Water vapor is a strong greenhouse gas, and accurate measurements of its concentration in the upper atmosphere (~8–25 km altitude) are crucial for reliable climate predictions. We investigated the performance of four airborne hygrometers, deployed on aircraft or stratospheric balloon platforms and based on different techniques, in a climate simulation chamber. The results demonstrate the high accuracy and reliability of the involved sensors for atmospheric monitoring and research applications.

We present, to our knowledge, the first relaxed eddy accumulation system explicitly tailored to a radiocarbon (¹⁴C)-based partitioning of fossil and non-fossil urban CO₂ fluxes. Laboratory tests and in-depth quality and performance checks prove that the system meets the technical requirements. A pilot application on a tall tower in the city of Zurich, Switzerland, demonstrates the ability to separate fossil and non-fossil CO₂ components within the typical precision of ¹⁴C measurements.