Opinion: Beyond Global Means: Novel Space-Based Approaches to Indirectly Constrain the Concentrations, Trends, and Variations of Tropospheric Hydroxyl Radical (OH)

The hydroxyl radical (OH) plays a central role in tropospheric chemistry as well as influencing the lifetimes of some climate gases, such as methane. Because of limitations in our ability to observe OH, we have historically relied on indirect methods to constrain its concentrations, trends, and variations, but only as annual global or semi-hemispheric averages. Recent methods demonstrated the feasibility of indirectly constraining tropospheric OH on finer spatio-temporal scales (e.g., seasonal, 1° latitude x 1° longitude), using satellite observations as proxies of the photochemical drivers of OH (e.g., nitrogen dioxide, formaldehyde, isoprene, water vapor, ozone). We found that there are currently reasonable satellite proxies to constrain about 75 % of the global source of tropospheric OH and about 50 % of the global sink. With additional research and investment in observing various volatile organic compounds, there is the potential to constrain an additional 10 % of the global source and 30 % of the global sink. In addition, these novel methods could be refined and made more robust by improvements in the capabilities of satellite instruments (e.g., signal-to-noise, spatial resolution) and retrieval algorithms that are used to develop data products. Another benefit of more robust data products is that they may be used to better constrain the chemical and dynamical processes in atmospheric chemical transport models that simulate the spatio-temporal variations of OH and OH drivers. Therefore, we propose steps forward for the development of a strategic and comprehensive space-based and suborbital observing strategy, which will improve our ability to indirectly constrain OH on much finer spatio-temporal scales than previously achieved. We discuss the strengths and limitations of such an observing strategy and potential improvements to current satellite instrument observing capabilities that would enable better constraint of OH. These improvements include ones that are obtainable with current technologies (e.g., more observations, co-located observations) as well as ones requiring additional technology development (e.g., to obtain vertically-resolved observations). Suborbital observations, which are required for information difficult to obtain from space and for validation of satellite-based OH estimates, will be an integral part of a comprehensive observing strategy.