Uncertainty in CO2 Gross Primary Productivity (GPP) Explains Discrepancies in a Critical Parameter for Quantifying GPP with Carbonyl Sulfide
Monday, 15 December 2014
Carbon dioxide gross primary productivity (GPP) is a critical component of the terrestrial biological carbon cycle, but quantifying GPP at regional and global scales is challenging. GPP cannot be observed directly because of large counterbalancing respiration fluxes, and process-based ecosystem models report a wide range of estimates. Carbonyl sulfide (COS) is metabolized by plants in parallel to CO2, and this relationship may provide a method to quantify GPP using COS observations. This COS tracer method depends on a thorough understanding of the leaf relative uptake (LRU) parameter of COS to CO2 by plants. Recent laboratory work suggests that the LRU parameter takes a much narrower range of values than previous atmospheric modeling studies have concluded. Here we conduct new atmospheric transport simulations using the data that underpinned several previous atmospheric modeling studies and demonstrate that GPP uncertainty can explain the relatively wide range of LRU values reported in these studies. We also find that atmospheric COS concentration is more sensitive to GPP uncertainty than to LRU uncertainty. These findings suggest that the COS tracer approach could be useful in quantifying regional and continental GPP.