Uncertain Natural Emissions Dampen the Increase in Tropospheric Hydroxyl Radical (OH) With Idealized Surface Warming

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Abstract The hydroxyl radical (OH) defines the oxidative capacity of the atmosphere and determines the lifetime of reactive greenhouse gases, including methane. The response of OH to climate warming is influenced by uncertain and compensating processes involving meteorological factors and temperature‐sensitive natural emissions, including soil N (SN) and biogenic volatile organic compounds (BVOC) emissions. However, separating individual processes that control the OH response to warming is challenging given the high dimensionality of both climate dynamics and emissions in fully coupled chemistry‐climate models. Here, we create an idealized chemistry‐climate model, Aqua‐chem, by prescribing annual mean emissions and zonally symmetric sea surface temperatures. We show that the net OH response to an idealized 2 K surface warming in Aqua‐chem depends on competing effects of moistening (a robust response to warming) and temperature‐sensitive BVOC emissions (a highly uncertain response). The 2 K surface warming increases water vapor, resulting in an increase in tropospheric OH through primary OH production (ozone photolysis followed by reaction of D with O). Temperature‐sensitive SN emissions further enhance OH via the NO + H reaction, but this additional increase is outweighed by the increase in temperature‐sensitive BVOC emissions. Amplified OH losses, through reactions with BVOCs and their oxidation byproducts, strongly dampen the increase due to atmospheric moistening with rising surface temperature. Our study underscores the importance of accurately quantifying the temperature sensitivity of natural emissions in order to constrain the OH response to climate warming.

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