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_OH) in the remote marine troposphere using comprehensive measurements from the NASA Atmospheric Tomography (ATom) airborne campaign. Proxy_OH 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_OH comprises only eight variables that are generally observed by routine ground- or satellite-based instruments. Proxy_OH 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_OH as a first-order approximation of the sensitivity of OH variations to individual terms. Two terms modulate within-region Proxy_OH 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_OH as a process-based approach for evaluating intermodel differences in remote marine tropospheric OH.