Photolysis rate coefficients in the upper atmosphere: Effect of line by line calculations of the O 2 absorption cross section in the Schumann–Runge bands

Abstract

A line by line (LBL) method to calculate highly resolved O 2 absorption cross sections in the Schumann–Runge (SR) bands region was developed and integrated in the widely used Tropospheric Ultraviolet Visible (TUV) model to calculate accurate photolysis rate coefficients ( J values) in the upper atmosphere at both small and large solar zenith angles (SZA). In order to obtain the O 2 cross section between 49,000 and 57,000 cm −1, an algorithm which considers the position, strength, and half width of each spectral line was used. Every transition was calculated by using the HIgh-resolution TRANsmission molecular absorption database (HITRAN) and a Voigt profile. The temperature dependence of both the strength and the half widths was considered within the range of temperatures characteristic of the US standard atmosphere, although the results show a very good agreement also at 79 K. The cross section calculation was carried out on a 0.5 cm −1 grid and the contributions from all the lines lying at ±500 cm −1 were considered for every wavelength. Both the SR and the Herzberg continuums were included. By coupling the LBL method to the TUV model, full radiative transfer calculations that compute J values including Rayleigh scattering at high altitudes and large SZA can now be done. Thus, the J values calculations were performed for altitudes from 0 to 120 km and for SZA up to 89°. The results show, in the J O 2 case, differences of more than ±10% (e.g. at 96 km and 30°) when compared against the last version of the TUV model (4.4), which uses the Koppers and Murtagh parameterization for the O 2 cross section. Consequently, the J values of species with cross sections overlapping the SR band region show variable differences at lower altitudes. Although many species have been analyzed, the results for only four of them (O 2, N 2O, HNO 3, CFC12) are presented. Due to the fact that the HNO 3 absorption cross section extends up to 350 nm this molecule was used to verify the consistency of the new TUV–LBL at lower altitudes. Thus, it shows differences up to 5.7% at 21 km but 0% in the troposphere. Because of the more accurate consideration of the Rayleigh scattering the distribution of the actinic flux in its direct and diffuse components (in the SR bands wavelength interval) is also modified.