Large uncertainties in global hydroxyl projections tied to fate of reactive nitrogen and carbon

The hydroxyl radical (OH) sets the oxidative capacity of the atmosphere and, thus, profoundly affects the removal rate of pollutants and reactive greenhouse gases. While observationally derived constraints exist for global annual mean present-day OH abundances and interannual variability, OH estimates for past and future periods rely primarily on global atmospheric chemistry models. These models disagree ± 30% in mean OH and in its changes from the preindustrial to late 21st century, even when forced with identical anthropogenic emissions. A simple steadystate relationship that accounts for ozone photolysis frequencies, Our best estimates of global mean abundance and interannual variability from OH rely on proxy measurements, particularly methyl chloroform (11), as the high reactivity and short lifetime of OH make direct measurement difficult and impractical for constraining spatial and temporal variability (12). On average, global atmospheric chemistry models cannot reproduce meridional gradients in carbon monoxide (CO) and other long-lived reactants, implying possible errors in simulated OH spatial and seasonal distributions (6, 13). They also overestimate global mean OH with respect to observational constraints from the