A Correlated‐K Parameterization for O2 Photolysis in the Schumann‐Runge Bands.

A recent study comparing ozone column depths and methane lifetimes at varied atmospheric O2 (pO2) levels calculated in the Kasting‐group 1‐D photochemical model and the Whole Atmosphere Community Climate Model version 6 (WACCM6) 3‐D model (Ji, Kasting, et al., 2023; https://doi.org/10.1098/rsos.230056) has exposed weaknesses in both models in parameterizing photolysis in the O2 Schumann‐Runge bands, 175–205 nm. WACCM6 does a good job for Earth's present atmosphere but neglects scattering, which becomes important at low pO2. The 1‐D model includes scattering but is based on an out‐of‐date band model, and it neglects the temperature dependence of photolysis at low pO2. We have revised and improved the 1‐D photochemical model by replacing the old O2 photolysis algorithm with a new correlated‐k parameterization, which improves accuracy for all O2 levels and all temperature profiles. The WACCM6 parameterization was also included in the 1‐D model for comparative purposes. The correlated‐k and WACCM6 photolysis algorithms agree well for both the present atmosphere and for an atmosphere containing 10−3 times the present O2 level, but only if multiple scattering is included at low pO2. The correlated‐k parameterization will be made available to photochemical modeling groups who might choose to adopt it. Plain Language Summary: In today's atmosphere, the absorption of incoming ultraviolet radiation by O2 plays a decisive role in creating O atoms that can react to form ozone (O3), as well as shielding other gases from photolysis. However, this absorption is difficult to parameterize in atmospheric models due to its complex structure at wavelengths of 175–205 nm, which we call the Schumann‐Runge bands. Good models for SR absorption in today's atmosphere already exist, but not all of them may be suitable for use in low‐O2 atmospheres in which multiple scattering is important. Here, we develop a new parameterization for SR absorption by employing a "correlated‐k" approach that has been widely used in climate models at both visible and infrared wavelengths. This approach accounts for the strong temperature dependence of the absorption and agrees well with existing models of O2 absorption in the present atmosphere. It should also be useful in simulations of low‐O2 atmospheres on early Earth, for which scattering must be included, as well as for atmospheres of putative Earth‐like exoplanets that astronomers hope to identify over the next several decades. Key Points: To parameterize O2 absorption, we generated a correlated‐k table based on detailed line‐by‐line calculations at 150, 200, 250, and 300 KThe new parameterization matches laboratory measurements and is well suited to include scattering at lower O2 levelsEarly earth photochemical models based on the old exponential sum fits should consider the new correlated‐k to achieve more accurate results [ABSTRACT FROM AUTHOR]