Dynamic Performance Evaluation of Optical and Capacitive Sensors for Atmospheric Water Vapor Monitoring

Atmospheric water vapor (H2O) is the most significant greenhouse gas; accurately quantifying it is a critical yet challenging task due to its high spatial and temporal variability. This study presents a comparative analysis of the LI-COR LI-7825 (LI-COR Biosciences, Lincoln, Nebraska), which utilizes OF-CEAS technology, and the Vaisala HMP7 (Vaisala Oyj, Vantaa, Finland) capacitive probe, which employs HUMICAP ® technology. Before testing the sensors under dynamic conditions, we performed a two-point calibration on the Vaisala HMP7. We then proceeded to evaluate the two technologies using a custom-made setup to put them in series and minimize atmospheric contamination. The results demonstrate a high statistical correlation (R2=0.995), though the Vaisala sensor exhibited systematic offsets at high-humidity levels. To better compare datasets, a low-pass digital filter was used to account for the different T90 values of the instruments. The results show that while spectroscopy provides superior precision and response speed, the environmental resilience of capacitive sensors makes them better suited for field work. The implementation of robust synchronization protocols is essential to develop effective hybrid monitoring networks that balance precision with scalability.