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

Abstract

Significance Reaction with the hydroxyl radical (OH) is the dominant loss mechanism for many atmospheric gases of interest for air quality, climate change, and stratospheric ozone. Understanding how and why OH may change in the future is therefore paramount for predicting changes in the societal impacts associated with such changes. Future models’ projections strongly disagree in how OH responds to changing emissions and climate—even in the sign. Here, we demonstrate that intermodel differences in OH are best explained by disparate implementations of chemical and physical processes that affect reactive oxides of nitrogen and organic chemical species. Targeted observations can reduce uncertainty in the chemical budgets of these key species to increase confidence in future projections of composition and its impacts. 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 steady-state relationship that accounts for ozone photolysis frequencies, water vapor, and the ratio of reactive nitrogen to carbon emissions explains temporal variability within most models, but not intermodel differences. Here, we show that departure from the expected relationship reflects the treatment of reactive oxidized nitrogen species (NOy) and the fraction of emitted carbon that reacts within each chemical mechanism, which remain poorly known due to a lack of observational data. Our findings imply a need for additional observational constraints on NOy partitioning and lifetime, especially in the remote free troposphere, as well as the fate of carbon-containing reaction intermediates to test models, thereby reducing uncertainties in projections of OH and, hence, lifetimes of pollutants and greenhouse gases.