Global southward shift in anthropogenic emisisons enhance tropospheric hydroxyl radicals during 1999-2019

The hydroxyl radical (OH), the main atmospheric oxidant, removes most pollutants including potent greenhouse gas methane. However, its short lifetime precludes direct observational quantification, so its temporal variability is poorly understood. Here, we used a three-dimensional chemical transport model GEOS-Chem, to investigate global tropospheric OH concentrations changes (ΔOH) from 1999 to 2019, driven by nitrogen oxides and carbon monoxide. We showed that ΔOH relative to 1999 increased rapidly from 2000 to 2008, dropped until 2010 and then continued to grow from 2011 to 2019, culminating in a 4.7% increase by 2019. The increase in ΔOH is primarily attributed to emissions from land (63%), followed by emissions from aircraft (24%) and shipping (13%). Notably, the tropics, particularly East Asia, Southeast Asia, South Asia, and Central America, contributed 74% of the global ΔOH burden in 2019. We also performed fractional simulations to separate the influence of land emissions influence from changes in the spatial distribution (Land_S) and magnitude of emissions (Land_M). We found that as land NO_x emissions shifted equatorward from middle and high latitudes to low latitudes, the influence of land_S increased persistently, exceeding Land_M by 2014, and the relative contribution of Land_S to ΔOH due to land emissions reached 58% in 2019. Looking forward, with the continued global southward shift in anthropogenic emissions, the role of Land_S in global OH levels should not be overlooked. These insights underscore the need to consider anthropogenic emission patterns in projecting future OH concentrations and developing climate mitigation strategies.