Catching Pac-Man: New Observational and Modeling Constraints on the Oxidizing Capacity of the Troposphere

August 27, 2026

Alex Turner

Hosted by Dien Wu

Abstract

The hydroxyl radical (OH) is the primary oxidant in the troposphere. It governs the lifetime of methane, ozone depleting substances, and other pollutants. Despite this importance, OH remains one of the most poorly constrained quantities in atmospheric chemistry. Direct OH observations are sparse and imprecise, proxy species such as methyl chloroform are losing their utility to constrain OH, and jointly simulating OH chemistry and climate is computationally expensive. These limitations make it difficult to determine, for example, whether recent changes in methane's growth rate reflect changes in emissions, changes in the sink, or both. Here I will discuss work from the FETCH₄ project aiming to address this problem. FETCH₄ is an international collaboration making new isotopologue measurements from a global air sampling network and an upcoming Greenland ice core campaign in 2027. In tandem, the FETCH₄ project is developing modeling tools to interpret these new measurements. One such tool is AuroraChem, an atmospheric foundation model that jointly simulates both chemistry and climate, reproducing the spatial and temporal structure of OH and other short-lived species at a fraction of the cost of conventional chemistry-climate models. Preliminary data assimilation experiments using AuroraChem show how we can jointly constrain both chemistry and meteorology. This represents a promising pathway to help constrain the oxidizing capacity of the troposphere and resolve ambiguities in the global methane budget.

Alex is an Assistant Professor in the Department of Atmospheric and Climate Science at the University of Washington. His research combines satellite remote sensing, numerical modeling, and Bayesian inference to investigate interactions between the carbon cycle and atmospheric chemistry. Before joining UW, he was a Miller Postdoctoral Fellow at UC Berkeley. He completed his PhD in atmospheric chemistry at Harvard in 2017 and his BS in mechanical engineering at the University of Colorado in 2012. He received the AGU Holton Award in 2020 for early-career research excellence and leads the FETCH₄ project, an international collaboration across 28 institutions and 8 countries focused on improving our understanding of the past and modern methane cycle.