A new model of the global biogeochemical cycle of carbonyl sulfide – Part 2: Use of carbonyl sulfide to constrain gross primary productivity in current vegetation models

摘要

Clear analogies between carbonyl sulfide (OCS) and carbon dioxide (CO)diffusion pathways through leaves have been revealed by experimental studies,with plant uptake playing an important role for the atmospheric budget ofboth species. Here we use atmospheric OCS to evaluate the gross primaryproduction (GPP) of three dynamic global vegetation models (Lund–Potsdam–Jena, LPJ; National Center for Atmospheric Research – Community Land Model 4,NCAR-CLM4; and Organising Carbon and Hydrology In Dynamic Ecosystems, ORCHIDEE). Vegetation uptake of OCS is modeled as a linear function ofGPP and leaf relative uptake (LRU), the ratio of OCS to CO depositionvelocities of plants. New parameterizations for the non-photosynthetic sinks(oxic soils, atmospheric oxidation) and biogenic sources (oceans and anoxicsoils) of OCS are also provided. Despite new large oceanic emissions, globalOCS budgets created with each vegetation model show exceeding sinks byseveral hundred Gg S yr. An inversion of the surface fluxes(optimization of a global scalar which accounts for flux uncertainties) ledto balanced OCS global budgets, as atmospheric measurements suggest, mainlyby drastic reduction (up to −50 %) in soil and vegetation uptakes.The amplitude of variations in atmospheric OCS mixing ratios is mainlydictated by the vegetation sink over the Northern Hemisphere. This allowsfor bias recognition in the GPP representations of the three selectedmodels. The main bias patterns are (i) the terrestrial GPP of ORCHIDEE at highnorthern latitudes is currently overestimated, (ii) the seasonal variationsof the GPP are out of phase in the NCAR-CLM4 model, showing a maximum carbonuptake too early in spring in the northernmost ecosystems, (iii) the overallamplitude of the seasonal variations of GPP in NCAR-CLM4 is too small, and(iv) for the LPJ model, the GPP is slightly out of phase for the northernmostecosystems and the respiration fluxes might be too large in summer in theNorthern Hemisphere. These results rely on the robustness of the OCSmodeling framework and, in particular, the choice of the LRU values (assumedconstant in time) and the parameterization of soil OCS uptake with smallseasonal variations. Refined optimization with regional-scale and seasonallyvarying coefficients might help to test some of these hypothesis.