Exploring Explainable Range of In-situ Portable CO2 Sensor Signatures for Carbon Stock Estimated in Forestry Carbon Project

Credible information regarding carbon stock is the fundamental underlying basis for forestry carbon trading. The current measurement, reporting, and verification (MRV) system uses various emission/absorption factors derived from biomass and land use/cover. However, this MRV system does not take into consideration the actual reduction in atmospheric CO2 concentration induced by the CO2 uptake of the above-ground biomass, which is closely related to the effects of on-site topographical factors on the capability of CO2 uptake of the above-ground biomass. This raises questions about the reliability of the actual atmospheric CO2 reduction of carbon stock presented in a project design document (PDD). The explainable range of 'ambient' CO2 concentrations measured using nondispersive infrared (NDIR) sensors from the ground level was evaluated to explore how the amount of carbon stock presented in the PDD reflects the variation in ground CO2 density in terms of the topographical above-ground biomass. Ground CO2 was measured using NDIR portable sensors at 182 points (August–September 2018) according to the World Data Centre for Greenhouse Gases (WDCGG) method. NDIR sensor signatures provide tangible quantitative values (correlation coefficient, R2 = 0.28) for differentiating the interactive relationships between the carbon stock presented in the PDD as a dependent variable and a set of independent variables (topographical above-ground biomass). It is shown that the sensor signal is not a measure of the amount of carbon accumulated in the above-ground biomass itself but is seriously affected by the surrounding topographical terrain parameters (low solar radiation, solar duration, slope, and elevation). The results of this study provide a valuable reference for verifying the measurable range of carbon concentrations in the atmosphere, which fluctuate according to the carbon absorption capability of the above-ground biomass in forestry carbon project sites.