We present a comparison of atmospheric transport models that simulate carbonyl sulfide (COS). This is part II of the ongoing Atmospheric Transport Model (ATM) Inter-comparison Project (TransCom–COS). Differently from part I, we focus on seven model intercomparison by transporting two recent COS inversions of NOAA surface data within TM5-4DVAR and LMDz models. The main goals of TransCom-COS part II are (a) to compare the COS simulations using the two sets of optimized fluxes with simulations that use a control scenario (part I) and (b) to evaluate the simulated tropospheric COS abundance with aircraft-based observations from various sources. The output of the seven transport models are grouped in terms of their vertical mixing strength: strong and weak mixing. The results indicate that all transport models capture the meridional distribution of COS at the surface well. Model simulations generally match the aircraft campaigns HIPPO and ATom. Comparisons to HIPPO and ATom demonstrate a gap between observed and modelled COS over the Pacific Ocean at 0–40$\degree$N, indicating a potential missing source in the free troposphere. The effects of seasonal continental COS uptake by the biosphere, observed on HIPPO and ATom over oceans, is well reproduced by the simulations. We found that the strength of the vertical mixing within the column as represented in the various atmospheric transport models explains much of the model to model differences. We also found that weak-mixing models transporting the optimized flux derived from the strong-mixing TM5 model show a too strong seasonal cycle at high latitudes.

A document by Luke Western. Click on the document to view its contents.

The monthly time series of carbonyl sulfide (OCS) atmospheric mole fractions measured at NOAA network stations (2000 to 2020) have been analyzed, and the long-term behaviour has been assessed based on the Empirical Mode Decomposition (EMD). EMD is a fully non-parametric analysis of frequency modes and trends in a given series and is based on the data alone. We have found that the OCS atmospheric mole fraction, after an increasing phase up to ~2015, with a temporary decline around 2009, is now decreasing at all stations, reflecting a recent imbalance in its total sources and losses. Our analysis has revealed a characteristic time scale for variation of 8-10 years. The variance associated with this long-term behaviour ranges from 15 to 40% of the total strength of the signal, depending on location. To our knowledge, this low-frequency mode is a novel result not assessed in previous studies. Apart from this complex long-term behaviour, the OCS time series show a strong annual cycle, which primarily results from summertime OCS uptake by vegetation. In addition, we have also found one more frequency of minor variance intensity in the measured mole fraction time-history, which corresponds to periods in the range of 2 to 3 years. This inter-annual variability of OCS may be linked to the Quasi-Biennial Oscillation or QBO.

Carbonyl sulfide (COS) was measured in firn air collected during seven different field campaigns carried out at four different sites in Greenland and Antarctica between 2001 and 2015. A Bayesian probabilistic statistical model is used to conduct multi-site inversions and to reconstruct separate atmospheric histories for Greenland and Antarctica. The firn air inversions cover most of the 20 century over Greenland and extend back to the 19 century over Antarctica. The derived atmospheric histories are consistent with independent surface air time-series data from the corresponding sites and the Antarctic ice core COS records during periods of overlap. Atmospheric COS levels began to increase over preindustrial levels starting in the 19 century and the increase continued for much of the 20 century. Atmospheric COS peaked at higher than present-day levels around 1975 CE over Greenland and around 1987 CE over Antarctica. An atmosphere/surface ocean box model is used to investigate the possible causes of observed variability. The results suggest that changes in the magnitude and location of anthropogenic sources have had a strong influence on the observed atmospheric COS variability.