A horizontal mobile measuring system for atmospheric quantities

Abstract. A fully automatic horizontal mobile measuring system (HMMS) for atmospheric quantities has been developed. The HMMS is based on the drive mechanism of a garden railway system and can be installed at any location and along any measuring track. In addition to meteorological quantities (temperature, humidity and short-/long-wave down/upwelling radiation), HMMS also measures trace gas concentrations (carbon dioxide and ozone). While sufficient spatial resolution is a problem even for measurements on distributed towers, this could be easily achieved with the HMMS, which has been specifically developed to obtain higher information density about horizontal gradients in a heterogeneous forest ecosystem. There, horizontal gradients of meteorological quantities and trace gases could be immense, particularly at the transition from a dense forest to an open clearing, with large impact on meteorological parameters and exchange processes. Consequently, HMMS was firstly applied during the EGER IOP3 project (ExchanGE processes in mountainous Regions – Intense Observation Period 3) in the Fichtelgebirge Mountains (SE Germany) during summer 2011. At a constant 1 m above ground, the measuring track of the HMMS consisted of a straight line perpendicular to the forest edge, starting in the dense spruce forest and leading 75 m into an open clearing. Tags with bar codes, mounted every metre on the wooden substructure, allowed (a) keeping the speed of the HMMS constant (approx. 0.5 m s⁻¹) and (b) operation of the HMMS in a continuous back and forth running mode. During EGER IOP3, HMMS was operational for almost 250 h. Results show that – due to considerably long response times (between 4 and 20 s) of commercial temperature, humidity and the radiation sensors – true spatial variations of the meteorological quantities could not be adequately captured (mainly at the forest edge). Corresponding dynamical (spatial) errors of the measurement values were corrected on the basis of well-defined individual response times of the sensors and application of a linear correction algorithm. Due to the very short response times (≤ 1 s) of the applied commercial CO₂ and O₃ analysers, dynamical errors for the trace gas data were negligible and no corrections were done.