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

Abstract

In this study, we update and integrate a line-by-line (LBL) module for calculating the O2 cross section (σO2) 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 σO2 calculations on the photolysis rate coefficient of O2 (JO2) and over 20 other molecules (J-values) at different altitudes. To compute σO2 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 JO2 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.