Recommendations for a robust observing strategy to indirectly constrain the distribution and spatio-temporal variability of the tropospheric hydroxyl radical

The hydroxyl radical (OH) plays a central role in tropospheric chemistry including by regulating the lifetime of some potent greenhouse gases, such as methane. Because of limitations in our ability to observe OH, we have historically relied on indirect methods to constrain its trends and variability, but only at the global or semi-hemispheric scale. Recent methods demonstrated the feasibility of indirectly constraining OH at finer spatio-temporal resolutions, using satellite observations of photochemical drivers and proxies of OH (e.g., nitrogen dioxide, formaldehyde, isoprene, water vapor, ozone). These novel tools show promise and 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. An additional 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 the drivers of OH. Therefore, we propose steps forward for the development of a strategic and comprehensive space-based and suborbital observing strategy of the drivers and proxies of OH, which will improve our ability to 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, new observations of volatile organic compounds). Suborbital observations, which are required for information difficult to obtain from space (e.g., fine vertical resolution of the drivers and proxies of OH) and for validation of satellite-based OH estimates, will be an integral part of a comprehensive observing strategy.