Towards understanding the variability in biospheric CO<sub>2</sub> fluxes: using FTIR spectrometry and a chemical transport model to investigate the sources and sinks of carbonyl sulfide and its link to CO<sub>2</sub>

Abstract

Abstract. Understanding carbon dioxide (CO 2 ) biospheric processes is of great importance because the terrestrial exchange drives the seasonal and interannual variability of CO 2 in the atmosphere. Atmospheric inversions based on CO 2 concentration measurements alone can only determine net biosphere fluxes, but not differentiate between photosynthesis (uptake) and respiration (production). Carbonyl sulfide (OCS) could provide an important additional constraint: it is also taken up by plants during photosynthesis but not emitted during respiration, and therefore is a potential means to differentiate between these processes. Solar absorption Fourier Transform InfraRed (FTIR) spectrometry allows for the retrievals of the atmospheric concentrations of both CO 2 and OCS from measured solar absorption spectra. Here, we investigate co-located and quasi-simultaneous FTIR measurements of OCS and CO 2 performed at five selected sites located in the Northern Hemisphere. These measurements are compared to simulations of OCS and CO 2 using a chemical transport model (GEOS-Chem). The coupled biospheric fluxes of OCS and CO 2 from the simple biosphere model (SiB) are used in the study. The CO 2 simulation with SiB fluxes agrees with the measurements well, while the OCS simulation reproduced a weaker drawdown than FTIR measurements at selected sites, and a smaller latitudinal gradient in the Northern Hemisphere during growing season when comparing with HIPPO (HIAPER Pole-to-Pole Observations) data spanning both hemispheres. An offset in the timing of the seasonal cycle minimum between SiB simulation and measurements is also seen. Using OCS as a photosynthesis proxy can help to understand how the biospheric processes are reproduced in models and to further understand the carbon cycle in the real world.

Response to Reviewer 1

“…This uncertainty is from the emissions of CS 2 , not the molar yield, for which a globally constant factor is used. The global DMS oxidation source of OCS was estimated by Barnes et al (1994) as 50.1-140.3 Gg S yr −1 , and subsequent budgets contain only revisions according to updated DMS emissions (Kettle et al, 2002;Watts, 2000). We suggest that the uncertainty in the production of OCS from DMS is underestimated.…”

“…The oceans are known to contribute to the atmospheric budget of OCS directly via OCS and indirectly via CS 2 (Fig. 8) (Chin and Davis, 1993;Watts, 2000;Kettle et al, 2002). Large uncertainties are still associated with current estimates of marine fluxes (Launois et al, 2015a;Lennartz et al, 2017, and references therein) and have led to diverging conclusions regarding the magnitude of their global role.…”

“…Indirect marine emissions from oxidation of the precursor gases CS 2 and possibly dimethyl sulfide (DMS) were hypothesized to be on the same order of magnitude as or larger than direct ocean emissions of OCS (Chin and Davis, 1993;Watts, 2000;Kettle et al, 2002). Production and loss processes of CS 2 in seawater are less well constrained than OCS production, and they include photoproduction, evidence for biological production (Xie et al, 1998(Xie et al, , 1999, and a slow chemical sink (Elliott, 1990).…”

“…Anthropogenic OCS sources include direct emissions of OCS and indirect sources from emissions of CS 2 . The dominant source is from rayon production , while other large sources include coal combustion, aluminum smelting, pigment production, shipping, tire wear, vehicle emissions, and coke production Chin and Davis, 1993;Du et al, 2016;Lee and Brimblecombe, 2016;Watts, 2000;Zumkehr et al, 2017).…”

“…We calculate a "bottom-up" global balance of OCS with several approaches, as presented in Table 4. Within the atmosphere, the tropospheric sink owing to oxidation by OH is estimated to be in the range 82-130 Gg S yr −1 (Berry et al, 2013;Kettle et al, 2002;Watts, 2000), and the stratospheric sink is in the range 30-80 Gg S yr −1 , or 50 ± 15 Gg S yr −1 (Barkley et al, 2008;Chin and Davis, 1995;Crutzen, 1976;Engel and Schmidt, 1994;Krysztofiak et al, 2015;Turco et al, 1980;Weisenstein et al, 1997). OCS concentrations are increasing roughly 0.5-1 ppt year −1 averaged over the last 10 years (Campbell et al, 2017a), suggesting approximately 2 to 5 Gg S yr −1 remains in the troposphere.…”