CHARACTERISATION OF THE INRIM LOW-FROST POINT GENERATOR OPERATING AT SUB-ATMOSPHERIC PRESSURE

In the framework of the EMRP project MeteoMet-2, to develop a metrological traceability for the measurement of several Essential Climate Variables (ECVS) identified by the Global Climate Observing System (GCOS), a new facility for the calibration of upper-air humidity sensors has been designed and constructed at INRIM. Water vapour is one of the ECVS; it represents a key greenhouse gas (GHG) in the atmosphere and it is also responsible for the global water and energy cycles on Earth. Accurate measurements of the upper air humidity and its profile in the atmosphere are essential in many areas of the atmospheric and climatological research. Through the years, several methods and instruments have been developed for the measurement of the humidity in atmosphere, such as radiosondes, airborne and balloon-borne chilled mirror hygrometer (CMH) and tunable diode laser absorption spectrometers (TDLAS). Considering that rising from the troposphere to the stratosphere, the water vapour amount may vary between some percent to few part per million, in order to obtain accurate and reliable humidity measurements minimizing at the same time any unwanted measurement bias or systematic error, a SI-traceable calibration, preferably carried out in conditions similar to those expected in the field, is essential. To satisfy such a need, a new calibration facility has been developed at INRIM based on a thermodynamic frost-point generator. The generator has been designed to achieve a complete saturation of the carrier gas in a single passage through an isothermal saturator. The humidity generator covers the frost point temperature range between -98 °C and -20 °C and is able to work at any controlled pressure between 200 hPa and 1100 hPa (corresponding to a barometric altitude range between the ground level and approximately 12000 m). The generated humid flow of nitrogen contains a water vapour amount fraction between 14∙10-9 mol/mol and 5∙10-3 mol/mol. A testing chamber expressly designed for reference radiosondes calibration (e.g. Vaisala R-10) has been realised in collaboration with the VTT MIKES; its operation will be reported elsewhere. In this paper the frost-point generator performance and the uncertainty evaluation of the generated frost-point temperature and water vapour amount fraction will be discussed. Preliminary experiments on radiosonde calibration will be presented, as well. The present work has been carried out within the European Joint Research Project “MeteoMet 2 - Metrology for Essential Climate Variables” co-funded by the European Metrology Research Programme (EMRP). The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union.