This study assesses the effect of the differential atmospheric transmission term in Raman lidar water vapour mixing ratio retrievals. Such issue is evaluated for a vibrational--rotational Raman nitrogen (387 nm) configuration and for a pure--rotational Raman molecular reference near 354 nm (nitrogen and oxygen). Both optical configurations use a vibrational--rotational water vapour channel at 408 nm. More than 300 aerosol profiles acquired by the University of Granada Raman lidar over the period 2010--2016 enabled the calculation of the aerosol contribution of the differential atmospheric transmission term, indicating that neglecting the total differential atmospheric transmission term can introduce systematic uncertainties in water vapour mixing ratio retrievals of approximately 5. 1\% and 15\% (18\% under high aerosol conditions) at 6 km for the first and second configuration respectively. Subsequently, in order to apply automatic differential transmission calculations, we developed a technique for estimating the aerosol contribution from sun photometer AOD measurements, yielding relative deviations in water vapour mixing ratio of 0. 10\% and 0. 40\% for 387 nm and 354 nm configurations when compared with cases where Raman lidar aerosol profiles were available. This approach transforms systematic uncertainties into random ones that can be reduced by increasing the number of measurements
Díaz-Zurita et al. (Fri,) studied this question.