Contributions of lightning to long-term trends and inter-annual variability in global atmospheric chemistry constrained by Schumann Resonance observations

Abstract. Lightning is a significant source of nitrogen oxides (NO_x ≡ NO + NO₂) in the free troposphere. Variations in global lightning activity influence long-term trends (LTT) and inter-annual variability (IAV) in tropospheric NO_x, ozone (O₃) and hydroxyl radicals (OH). However, accurately quantifying these impacts is hindered by uncertainties in representing year-to-year fluctuations of global lightning activity in models. Here, we apply Schumann Resonance (SR) observations, which are sensitive to changes in global lightning activity, to better constrain inter-annual variations in lightning NO_x (LNO_x) emissions. By integrating this update into an atmospheric chemical transport model, we assess the contributions of lightning to both LTT and IAV in global atmospheric chemistry from 2013 to 2021. The updated parameterization predicts an insignificant trend in global LNO_x emissions, contrasting with a significant increase of 6.4 % dec^-1 (P < 0.05) by the original parameterization, reducing lightning contributions to LTT in NO_x, O₃, and OH. The updated simulation better aligns with satellite-observed trends in global and Northern Hemispheric NO₂, but further underestimates tropospheric O₃ increases. The updated parameterization reveals twice the IAV in global LNO_x emissions but 20 % smaller IAVs in global O₃ and OH, because lightning generally counteracts other sources of natural variability. A ~10 % decline in lightning in 2020 relative to 2019 led to ~2 % decrease in global OH, explaining half of observed annual methane growth. These findings highlight the value of Schumann Resonance observations in constraining global lightning activity, thereby enhancing our understanding of lightning’s role in atmospheric chemistry.