Atmospheric oxidation capacity at northern mid-latitude regions is reaching a turning point

The hydroxyl radical (OH) constitutes the primary atmospheric oxidant, governing the removal of short-lived greenhouse gases such as methane and the formation of secondary pollutants, including ozone. Characterizing long-term OH variability is critical for understanding atmospheric oxidation capacity and formulation of air quality improvement strategies. Although previous studies have linked OH increases to anthropogenic emissions, particularly nitrogen oxides (NO_x), the historical evolution of OH remains poorly constrained. Here, we develop an observation-based method (OBM) to quantify spatiotemporal variability of surface OH concentration and identify its key turning points for China, Europe and the United States. Relative to clean regions, OH concentrations have increased by about 300% in all three regions, mainly attributable to enhanced NO_x emissions. However, OBM-derived trends reveal that OH levels have reached a turning point, with statistically significant decreases observed at ∼10% of stations in Europe and the United States, while ∼85% of stations continue to increase. At hemispheric mid-latitudes, OH levels have remained near peak values of (5-9) × 10⁶ cm^-3 over the past 50-60 years, and it may take another several decades to fully return to the NO_x-limited regime, beyond the turning point. These findings challenge prior assessments of OH increases or stability and suggest an emerging decline in mid-latitude atmospheric oxidation capacity, as well as provide a novel indicator of regional ozone control strategies.