An observation-based, reduced-form model for oxidation in the remote marine troposphere

The hydroxyl radical (OH) fuels atmospheric chemical cycling as the main sink for methane and a driver of the formation and loss of many air pollutants, but direct OH observations are sparse. We develop and evaluate an observation-based proxy for short-term, spatial variations in OH (Proxy) in the remote marine troposphere using comprehensive measurements from the NASA Atmospheric Tomography (ATom) airborne campaign. Proxy is a reduced form of the OH steady-state equation representing the dominant OH production and loss pathways in the remote marine troposphere, according to box model simulations of OH constrained with ATom observations. Proxy comprises only eight variables that are generally observed by routine ground- or satellite-based instruments. Proxy scales linearly with in situ [OH] spatial variations along the ATom flight tracks (median r² = 0.90, interquartile range = 0.80 to 0.94 across 2-km altitude by 20° latitudinal regions). We deconstruct spatial variations in Proxy as a first-order approximation of the sensitivity of OH variations to individual terms. Two terms modulate within-region Proxy variations-water vapor (H₂O) and, to a lesser extent, nitric oxide (NO). This implies that a limited set of observations could offer an avenue for observation-based mapping of OH spatial variations over much of the remote marine troposphere. Both H₂O and NO are expected to change with climate, while NO also varies strongly with human activities. We also illustrate the utility of Proxy as a process-based approach for evaluating intermodel differences in remote marine tropospheric OH.