Spatial And Temporal Distribution Characteristics Of Air CO₂ Concentration In Karst Caves And Its Response Relationship

The operation of karst dynamics system is accompanied by carbon migration changes and the carbon migration in each phase is accompanied by concentration changes and isotopic fractionation,and with the rise of cave tourism,the air environment inside the cave is gradually affected by various effects.Through field monitoring and indoor experiments on different time scales of the cave air environment,CO₂ concentration of the overlying soil and atmospheric characteristics outside the cave,and the analysis and investigation of the spatial and temporal distribution characteristics by using GIS analysis and relevant mathematical and statistical analysis methods,the spatial and temporal distribution patterns of the air CO₂ concentration inside the cave were clarified.At the same time,the spatial and temporal variation of CO₂ in the air inside and outside the cave,the stable carbon isotope variation characteristics of the overlying soil air and the construction of a model for the variation of CO₂ concentration in the air inside the cave are combined to clarify the spatial and temporal variation of CO₂ in the air inside the tourist cave and the corresponding mechanism.It provides a theoretical basis for the study of the interaction between the variable external environment and the relatively stable cave internal air environment,the mechanism of cave air environment changes and the impact on secondary sediments,and then provides a theoretical basis and reference for the study of"carbon sink"in karst areas.The monitoring period was from December 2018 to November 2019 and the National Day 2018（October1-7,2018）,and the study found that.（1）Affected by the ventilation effect of the cave,the CO₂concentration in the air in Dafeng Cave mainly shows that the CO₂concentration in summer and autumn is higher than that in winter and spring in time.The CO₂ concentration gradually increases from the entrance to the cave in space.,And gradually tend to be stable.The main manifestations are:the air in the cave is mainly restricted ventilation mode in summer and autumn,the gas exchange inside and outside the cave is suppressed,so CO₂ gas is not easy to diffuse out,and the CO₂ gas brought by tourists’breathing and movement makes the CO₂ in the cave a cumulative effect;In spring,the air in the cave is mainly in a state of active ventilation,and the gas exchange inside and outside the cave is strong,so the CO₂ concentration in the cave is similar to the atmospheric CO₂ concentration outside the cave.However,different monitoring points have different ventilation effects due to their distance from the entrance of the cave and the difference in their tunnel structure.The closer the monitoring point is to the entrance,the ventilation is better,and the ventilation of the monitoring point with a straight tunnel structure is relatively relatively.（2）The continuous monitoring during the National Day of Shenquan2#and Yemingzhu found that the air CO₂ in the cave during the continuous monitoring period showed that the CO₂ concentration during the day was higher than that at night,and the CO₂ concentration gradually increased from the entrance to the cave in space.And gradually tend to be stable.The data shows that Shenquan 2#and Yemingzhu are both must-go points for tourists,but due to the difference in tunnel structure and altitude,the CO₂ concentration of Shenquan 2#is basically higher than that of Yemingzhu,and the two monitoring points have the highest CO₂ concentration Most of the values appear in the 15:30-18:30 time period of the day,and then gradually decrease to the background value of the air environment due to the self-purification ability of the cave air environment.The main reason is that the air in the cave has cave ventilation effect and self-purification ability,so the CO₂ concentration of cave air shows obvious diurnal changes under the mutual adjustment of the two.（3）The conceptual model of CO₂ concentration change shows that the ventilation effect of the air inside and outside the cave and visitors have a greater impact on the CO₂ concentration in the cave,and the ventilation effect interacts with visitors,the CO₂ concentration mainly undergoes four stages of changes:a small number of tourists enter,a large number of tourists enter or stay,tourists leave and no tourists stage.While the CO₂ concentration is increased by the respiration of tourists,the CO₂concentration will also change due to the activities of the tourists.The air exchange generated by the activities of the tourists is much larger than the natural air exchange,and can even reach nearly 55 times.The airflow generated by the movement of visitors will also change the ventilation mode of the cave.（4）In general,the temporal and spatial distribution of air CO₂concentration in Dafeng cave is affected by cave ventilation effect,tunnel structure,and the number of tourists.The CO₂ concentration of the soil overlying the cave also has a certain influence on the change of cave air CO₂ concentration.The air CO₂ concentration at the soil point over the cave and Shenquan 2#and Yemingzhu corresponding to Yemingzhu generally shows a change pattern of summer and autumn higher than winter and spring,and there are obvious seasonal changes,which correspond to the changes in the CO₂ concentration of the monitoring points in the cave.The situation is consistent,indicating that the change of the air CO₂ concentration in the cave has a greater relationship with the CO₂ concentration of the overlying soil.