Using solar FTIR spectrometry to investigate the sources and sinks of carbonyl sulfide and its application to the carbon cycle

Abstract

Understanding carbon dioxide (CO2 ) biospheric processes is of great importance because the terrestrial exchange drives the seasonal and inter-annual variability of CO2 in the atmosphere. Atmospheric inversions based on CO2 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 CO2 and OCS from measured solar absorption spectra. Here, we exploit the FTIR measurements of OCS and CO2 to study their atmospheric relationship. The OCS columns are retrieved from the measured spectra at twelve stations spanning both Northern and Southern Hemisphere. The CO2 FTIR data in the Northern Hemisphere are also used. The OCS measurements were compared to forward simulations using a chemical transport model (GEOS-Chem) driven by different land biosphere fluxes to reproduce the seasonality of the measurements. Increasing the plant uptake of Kettle et al. (2002a) by a factor of three resulted in the best comparison with the measurements. The simulation with OCS land fluxes from the simple biosphere model (SiB) underestimated the seasonal amplitude in the high latitudes of the Northern Hemisphere, indicating that the latitudinal flux distributions in SiB need to be adjusted. There are discrepancies in the low latitudes when comparing with HIPPO (HIAPER Pole-to-Pole Observations) data spanning both hemispheres, which implies a missing source in that region. OCS flux inversions were performed to gain better flux maps. The inversion with SiB land fluxes and Campbell et al. (2015) anthropogenic emissions leads to the best agreement with the measurements. However, the validation with HIPPO measurements shows mismatches in the tropics as well as Northern temperate region, where the measurements are too sparse to constrain the fluxes. Inclusion of FTIR measurements did not improve the inversion, because there is an offset between these two data sets, which makes it not straight forward. The simple biosphere model (SiB) simultaneously calculates the biospheric fluxes of both OCS and CO2. Therefore the CO2 biosphere fluxes in SiB can be evaluated with the helpof OCS. The CO2 simulation with SiB fluxes agrees with the measurements well, while the OCS simulation reproduced a weaker drawdown than the measurements at selected Northern Hemispheric sites, and a smaller latitudinal gradient in the Northern Hemisphere during growing season. It suggests that the photosynthesis is underestimated in the boreal region in SiB. An offset in the timing of the seasonal cycle minimum between SiB simulation and measurements is also seen in both CO2 and OCS. These phase differences offer another aspect that can be used to evaluate the photosynthesis and respiration in SiB. The OCS was also used to study the contributions of photosynthesis and respiration on the inter-annual variation of atmospheric CO2. The heatwave event in 2010 was taken for a case study. The analysis of OCS indicates that the photosynthesis decreased during the heatwave, which is underestimated in SiB. 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.