OXYGENATED species in the atmosphere are important sources of free radicals and are intricately linked with the fate of nitrogen oxides (NOx), which are themsel es necessary for tropospheric ozone formation(1,2). With the exception of formaldehyde, oxygenated hydrocarbons have rarely been measured in the free troposphere. Here we report airborne measurements indicating the presence of high concentrations (compared to those of routinely measured C-2-C-6 tropospheric hydrocarbons(3,4)) of acetone and methanol. We use a three-dimensional model to show that acetone photochemistry provides a quantitatively significant (up to 50%) pathway for sequestering NOx in the form of peroxyacetylnitrate, a relatively unreactive temporary reservoir of NOx. Furthermore, in the dry regions of the upper troposphere, acetone can provide a large primary source of HOx (OH + HO2) radicals, resulting in increased ozone production. This surprisingly significant contribution of such oxygenated hydrocarbons to tropospheric NOx, HOx and ozone cycling is likely to be affected by their changing natural and anthropogenic emissions due to land-use change, biomass burning and alcohol-based biofuel use.