Observation of a new absorption band of HOBr and its atmospheric implications

A new absorption band of HOBr centered near 440 nm is detected by monitoring the yield of OH radicals as the wavelength of an excitation laser is scanned over the region from 440 to 650 nm. The band is believed to arise from excitation to a triplet state of HOBr, and although its peak absorption cross section is fairly modest, sigma(max) similar to 9 x 10(-21) cm(2), its influence on determining the photochemical lifetime of HOBr is large due to its proximity to the peak of the solar actinic flux. Preliminary estimates suggest that inclusion of absorption by this new band system will shorten the photochemical lifetime of tropospheric HOBr in the polar regions by a factor of 2 compared to the recently recommended value based on the near-UV absorption bands alone.