oxidizing power of the Arctic atmosphere.