Simulating dynamics of –¹³C of CO₂ in the planetary boundary layer over a boreal forest region: covariation between surface fluxes and atmospheric mixing

Abstract

Stable isotopes of CO₂ contain unique information on the biological and physical processes that exchange CO₂ between terrestrial ecosystems and the atmosphere. Ecosystem exchange of carbon isotopes with the atmosphere is correlated diurnally and seasonally with the planetary boundary layer (PBL) dynamics. The strength of this kind of covariation affects the vertical gradient of δ¹³C and thus the global δ¹³C distribution pattern. We need to understand the various processes involved in transport/diffusion of carbon isotope ratio in the PBL and between the PBL and the biosphere and the troposphere. In this study, we employ a one-dimensional vertical diffusion/transport atmospheric model (VDS), coupled to an ecosystem isotope model (BEPS-EASS) to simulate dynamics of ¹³CO₂ in the PBL over a boreal forest region in the vicinity of the Fraserdale (FRD) tower (49°52’29.9”N, 81°34’12.3”W) in northern Ontario, Canada. The data from intensive campaigns during the growing season in 1999 at this site are used for model validation in the surface layer. The model performance, overall, is satisfactory in simulating the measured data over the whole course of the growing season.We examine the interaction of the biosphere and the atmosphere through the PBL with respect to δ¹³C on diurnal and seasonal scales. The simulated annual mean vertical gradient of δ¹³C in the PBL in the vicinity of the FRD tower was about 0.25‰ in 1999. The δ¹³C vertical gradient exhibited strong diurnal (29%) and seasonal (71%) variations that do not exactly mimic those of CO₂. Most of the vertical gradient (96.5% ±) resulted from covariation between ecosystem exchange of carbon isotopes and the PBL dynamics, while the rest (3.5%±) was contributed by isotopic disequilibrium between respiration and photosynthesis. This disequilibrium effect on δ¹³C of CO₂ dynamics in PBL, moreover, was confined to the near surface layers (less than 350 m).