Refinement and Application of the Absorption Cross-Section of Molecular Bromine with Relevance to Atmospheric Chemistry.

Bromine (Br2) plays a crucial role in atmospheric oxidative chemistry, particularly in polar, marine, and stratospheric environments, where it significantly impacts ozone depletion and mercury oxidation. In this study, high-resolution measurements of the Br2 absorption cross section were evaluated across 325-850 nm (11765 to 30,769 cm-1), with emphasis on features due to the B3Πou ← X1Σg + transition at 530 nm (18,867 cm-1). UV-vis spectra revealed well-resolved vibronic features, enabling the accurate assignment of absorption peaks. Simulated rovibronic spectra reproduced the measured features and confirmed spectral assignments. The data set produced in this work shows improved signal-to-noise and peak distinction compared to prior studies and reveals differences of up to 25% relative to literature values, and to 6.7% relative to the JPL evaluation values, in structured regions of the spectrum, which directly alters calculated Br2 photolysis rates used in atmospheric chemistry models. These refined cross sections serve as robust reference data for laboratory and atmospheric research, supporting improved photochemical modeling of reactive halogen species. A broadband cavity-enhanced absorption spectroscopy (BBCEAS) instrument was employed to test the application of the evaluated absorption cross-section in a field-ready instrument and setting. The BBCEAS method achieved a detection limit of 97 ppt and an effective optical path length of 22.6 km, facilitating sensitive detection suitable for field applications.