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 moni toring the yield of OH radicals as the wavelength of an excitation las er 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 lif etime of HOBr is large due to its proximity to the peak of the solar a ctinic flux. Preliminary estimates suggest that inclusion of absorptio n by this new band system will shorten the photochemical lifetime of t ropospheric HOBr in the polar regions by a factor of 2 compared to the recently recommended value based on the near-UV absorption bands alon e.