High concentrations and photochemical fate of oxygenated hydrocarbons in the global troposphere

Abstract

OXYGENATED species in the atmosphere are important sources of free radicals and are intricately linked with the fate of nitrogen oxides (NO_x), which are themselves 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 C₂–C₆ 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 NO_x in the form of peroxyacetylnitrate, a relatively unreactive temporary reservoir of NO_x. Furthermore, in the dry regions of the upper troposphere, acetone can provide a large primary source of HO_x (OH + HO₂) radicals, resulting in increased ozone production. This surprisingly significant contribution of such oxygenated hydrocarbons to tropospheric NO_x, HO_x 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.