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Upper Tropical Tropospheric COS

October 1st, 2007 by tanya · No Comments

For the ATOC Journal Club I discussed a paper entitled Enhance Upper Tropical Tropospheric COS: Impact on the stratospheric aerosol layer, by J. Notholt et al. 2003.
First what is COS, well it is carbonyl sulfide, which is created when carbonyl (carbon and oxygen) forms a double bond with sulfur. COS converts to sulfur dioxide then sulfate when in the stratosphere. Thus COS is important due to its impact on the stratospheric aerosol layer, as hinted at in the title of the paper. An increase in the number of aerosol particles due to COS can cool the stratosphere through the scattering of light. In addition ozone depletion is accelerated by providing more heterogeneous surfaces for chemical reactions.

Now, how does COS get up into the stratosphere in the first place. First, the study is conducted during quiescent periods, times with out volcanic eruptions. During these chunks of time COS gets into the stratosphere due to its long lifetime, unlike other particles such as carbon disulfide and DMS, both sulfur compounds. Both of these molecules contribute to the creation of COS through oxidation but do not get to the stratosphere due to their destruction in the troposphere. Carbon disulfide is emitted naturally from marshes and volcanoes. DMS is produced from phytoplankton, thus coming from the ocean. COS is itself directly emitted from deep sea vents and volcanoes. So there are many avenues for the production of COS, indirectly and directly, but this paper focuses on COS from biomass burning.

For completeness they compared studies from two different sources, one ground-based and one space-borne. Ground-based data was gathered by a German research vessel, called the Polarstern. The set of data extended from October to November in 1996 and December to January in 1999-2000 in the Atlantic. Space-borne data was gathered by the ATMOS instrument on the space shuttle. Observations from November 1994 over the Pacific Ocean are used in this study. Both of these instruments use passive absorption spectrometry in the infrared, with the Sun as a light source.

First the COS volume mixing ratios over tropical regions from Polarstern are analyzed. In addition, thermal tropopause altitudes are given, as determined from the pressure-temperature profiles, measured by balloon sondes daily during the cruises. Results are highlighted in the below figure.Figure 1
Both years studied, 1996 and 1999-2000, show an enhancement of 600 pptv for COS between 10 km and 18 km, where altitudes greater than 13 km is considered the tropical tropopause layer (TTL). Recent models assume an enhancement of only 400 to 500 ppbv, which is less than what is shown in figure 1. What is the source for this observed COS increase? The enhanced CO in the right half of figure 1 suggests biomass burning, but requires confirmation.

Part of the confirmation comes from analyzing space-borne data from ATMOS. Results from the analysis are shown in the below figure.Figure 2
The upper graph, (A), is the volume mixing ratio profiles of COS, CO, and HCN retrieved from solar occultation measurements recorded on 14 November 1994 above the Pacific (6°N, 151°W) by the ATMOS instrument flown on the space shuttle. This plot, like Polarstern, shows enhancement of COS and CO between 10 km and 18 km, further evidence of biomass burning as the source of COS. Additionally, HCN is plotted, which is a product of biomass burning and is enhanced, like CO. The lower graph, (B), is the temperature profile for these occultations determined from the ATMOS instrument and the ratio C2H2/CO, which is indicative of the age of the air masses (26). A large ratio would indicate a young air mass. So air entering the upper tropical troposphere moves in quickly, then slowly ascents into the stratosphere, where its mixing ratio decreases due to chemical reactions. Vigorous convective systems could provide rapid movements of air. These convective systems are produced in a variety of way, one of which is biomass burning.

Thus a possible mechanism for transport has been established, now the source (biomass burning) must be proven. The below figure will do just that.Figure 3
Both figures show backward trajectories ending at 14-km altitude (25). The lower part of each panel shows the altitude history of the trajectories (in meters). The labels on the trajectories indicate zero UT. The results of the satellite images of the ATSR World Fire Atlas (24) from October 1996 and December 1999 have been incorporated into the two panels.

Rapid fluctuations in altitude, indicated by the arrows, suggest the presence of unresolved convection in these regions. Backward Trajectories are calculated on isentropic levels, using the HYSPLIT4 model, from individual data points within a region of enhanced COS. ATSR shows regions of intense fires over Africa and Brazil, which is the same region HYSPLIT4 indicates is the source of COS enhancement.

One part I am unsure of is the Backward Trajectories calculation. I’ve had very little experience with such models and was curious as to how they work and their accuracy. Does anyone has information about such calculations? This seems to be the crux of their study, how the authors directly connect the enhanced COS to biomass burning. Thus I think it is the most important part to understand.

Now, there are three different measurements which indicate biomass burning as the source for COS enhancement.
1). Observations of CO and HCN, emitted during biomass burning, show enhancement in the same altitude range.
2). Large C2H2/CO ratio points toward a young air mass, which could be produced from intense fires, hence biomass burning.
3). Calculated backward trajectories show COS originates over Brazil. ATSR World Fire Atlas shows intense fires in Brazil during years studied.
So there is a confirmed source and mechanism for transport of COS intro the upper tropical troposphere and eventual ascent into the stratosphere.

The main conclusions are:
More COS comes from biomass burning than previously thought.
Results could change our understanding of the origin of upper tropospheric air.

Here is a Power Point of my talk, available for download.

Tags: aerosols · ATOC Journal Club · stratosphere

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