Natural and Anthropogenic Impacts on Global Tropospheric OH and Methane Lifetime: A Multi‐Model Analysis From AerChemMIP

This study assesses the influence of natural and anthropogenic emissions on tropospheric hydroxyl radical concentrations ([OH]) and methane (CH 4 ) lifetime using multi‐model simulations from the AerChemMIP experiments of CMIP6. Compared to the preindustrial era, we find that 2014‐level anthropogenic emissions, which increase global [OH] by 1.1 (0.7–2.1) × 10 5 molecules cm −3 , and doubled natural emissions, which decrease global [OH] by 0.9 (0.2–1.4) × 10 5 molecules cm −3 , have comparable impacts on global [OH] and CH 4 lifetime. The impact of both anthropogenic and natural emissions reflects a complex counterbalance among different sources. Doubling biogenic non‐methane volatile organic compounds (NMVOCs) and lightning NO x (LNO x ) results in a 2.5 (1.3–3.0) × 10 5 molecules cm −3 decrease and a 2.1 (1.6–2.6) × 10 5 molecules cm −3 increase in global [OH], respectively, and these are the primary natural factors regulating [OH]. Anthropogenic emissions primarily affect [OH] through near‐term climate forcers (NTCFs), increasing [OH] by 2.5 (2.0–3.7) × 10 5 molecules cm −3 , partially offset by well‐mixed greenhouse gases. Among NTCFs, NO x and NMVOC emissions dominate, increasing global [OH] by 4.5 (3.2–6.1) × 10 5 molecules cm −3 and decreasing it by 1.8 (1.5–2.3) × 10 5 molecules cm −3 , respectively. Regionally, anthropogenic emissions increase the North to South ratio of [OH], whereas natural emissions decrease it. The global [OH] under preindustrial conditions shows 1%–2% interannual variability, with upper‐tropospheric variations primarily driven by lightning NO x . The global [OH] is nearly five times more sensitive per Tg N to LNO x than to anthropogenic NO x , highlighting the importance of accurately representing natural emissions to better constrain [OH].