Measuring water vapour in the UTLS region has been challenging for decades. The method presented here provides a breakthrough for future high precision measurements at the low concentrations and under the conditions found in the UTLS region. Such measurements are essential given the critical role of water vapour in this region for the Earth's radiative balance.

From the handling editor : I think this paper is of particular interest to the geoscience community (following peer review) though likely too arcane for broader public/media. There's been a lot of discussion of the potential for preferentially oriented dust particles and this has impacts for remote sensing and modeling. So this seems to be an important step towards real characterisation of them. I agree with the authors' provided comment here so reproduce it below: Atmospheric dust particles with irregular shapes may present a preferential orientation in the Earth's atmosphere. This alignment can be detected from the resulting dichroic extinction of the transmitted sunlight through the elevated dust layers. Since mineral dust is one of the most abundant atmospheric aerosols and plays a significant role to the radiative forcing of the global climate, the somewhat elusive observation of particle orientation will be a game changer to existing remote sensing measurement techniques and the implementation of particle dynamics in desert dust transport models. With our research, we utilize a specifically designed direct sun Solar Polarimeter (SolPol), installed and operated on a remote observatory in Greece, and report on unique observations of enhanced linear polarization during dust events. We aim to interpret our measurements as the first observations of atmospheric dust particle orientation by measuring direct sunlight.