Experimental studies and physically substantiated model of carbon dioxide emission from the exposed cultural layer of Velikii Novgorod

Abstract

The results of quantitative assessment and modeling of carbon dioxide emission from urban pedolithosediments (cultural layer) in the central part of Velikii Novgorod are discussed. At the first stages after the exposure of the cultural layer to the surface in archaeological excavations, very high CO₂ emission values reaching 10–15 g C/(m² h) have been determined. These values exceed the normal equilibrium emission from the soil surface by two orders of magnitude. However, they should not be interpreted as indications of the high biological activity of the buried urban sediments. A model based on physical processes shows that the measured emission values can be reliably explained by degassing of the soil water and desorption of gases from the urban sediments. This model suggests the diffusion mechanism of the transfer of carbon dioxide from the cultural layer into the atmosphere; in addition, it includes the equations to describe nonequilibrium interphase interactions (sorption–desorption and dissolution–degassing of CO₂) with the first-order kinetics. With the use of statistically reliable data on physical parameters—the effective diffusion coefficient as dependent on the aeration porosity, the effective solubility, the Henry constant for the CO₂ sorption, and the kinetic constants of the CO₂ desorption and degassing of the soil solution—this model reproduces the experimental data on the dynamics of CO₂ emission from the surface of the exposed cultural layer obtained by the static chamber method.