Evaluation of Five Gas Diffusion Models Used in the Gradient Method for Estimating CO2 Flux with Changing Soil Properties

The gradient method used to estimate soil CO2 flux is distinctive because it can provide additional information about CO2 production and consumption of soil profile. However, choosing an appropriate gas diffusion model with confidence with the gradient method is a big challenge. There is no universal optimal diffusion model but only the most suitable model in specific soils. This paper evaluates the applicability of five commonly used diffusion models in laboratory with changing soil properties and in a forest farm, respectively. When soil moisture, bulk density and fertility status were changed in the laboratory, the applicability of the five diffusion models was discussed. Moreover, this paper shows diurnal variation of soil CO2 flux estimated by the gradient method under four different climatic conditions in the forest farm, and the applicability of the five models was also analyzed. Both laboratory and forest experimental results confirm that the estimating accuracy of the Moldrup model is the highest, followed by the Millington–Quirk model, while those of the Penman, Marshall and Penman–Millington–Quirk models are poor. Furthermore, the results indicate that soil CO2 flux estimated by the gradient method is highly sensitive to the diffusion model and insensitive to the changes of soil properties. In general, the gradient method can be used as a practical, cost-effective tool to study soil respiration only when the appropriate diffusion model is first determined.

The Monitoring of CO2 Soil Degassing as Indicator of Increasing Volcanic Activity: The Paroxysmal Activity at Stromboli Volcano in 2019–2021

Since 2016, Stromboli volcano has shown an increase of both frequency and energy of the volcanic activity; two strong paroxysms occurred on 3 July and 28 August 2019. The paroxysms were followed by a series of major explosions, which culminated on January 2021 with magma overflows and lava flows along the Sciara del Fuoco. This activity was monitored by the soil CO2 flux network of Istituto Nazionale di Geofisica e Vulcanologia (INGV), which highlighted significant changes before the paroxysmal activity. The CO2 flux started to increase in 2006, following a long-lasting positive trend, interrupted by short-lived high amplitude transients in 2016–2018 and 2018–2019. This increasing trend was recorded both in the summit and peripheral degassing areas of Stromboli, indicating that the magmatic gas release affected the whole volcanic edifice. These results suggest that Stromboli volcano is in a new critical phase, characterized by a great amount of volatiles exsolved by the shallow plumbing system, which could generate other energetic paroxysms in the future.

A Threshold Line for Safe Geologic CO2 Storage Based on Field Measurement of Soil CO2 Flux

Carbon capture and storage (CCS) is an established and verified technology that can implement zero emissions on a large enough scale to limit temperature rise to below 2 °C, as stipulated in the Paris Agreement. However, leakage from CCS sites must be monitored to ensure containment performance. Surface monitoring of carbon dioxide (CO2) concentrations at onshore CCS sites is one method to locate and quantify CCS site leakage. Employing soil accumulation chambers, we have established baseline data for the natural flux of CO2 as a threshold alert to detect CO2 leakage flux to ensure the safety of onshore CCS sites. Within this context, we conducted on-site CO2 measurements at three different locations (A, B, and C) on the INAS test field at the Ito campus, Kyushu University (Japan). Furthermore, we developed a specific measurement system based on the closed-chamber method to continuously measure CO2 flux from soil and to investigate the correlation between CO2 flux from the soil surface and various parameters, including environmental factors and soil sample characteristics. In addition, gas permeability and the effect of different locations on soil CO2 flux are discussed in this study. Finally, we present an equation for estimating the soil CO2 flux used in the INAS field site that includes environmental factors and soil characteristics. This equation assists in defining the threshold line for an alert condition related to CO2 leakage at onshore CCS sites.

Short-Term Monitoring of Geogenic Soil CO2 Flux in a Non-Volcanic and Seismically Inactive Emission Site, South Korea

Temporal changes of soil CO2 flux (FCO2) and soil CO2 concentration ([CO2]v) were surveyed in a natural CO2 emission site to characterize the factors controlling the short-term temporal variation of geogenic FCO2 in a non-volcanic and seismically inactive area. Due to a lack of long-term monitoring system, FCO2 was discontinuously measured for three periods: Ⅰ, Ⅱ at a high FCO2 point (M17) and Ⅲ about 30 cm away. Whereas [CO2]v was investigated at a point (60 cm depth) for all periods. A 2.1 magnitude earthquake occurred 7.8 km away and 20 km deep approximately 12 h before the period Ⅱ. The negative correlation of FCO2 with air pressure suggested the non-negligible advective transport of soil CO2. However, FCO2 was significantly and positively related with air temperature as well, and [CO2]v showed different temporal changes from FCO2. These results indicate the diffusive transport of soil CO2 dominant in the vadose zone, while the advection near the surface. Meanwhile [CO2]v rapidly decreased while an anomalous FCO2 peak was observed during the period Ⅱ, and the CO2 emission enhanced by the earthquake was discussed as a possible reason for the synchronous decrease in [CO2]v and increase in FCO2. In contrast, [CO2]v increased to 56.8% during the period Ⅲ probably due to low gas diffusion at cold weather. In addition, FCO2 was low during the period Ⅲ and showed different correlations with measurements compared to FCO2 at M17, implying heterogeneous CO2 transport conditions at the centimeter scale. The abnormal FCO2 observed after the earthquake in a seismically inactive area implies that the global natural CO2 emission may be higher than the previous estimation. The study result suggests a permanent FCO2 monitoring station in tectonically stable regions to confirm the impact of geogenic CO2 to climate change and its relation with earthquakes.