Interannual changes in atmospheric oxidation over forests determined from space

The hydroxyl radical (OH) is the central oxidant in Earth’s troposphere, but its temporalvariability is poorly understood. We combine 2012-2020 satellite-based isoprene and formaldehyde measurements to identify coherent OH changes over temperate and tropical forests with attribution to emission trends, biotic stressors, and climate. We present the first demonstration of a multi-year OH decrease over the southeast United States and show that with increasingly hot/dry summers the regional chemistry could become even less oxidizing depending on competing temperature/drought impacts on isoprene. Furthermore, while global-mean OH decreases during El Niño, we show that near-field effects over tropical rainforests can alternate between high/low OH anomalies due to opposing fire and biogenic-emission impacts. Results provide insights into how atmospheric oxidation will evolve with changing emissions and climate.