Global trends in Carbonyl Sulfide from 20 NDACC FTIR station

Carbonyl sulfide, OCS, is the largest sulfur containing species in the atmosphere.  OCS has a complex array of sources and sinks that are not completely understood. Primary global sources are seasonally varying oceanic that are direct emissions and indirectly via oxidation of dimethyl sulfide and CS2 [Chin & Davis,1993]. The lifetimes of oceanic DMS and CS2 are short, days to weeks, and primarily control the OCS concentration in the Southern Hemisphere. The Northern Hemispheric budget is also sourced via oceanic processes but there is also a drawdown, seasonally varying from vegetation uptake and direct uptake from soils [Kettle et. al., 2002]. Relatively fast oxidation processes and drawdown act near the earth’s surface having less overall effect in the column of OCS.  There is consequently some short term variability nearer the surface and vertical gradients in OCS concentration that are larger in the north then in south [Montzka et al., 2008]. Recently a study into a wider range of anthropogenic sulfur containing species showed that there was generally an under reporting of anthropogenic sulfur emissions that would ultimately contribute to the total free tropospheric OCS burden [Lee & Brimblecomb, 2016].

Free tropospheric lifetime of OCS is up to 6 years and so is the largest contributor of sulfur to the stratosphere where it is converted to sulfate aerosol and is radiatively active. Understanding the sulfur loading in the stratosphere hence has large climate implications. For many years the OCS concentrations were observed constant and budgets to be relatively in balance but a recent investigation has shown this to not be case specifically for the southern hemisphere [Kremser et al, 2015]. Due to the complexity of emissions and sinks near surface observations they may not capture the free tropospheric trends. 

IRWG-NDAC Participating SitesFigure 1

Our recently initiated work is the use data from 21 NDACC1 FTIR2 sites to make a global assessment of the total, lower and upper OCS burdens. Figure 1 shows the participating stations dispersed over the globe. Each station processed data in a consistent manner and is currently active. Start dates though vary from station to station and some datasets are not continuous.

Figure 2

Figure 2 shows the mean weighted mixing ratios for the total, tropospheric partial and stratospheric partial columns for all sites plotted versus latitude. The top panel are all years (that are different) for all sites. Previous work has shown that at many sites there are inflections in trends so the next three panels are broken into these general time regimes. These data show the total and troposphere concentrations to be highest in the norther mid-latitudes with a draw down between 45º and 50º N similar to Montzka et al. Total and tropospheric concentrations fall toward both poles away from sources. There is very low stratospheric concentration at Paramaribo (6ºN) vs MLO at 20ºN. We may soon have data from Palau at 7ºN and Addis Ababa at 9ºN to investigate this.

Figure 3

Figure 3 are trends for each site for the same time periods. These data tend to confirm the measurements that there is a small increase at all but one station of the long term trend (left panel) of ~ 0.2 to 0.5 %/y (except St Denis and Maido each have only 3y of recent data). We also show an increase in the stratosphere in the long term for most sites. In the most recent 8 years (right panel) show slightly larger increases at most stations in the lower atmosphere mixed trends in the NH but strong increase in the stratospheric burden in the SH. We hope this study in progress will help understand the what is driving the most recent increase.

 

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1 NDACC Network for the Detection of Atmospheric Composition Change

2 FTIR: Fourier Transform Infrared spectrometers

  • Chin, M. and Davis, D. D. (1993). Global sources and sinks of ocs and cs2 and their distributions. Global Biogeochemical Cycles, 7(2):321–337.
  • Kremser, S., Jones, N. B., Palm, M., Lejeune, B., Wang, Y., Smale, D., and Deutscher, N. M. (2015). Positive trends in Southern Hemisphere carbonyl sulfide. Geophysical Research Letters, 42(21):9473–9480. 2015GL065879.
  • Lee, C.-L. and Brimblecombe, P. (2016). Anthropogenic contributions to global carbonyl sulfide, carbon disulfide and organosulfides fluxes. Earth-Science Reviews, 160:1 – 18.
  • Montzka, S. A., Calvert, P., Hall, B. D., Elkins, J. W., Conway, T. J., Tans, P. P., and Sweeney, C. (2007). On the global distribution, seasonality, and budget of atmospheric carbonyl sulfide (COS) and some similarities to CO2. Journal of Geophysical Research: Atmospheres, 112(D9). D09302.
Image: 
Carbonyl Sulfide Sources
Source: 
ACOM
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ACOM

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