Broadband absolute absorption measurements of atmospheric continua with millimeter wave cavity ringdown spectroscopy

An instrument for the measurement of continua effects in the millimeter and submillimeter (a.k.a. terahertz) spectral region for mixtures of gases is described. In addition to their scientific interest, these continua are important because they contribute more than 50% of the opacity in the atmospheric windows important in remote sensing and communications. The instrument is based upon a fast scanning cavity ringdown approach that makes possible the measurement of continua and broad line absorptions at ∼6000 distinct cavity frequencies from 170to260GHz in ∼3s. The instrument’s 10m Fabry–Perot cavity (Q∼5×106) is in a chamber whose temperature can be controlled between −80 and +100°C and whose pressure can be varied between ∼10−5Torr and 10atm. The 3s scan results in statistical uncertainty in the absorptions of ∼2×10−7cm−1 at each of the 6000 frequencies. From these data an uncertainty of less than 1% for the weakest atmospheric continua (the dry air continuum) can be determined. For comparison, the conti...