Impacts of interannual isoprene variations on methane lifetimes and trends

Abstract

Methane is a potent greenhouse gas that has an atmospheric lifetime of 9-12 years due to its reaction with the hydroxyl radical (OH). Recent trends in methane indicate anomalously high methane growth in 2020, which has been attributed to a combination of increased wetland emissions and a decrease in OH from decreased anthropogenic nitrogen oxide (NOx) emis-sions during the COVID-19 lockdowns. However, NOx is not the only atmospheric species that affects global OH concentrations—isoprene, the most significant non-methane hydrocarbon by total emissions, is oxidized by OH, which can deplete OH during periods of high isoprene emissions. Using satellite isoprene retrievals from the Cross-track infrared sounder (CrIS) instrument, we find interannual variability in isoprene columns and anomalously high isoprene columns during 2020, coincident with anomalously high methane growth. Although isoprene is concentrated in source regions such as the Amazon, its atmospheric oxidation produces carbon monoxide, which can be transported over longer distances and thus impact the global atmospheric oxidative capacity. Elevated isoprene concentrations may have contributed 13% (bounds: 10% - 28%) of the total methane growth in 2020 if we assume no change in NOx emissions between 2019 and 2020. Given that COVID-19 lockdowns also decreased anthropogenic NOx emissions during this same period, this estimate of isoprene emissions on the methane lifetime during 2020 is an upper-limit, and the resulting change in the oxidative capacity may be sensitive to whether isoprene emissions or NO emissions drove this isoprene anomaly. This study illustrates how the biosphere may impact the atmospheric oxidative capacity and, in turn, the methane lifetime through interannual changes in biogenic emissions that are forced by meteorological and climate variability.