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So does the water vapour of a wingtip contrail still form an ice crystal?
Are you saying that without the ice crystals a persistent contrail would still form with the right relative humidity levels?
Originally posted by weedwhacker
As to the viability of "adding" sulphur to the fuel? I have some doubts about that, and whether it would make it through the entire process, through filters and to combustion and exhaust, without having grave effects on engine performance and long-term mechanical reliability.....sulphur is in the "metals" family, on the Periodic Table of elements, I believe?
Have to check its melting point, and stuff like that. It might be the type of material that would melt, and then coat all the internal parts of the engine....the turbine blades, for example...and bearings, etc...effectively destroying an engine. This is what happens when jet engines encounter volcanic ash, in flight for example.......
Aviation and the Global Atmosphere:
The impact of the trend to use low-sulfur diesel fuels is not clear. Many refineries worldwide do not have the hydro-treating capability to make low-sulfur fuels. The API/NPRA survey for 1996 reported that 46% of the jet fuel blendstock in the United States was straight-run material that was not hydro-treated (API/NPRA, 1997). For many of these refineries with limited hydro-treating capability, the most economical approach may be to shift blending stocks with higher sulfur content to jet fuel, saving streams with lower sulfur for diesel fuel.
Coal accounts for most US sulfur dioxide (SO2) emissions and a large portion of NOx emissions. Sulfur is present in coal as an impurity, and it reacts with air when the coal is burned to form SO2. In contrast, NOx is formed when any fossil fuel is burned.
Originally posted by Uncinus
The role of soot is quite complicated, but it's really only a major factor in borderline conditions. This paper goes into it in some detail:
It would certainly be possible to manipulate the content of fuel and the operation of engines to make denser contrails, but given that this would decrease aircraft efficiency, and actually increase global warming (both via more cirrus clouds cover, and increase CO2 emissions), there seems very little incentive to do it. Plus there's no actual evidence that suggests any such manipulation has happened.
Increasing Sulfur Content of Jet Fuel in Commercial Fleet
This option involves increasing the sulfur content of jet fuel for the commercial fleet of jet aircraft (around 20,000 planes today) from 0.04% to 0.6 and increasing to 0.9% by 2050. Sulfur dioxide gas is emitted in the turbine exhaust and ideally, nearly all of it converted to sulfuric acid gas and then to sulfuric acid aerosol. The sulfuric acid aerosol floats around in the stratosphere for 1-2 years and reflects sunlight. The level in jet fuel is raised each year to match increased greenhouse gas emissions.
As for the sulfur in fuel option, current jet fuel sulfur levels are around 400ppm, mostly due to the removal of sulfur by use of hydroprocessing to meet other parts of the jet fuel specification or the fact that low sulfur feedstock is available (27-29). Jet aircraft have a specification limit of 3000ppm in their fuel in the U.S. and similar elsewhere, so the level could be increased by a factor of almost 8 without any modification to the specifications (30). I don’t know the origin of the 3000ppm limit, i.e., whether it is set for environmental or performance reasons and whether or not increasing it is feasible. Inquiries as to this have been made and will be reported in a revision to this commentary. The refiners would have to reformulate the fuel, but ironically, since their recent problems have largely been how to remove sulfur, this would require modifying their refining process to add more, a job that might take 5 years or so to complete (31, 32) or reverse, locking in the strategy. Thus, if this strategy were to be adopted, it would have to be continued for some time, even if the results were unacceptable.
Originally posted by Hally Burton
But as you have pointed out and as was posted earlier, the increase in contrail created cirrus appears to increase global warming and has not resulted in cooling.
Could the reason for this be that the formation of contrails is scattered due to atmospheric conditions during high air traffic and is therefore trapping heat locally for several days or maybe even weeks at a time and not reflecting sunlight on a global scale?
As the article above seems to claim "The sulfuric acid aerosol floats around in the stratosphere for 1-2 years and reflects sunlight." does this mean that contrails do not even need to form to create a reflective barrier and only needs to be dispersed as a sulfuric acid aerosol to float around in the atmosphere for 1-2 years, and that it is only because of the atmospheric conditions that we see the result as denser contrails. (if indeed the jet fuel has a high sulfur content)
What I would really like to know is if the sulfuric acid aerosol accumulates and floats around in the stratosphere above the poles, the north pole in particular since most air traffic is concentrated in the northerm hemisphere.
Originally posted by Uncinus
The biggest problem with the theory is that if it were true, they you would expect global dimming to increase. In fact global dimming has steadily been decreasing since 1993 (Mt Pinatubo eruption). Or really since the 1970s if you factor out volcanos. This has been mostly due to the clean air acts in most developed nations.
Levels of black carbon (soot) and sulphate aerosol in the Arctic have also decreased steadily in the last 20 years:
EU's airline emission goals under scrutiny "When the European Commission unveiled plans to slash transport CO2 emissions by 60% by 2050 last week, many assumed the figure would apply to road, rail and air travel in the same way. But EurActiv has learned that the cut for aviation is only 34%, a target both environmentalists and industry sources say is unrealistic."
"Sandro Santamato, head of the alternative fuel policy unit at the Commission's transport department, told EurActiv that "CO2 emissions from aviation will not decrease by 60% but by only 34% between 2005 and 2050". Other transport sectors will be measured against a 1990 baseline but in the 15 years to 2005, air emissions soared by some 81%. So the 2050 figure should be more easily achieved for airlines."