Atmospheric self-cleansing capacity at northern midlatitude regions is reaching a turning point

The hydroxyl radical, often called the "tropospheric vacuum cleaner", reacts with hundreds of gases, both natural and anthropogenic, determining how long pollutants and greenhouse gases like methane (CH₄) remain in the air. The collective abundance of OH defines the "atmospheric oxidation capacity", a key factor linking air quality and climate change. Due to limitations in field measurements, the quantification of OH has primarily relied on approaches such as atmospheric chemical models and halogenated hydrocarbon inversion. However, different studies have yet to reach a consensus on its long-term trend.

A new study published in National Science Review now provides a robust observation‑based approach to assess OH radical. The research team, led by Professor Keding Lu from Peking University, along with Professor Shaw Chen Liu from Jinan University, developed an observation‑based method (OBM) to quantify the spatiotemporal variability of surface OH concentration and identify its key turning points for China, Europe, and the United States.

Researchers found that OH concentrations in China, Europe, and the US are approximately 300% higher than in clean regions, largely attributable to increased NO_x emissions. This confirms the significant impact of human activities on atmospheric oxidation capacity, a finding consistent with earlier studies but now quantified with greater spatial and temporal scales.

However, this increase will not continue indefinitely. Researchers found that in Europe and the United States, approximately 10% of monitoring stations have already shown statistically significant decreasing trends in OH, while about 85% of stations continue to exhibit increasing trends. By contrast, OH levels in China are still generally in a growth phase.

The study further points out that as NO_x emissions continue to decline in the future, regional OH concentrations are expected to gradually cross a critical threshold and enter the NO_x-limited regime. At that point, OH levels will transition from a stable high level to a declining trajectory. This implies that the atmospheric lifetimes of air pollutants and greenhouse gases such as methane may be prolonged, posing new challenges for future regional air quality management and global climate governance.

"In the past, trends in the critical oxidant OH are less certain," said one of the researchers. "Now, things are different. With an observation-based method, we can quickly and reliably assess OH concentrations using only routine observation data, and also see how OH responds to anthropogenic emissions. This has provided tremendous support for pollution control and addressing global climate issues."

Their finding reveals that the atmospheric oxidation capacity at the northern midlatitude regions may be approaching a critical turning point. The observation-based model offers a more reliable characterization of long-term hydroxyl (OH) changes than traditional model simulations, filling a key gap in the regional-scale quantitative assessment of how anthropogenic emissions affect OH levels. This breakthrough also provides a new scientific foundation for the coordinated response to regional air pollution control and climate change.

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