publications

2026

1. 

   Inferring drivers of tropical isoprene: competing effects of emissions and chemistry

   James Young Suk Yoon, Kelley C. Wells, Dylan B. Millet, and 5 more authors

   Atmospheric Chemistry and Physics, Apr 2026

   Abs DOI

   Isoprene is the most significant non-methane hydrocarbon by total emissions and an important control on the tropospheric oxidative capacity. In the atmosphere, isoprene is oxidized by the hydroxyl radical (OH) on the order of hours depending on local OH concentrations. Using isoprene retrievals from the Cross-track infrared sounder (CrIS), we monitor global isoprene column variability and observe differing isoprene column responses to El Niño-Southern Oscillation across three tropical regions: Amazonia, the Maritime Continent, and equatorial Africa. We find correlations between isoprene column variability and temperature over Amazonia, which suggests that isoprene emissions drive Amazonian isoprene variability (“emissions-controlled”). In the Maritime Continent, we find strong correlations between isoprene columns, precipitation and soil moisture, as well as an anti-correlation between isoprene and formaldehyde retrievals. These correlations suggest that isoprene columns may be modulated by non-anthropogenic NOx emissions, namely soil and biomass burning NOx (“chemistry-controlled”), although convection and lightning NOx may also modulate isoprene column retrievals if the lofted isoprene flux is large enough. In equatorial Africa, both biomass burning and temperature can explain isoprene variability during different periods, representing an intermediate regime with contributions from emissions and chemistry. We suggest that these isoprene regimes are caused by differences in the dynamic temperature and oxidant range between the three regions, and we specifically highlight oil palm plantations in the Maritime Continent as an area of co-located isoprene and soil NOx fluxes. By leveraging CrIS isoprene retrievals, we can study interactions between VOC and NOx sources over tropical areas with few in-situ observations.

2. 

   Airborne Observations Reveal Underestimated Riverine Methane Emissions Across the Amazon

   Linda Ort, Christian Frankenberg, Nikhil Dadheech, and 9 more authors

   Geophysical Research Letters, 2026

   Abs DOI

   Strongly increasing trends in global methane in recent decades highlight the importance of understanding source-sink attributions and climate feedback mechanisms, especially for large natural sources like wetlands. Here, we present an analysis of airborne methane observations above the Amazon Basin conducted from December 2022 through January 2023 that reveal enhanced methane concentrations, especially below 6 km. Using Bayesian inversion with the atmospheric transport model STILT, we optimize surface fluxes to match our observations, starting from prior bottom-up flux estimates by WetCHARTs. The posterior suggests underestimated emissions across the Amazon Basin. The largest underestimates occur along rivers and tributaries, where emissions exceeded model values by up to a factor of four. River deltas make up 26% of the underestimated fluxes with reservoirs, and regularly flooded riverine areas contribute 19% and 13%, respectively. This highlights the need for improved understanding of the continuum across wetlands and freshwaters for tropical methane emissions.

2025

1. 

   Impacts of Interannual Isoprene Variations on Methane Lifetimes and Trends

   James (Young Suk) Yoon, Kelley C. Wells, Dylan B. Millet, and 3 more authors

   Geophysical Research Letters, 2025

   Abs DOI

   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 (NOx) emissions. NOx 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 NOx emissions in 2020. Since COVID-19 decreased anthropogenic NOx emissions, this estimate is an upper-limit and may depend on whether isoprene or NO emissions drove this isoprene anomaly.

2024

1. 

   Sensitive Response of Atmospheric Oxidative Capacity to the Uncertainty in the Emissions of Nitric Oxide (NO) From Soils in Amazonia

   Ben H. Lee, J. William Munger, Steven C. Wofsy, and 5 more authors

   Geophys. Res. Lett, 2024

   Abs DOI

   Soils are a major source of nitrogen oxides, which in the atmosphere help govern its oxidative capacity. Thus the response of soil nitric oxide (NO) emissions to forcings such as warming or forest loss has a meaningful impact on global atmospheric chemistry. We find that the soil emission rate of NO in Amazonia from a common inventory is biased low by at least an order of magnitude in comparison to tower-based observations. Accounting for this regional bias decreases the modeled global methane lifetime by 1.4%–2.6%. In comparison, a fully deforested Amazonia, representing a 37% decrease in global emissions of isoprene, decreases methane lifetime by at most 4.6%, highlighting the sensitive response of oxidation rates to changes in emissions of NO compared to those of terpenes. Our results demonstrate that improving our understanding of soil NO emissions will yield a more accurate representation of atmospheric oxidative capacity.