REA-GC-PID for long-term isoprene flux measurements in tropical forest

Biosphere-Atmosphere interaction studies in tropical forests are extremely important to understand global climate change. The motivation of this study is to introduce a robust observational system which will be used to investigate long-term biosphere-atmosphere interactions to advance our understanding of reactive gases in the atmosphere and improve the predictability of air pollution and climate models. The system is a Relaxed Eddy Accumulator coupled to a Gas Chromatograph with Photoionization Detector module (REA-GC-PID) and was developed to measure the canopy-scale fluxes of isoprene to the atmosphere. The Relaxed Eddy Accumulation technique (REA) provides an opportunity to make a network to investigate the large variability of isoprene fluxes above ecosystem, since the Eddy Covariance (EC) technique has disadvantages such as high financial costs, high power consumption and low portability. This low-cost instrument features a direct (on-line) connection to a gas analyzer (GC-PID) collecting air from updrafts and downdrafts in separate sampling bags. After collecting air for a predefined period of time, typically 30 minutes, the isoprene concentrations in the bags are analyzed by the GC-PID (Bolas et al., 2020) and the average flux is calculated. We have developed and tested an initial system targeting isoprene. The REA approach has been used for a wide range of other atmospheric constituents including other volatile organic compounds (VOCs), trace gases, aerosols and isotopes. In addition, the modular design and the external casing make for a portable and robust system. The system can operate either from 12V DC or 110V/220V AC and is therefore suitable for a wide range of field sites. The meteorological data is generated every 0.1 second by a anemometer, the b-coefficients and heat fluxes are calculated by the controller, and the isoprene fluxes are calculated every 30 to 60 mins. The data is saved onto an SD card and can also be accessed remotely. We evaluate the performance of our REA-GC-PID system by comparing it to a PTR-MS-based Eddy Covariance System.