Impact of the overlapping O2 Schumann-Runge and Herzberg continua with Schumann-Runge bands on photolysis rate coefficients: comparing the Koppers & Murtagh parameterization with line-by-line cross section

In this study, we update and integrate a line-by-line (LBL) module for calculating the O₂ cross section (σ_O2) within the Schumann-Runge Bands (SRB, 175.4-206.2 nm) into the Tropospheric Ultraviolet Visible (TUV) model V5.4. This coupling allows us to analyze the impact of precise σ_O2 calculations on the photolysis rate coefficient of O₂ (J_O2) and over 20 other molecules (J-values) at different altitudes. To compute σ_O2 within the SRB, the LBL module uses the HIgh-resolution TRANsmission molecular absorption (HITRAN) 2020 database and the local O₂ absorption, while the standard TUV model uses the Koppers and Murtagh (K&M) parameterization, which considers the overhead O₂ slant path. In addition to correcting previously unnoticed errors in HITRAN and K&M datasets, the updated model now allows a separate handling of the underlying Herzberg (HZC) and Schumann-Runge (SRC) continuum in the wavelength region where they overlap with SRB (175.4-180 nm and 190-206.2 nm). Through TUV calculations, we found that J_O2 differences between the standard K&M and LBL versions are up to 6% in the upper mesosphere and –2 % in the middle-upper stratosphere. These variations affect the tropospheric and/or stratospheric J-values of several long-lived and short-lived halogenated species differently, depending on their individual cross sections and the extent of overlap with SRB. Despite variability, persistent altitude-dependent patterns are identified and explained by absorption contributions within the underlying SRC and HZC ranges. In particular, the LBL J-values of HCFC-22, CFC-115 and CHCl₃ are 12%, 8% and 2% lower in the middle-upper stratosphere, respectively, and up to 24% larger in the lower stratosphere, which is of relevance for the evolution of the ozone layer.