Climate and Tropospheric Oxidizing Capacity

Abstract

The hydroxyl radical (OH) largely controls the tropospheric self-cleansing capacity by reacting with gases harmful to the environment and human health. OH concentrations are determined locally by competing production and loss processes. Lacking strong observational constraints, models differ in how they balance these processes, such that the sign of past and future OH changes is uncertain. In a warmer climate, OH production will increase due to its water vapor dependence, partially offset by faster OH-methane loss. Weather-sensitive emissions will also likely increase, although their net impact on global mean OH depends on the balance between source (nitrogen oxides) and sink (reactive carbon) gases. Lightning activity increases OH, but its response to climate warming is of uncertain sign. To enable confident projections of OH, we recommend efforts to reduce uncertainties in kinetic reactions, in measured and modeled OH, in proxies for past OH concentrations, and in source and sink gas emissions. ▪ OH is strongly modulated by internal climate variability despite its lifetime of a few seconds at most, with implications for interpreting trends in methane. ▪ Improved kinetic constraints on key reactions would strengthen confidence in regional and global OH budgets, and in the response of OH to climate change. ▪ Future OH changes will depend on uncertain and compensating processes involving weather-sensitive chemistry and emissions, plus human choices. ▪ Technological solutions to climate change will likely impact tropospheric oxidizing capacity and merit further study prior to implementation. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 52 is May 2024. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Enhancing Long-Term Trend Simulation of Global Tropospheric OH and Its Drivers from 2005–2019: A Synergistic Integration of Model Simulations and Satellite Observations

“…The hydroxyl radical (OH) regulates the lifetimes of a vast number of key atmospheric compounds, such as sulfur dioxide (SO2), nitrogen dioxide (NO2), volatile organic compounds (VOCs), carbon monoxide (CO), and methane (CH4). Despite its outsized importance for atmospheric chemistry and climate, our knowledge on both the abundance and long-term trends of OH is limited due to its sparse observations, manifesting in large discrepancies between simulated OH among global models (e.g., Naik et al, 2013;Zhao et al, 2019;Murray et al, 2021;Fiore et al, 2024). Particularly, these discrepancies can introduce large uncertainties when it comes to precisely representing methane (Holmes et al, 2013;Nguyen et al, 2020), a potent greenhouse gas.…”