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 (NOx≡NO+NO2) in the free troposphere. Variations in global lightning activity influence long-term trends (LTTs) and inter-annual variability (IAV) in tropospheric NOx, ozone (O3), and hydroxyl radicals (OH). However, accurately quantifying these impacts is hindered by uncertainties in representing year-to-year fluctuations in 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 NOx (LNOx) 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 2013–2021. The updated parameterisation predicts an insignificant trend in global LNOx emissions, contrasting with a significant increase of 6.4 % dec−1 (P<0.05) by the original parameterisation, reducing lightning contributions to LTTs in NOx, 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 parameterisation reveals twice the IAV in global LNOx 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 a ∼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.