Measurements of weak absorptions by O ₃ and O ₃-H ₂O clusters using cavity enhanced spectroscopy

Weak spectral absorptions play an important role in the radiative transfer of the Earth's atmosphere and are necessary for accurate satellite retrievals; additionally, weak electronic features in the near-UV are important for tropospheric radical production. This presentation focuses on ozone's near-UV spectrum, which is poorly characterized, with reported discrepancies up to one order of magnitude in the weakly absorbing spectral region between 350-450 nm. Theoretical calculations predict that O ₃ will form O3-H ₂O clusters that absorb in this region and may act as an atmospheric source of hydroxyl radicals. In this investigation we designed, constructed, and characterized a two channel incoherent broad-band cavity-enhanced absorption spectrometer (IBBCEAS) to provide accurate absolute absorption cross-sections for O ₃ in the 350-450nm region. The instrument has an effective path length of several kilometers allowing for high sensitivity measurements of absorption cross sections in the near-UV, even for dilute trace gas levels. We have investigated the absolute absorption cross-section for ozone and the ozone-water complex in the region of 350 to 450 nm as a function of relative humidity. Results of this study will be discussed in light of their importance in atmospheric chemistry and atmospheric trace gas measurments.