An airborne diode laser spectrometer for the simultaneous measurement of H2O and HNO3 content of stratospheric cirrus clouds

We describe a tunable diode laser spectrometer for the in situ simultaneous detection of the nitric acid and the water vapour by means of high resolution absorption spectroscopy on rovibronic lines, using a single laser emitting in the mid-infrared (5.8?m) and a multipass cell. The instrument was designed to be installed on a high altitude aircraft, as a part of a composite payload for atmospheric aerosol studies. The instrument design criteria are discussed and the expected performances are compared with results of laboratory and field operation. Due to the high chemical activity of HNO3, that makes difficult to use a reference cell, a fast sweep detection of direct absorption was used for the measurement. An absorption sensitivity of about 2 × 10-4 was achieved with an integration time of 2.5s, corresponding to a concentration of about 4ppb of HNO3 in the cell and 0.1ppb in the external aerosol. The data acquisition and processing techniques, based on the full molecular lineshape fitting are also shown and discussed, including some examples of the data acquired during the scientific flights.