The variation of pressure, temperature and transmission within a pressure modulator: Measurements with a high-compression modulator

Analysis of data from atmospheric measurements with pressure modulators has relied on computer models of their pressure and transmission cycles, some measurements of the pressure cycles with low-compression (SAMS) modulators, and laboratory measurements of the transmission of the absorbing gas. The measured and calculated transmissions always agreed within 20% or better, but the laboratory measurements did not always cover the full range of stratospheric temperatures. During the Balloon Intercomparison Campaigns (BIC) of 1982 and 1983, differences between simultaneous measurements of NO₂ by the Balloon-borne Pressure Modulator Radiometer (BPMR) and some other remote sensors exceeded a factor three. To try to resolve these differences in NO₂ results and in the search for better accuracy from pressure modulators containing other gases, the cycles of pressure, temperature and transmission have now been measured with the high-compression balloon-borne modulator. Pressure cycles were observed with a variety of gases; temperature and transmission cycles were observed with NO₂ in the modulator. The maximum amplitude of the temperature cycle was ±10 K. At the filling pressures used in the balloon flights (about 4 mbar), there was excellent agreement between these measurements and the calculations of modulator behaviour used in the analysis of previous data. The large differences of BIC cannot be attributed to uncertainties in the pressure and temperature cycles within the modulator. Although the evidence is ambiguous, assumptions used in representations of thermal relaxation in earlier calculations may not be valid. If so, these earlier calculations would overestimate the temperature amplitude at higher pressures (e.g. in some modulators in the Improved Stratospheric And Mesospheric Sounder, ISAMS), and this representation should not be used to estimate their temperature cycles. Alternatively, if the earlier assumptions are correct, some ISAMS modulators may have temperature amplitudes which would significantly affect spectral parameters of absorption lines within them. The validity of the assumptions cannot be established without detailed comparisons of these measurements with new calculations. A computer model of the behaviour of the modulator when empty has been constructed. Its predictions agree well with the measurements. Previous models of modulator behaviour were numerically unstable with an empty system. By combining the features of this model with those of previous ones, a new model can be constructed with a minimum of empirical approximations. Such a model is planned in the near future. © 1989.