Impacts of Interannual Isoprene Variations on Methane Lifetimes and Trends

Recent observations show anomalously high methane growth in 2020, which has been attributed to increased wetland emissions and decreased OH from lower COVID-19 nitrogen oxide (NO_x) emissions. NO_x is not the only species that affects OH—isoprene, the most significant non-methane hydrocarbon by total emissions, is oxidized by OH, which can deplete OH during periods of high emissions. Using satellite isoprene retrievals from the Cross-track infrared sounder (CrIS), we find anomalously high isoprene columns during 2020, coincident with high methane growth. Isoprene's oxidation produces carbon monoxide, which can be transported over longer distances and decrease OH outside of isoprene source regions. Elevated isoprene concentrations may have contributed 13% (bounds: 10%–28%) of 2020's methane growth if we assume no change in NO_x emissions in 2020. Since COVID-19 decreased anthropogenic NO_x emissions, this estimate is an upper-limit and may depend on whether isoprene or NO emissions drove this isoprene anomaly.