Water vapor is an important constituent in the Earth’s atmosphere. It plays a key role in the global radiative budget and in energy transport mechanisms and is one of the main contributors to the greenhouse effect. Moreover, it affects cloud formation and microphysics and modifies aerosol particle properties through hygroscopic growth, thus indirectly contributing to changes in the atmospheric radiative forcing.
Raman lidars have emerged in the last decades as powerful tools for providing detailed water-vapor profiles with high vertical and temporal resolution. The water vapor Raman lidar technique consists in obtaining the range-resolved ratio of rotational-vibrational Raman scattering intensities from water-vapor and nitrogen molecules, which is proportional to the water-vapor mixing ratio and permits retrieving atmospheric relative humidity profiles if simultaneous temperature and pressure data are also available.
The UPC Raman lidar instrument is equipped with both nitrogen and water vapor Raman channels, which permit measuring water vapor mixing ratio profiles. The receiving wavelengths are the ones corresponding to the rotational-vibrational Raman scattering of nitrogen (387 nm) and water vapor (407 nm), when the sounding volume is illuminated with 355 nm radiation. The UPC lidar group has developed calibration methods and retrieval algorithms for reliable and accurate water vapor profiling and is currently performing regular nighttime humidity measurements for aerosols hygroscopicity analysis and characterization in the Barcelona region.
Fig. 1. UPC Raman lidar optical head and wavelength-selective receiving set-up, with nitrogen (387 nm) and water vapor (407 nm) channel detectors.
Fig. 2. Example of lidar water vapor mixing ratio retrieval and comparison with reference radiosonde measurement.