QUOTE
not like the gas carbon dioxide for which only 'crazies' consider a pollutant.
I suppose it could be said astronauts pollute their environment. They do not need to use the larger biosphere("to clean the air") which should tell you how easy it is. In addition to ground transport, air transport is a consideration. (We can leave sea for another day... a large place to hide or dump trash for a time)
I did some editing and put in bold a comment about a or the rate-determining step involving a layer of atmosphere. The rds is a chemical term i searched for a few days ago and just got around to reading. You also will note that they do not use the word saturation but instead speak of a new equilibrium.
Table 4 (CONCAWE (1997), EC (1996)) shows how the emissions of CO, hydrocarbons, NOx and particulate matter have been reduced in Europe, reflecting the ability of technology to deliver reductions in emissions.
The data show how the largest reductions in emissions have already taken place, with projections that further reductions will be possible by the introduction of on-board diagnostic systems, in-service emissions testing, recall programmes and fuel quality improvements (CONCAWE, 1997). These reductions in petrol and diesel engined vehicle emissions are sufficient to leave little room for improvement by switching to alternative hydrocarbon fuels such as natural gas or vegetable oil.
The only cleaner option, as far as local emissions are concerned, is for a zero-emissions vehicle powered by electricity or hydrogen fuel cells. For such vehicles, it is important to consider, however, the total environmental impact of their use, as the air pollution emissions from remote generation of electricity or production of hydrogen fuel
could possibly exceed the exhaust emissions that a conventional vehicle would produce. The main advantage of zero-emission vehicles is that the emissions can be relocated to where they are further from human receptors, so benefits to human health can be obtained
while other environmental impacts are not reduced (see Fig. 1).
Many decades, they say. You can probably take that with a grain of salt.When comparing different impacts of aircraft upon the global atmosphere with each other, and with the effect of emissions from other transport sectors and non transport related activity, the most challenging aspect of CO2 is perhaps the time scale over which it has an effect. CO2 is chemically sufficiently unreactive for its dominant removal process to be physical.
Solution in the water of the upper ocean and exchange of carbon between the atmosphere and terrestrial biomass are relatively rapid, with the combined annual flux amounting to 20% of the atmospheric carbon
reservoir mass of 750 GT (Houghton et al., 1996), but these fluxes are bi-directional. The rate determining step for net removal of carbon is mixing from the surface and intermediate ocean to the much larger carbon reservoir of the deep oceans. At the turn of the 21st Century, anthropogenic carbon emissions of 7 to 8 GT per year (including deforestation) are greater than the equilibrium rate of removal at current atmospheric and surface ocean concentrations, such that an
amount of carbon equal to around half the emissions each year are removed and the imbalance results in a steady increase in atmospheric carbon dioxide levels. Were emissions to remain constant at today’s rate, the atmospheric concentration would reach an equilibrium level about one third higher than today’s value towards the end of the 21st Century.
The global total emissions of CO2 from aviation in 1990 was about 450 million tonnes of carbon (Barrett, 991), which was less than 20% of global road transport emissions and about 3% of total anthropogenic emissions. Furthermore, historical emissions of CO2 from aviation are almost zero going back just a few decades into the mid 20th Century, while around half the carbon dioxide from all anthropogenic sources currently in the atmosphere was emitted before 1980, so the
overwhelming majority of the total is from non-aviation sources.
The small contribution of aviation is, however, increasing, and the small amounts of CO2 being emitted by aircraft now will remain in the air for many decades.
Finally, water vapour from jet engines can also form line-shaped clouds in the free troposphere. The temperature of these clouds is lower than that of Earth’s surface, so their black body radiation is less than what would be emitted from Earth’s surface were the clouds not there, resulting in net warming. This is more significant than the amount of incoming solar radiation reflected, so that overall the contrails have a warming effect on climate at the surface. Usually, contrails evaporate again within minutes or even seconds
such that their impact is negligible, but under certain meteorological
conditions they can be sufficiently persistent [and] a large part of the sky can become obscured continually along a major flight path until weather conditions change many hours or days later. In the stratosphere, contrails are never persistent because of the low ambient relative humidity there, although the water vapour from aircraft is not removed rapidly by precipitation as it is in the troposphere so has a small warming effect on climate because of its greenhouse gas properties.
-Current ability to quantify impact and major sources of uncertainty-
In theory, the impact of aircraft emissions on upper troposphere and lower stratosphere chemistry can be quantified using global models of circulation and chemistry (such as Johnson et al., 1999). However, despite the fact that the reaction mechanisms are now qualitatively understood, quantifying the impact of aircraft emissions remains elusive. There are two main reasons for this:
Firstly, the chemical reaction cycles are complex, as different gas-phase and heterogeneous pathways become more important at different temperatures. Small errors in the predicted mix of different pollutants can propagate via resulting errors in the relative rates of two or more competing reactions to end up with quite unrealistic simulated O3 concentrations. Not only must the chemical composition of the upper troposphere and stratosphere be simulated accurately, but rates of mixing between layers as well as chemistry determines the composition, the temperature needs to be known to determine where heterogeneous processes occur, and the temperature has a large influence on the mixing. The whole process of stratospheric O3 destruction in particular is a highly non-linear catastrophic process.
Secondly, emissions of aircraft in the upper troposphere and stratosphere occur along highly localised flight paths that vary in time and space. The physical size of these is much less than the resolution of the global-scale models that are required to simulate chemistry in the upper troposphere and stratosphere. This problem of scale is added to the fact that the total emissions from aircraft are at least as difficult to quantify as emissions for road traffic are on the ground. It is exacerbated by the fact that other sources of the same pollutants in the upper troposphere and lower stratosphere, such as lightening and mixing from the lower troposphere, are also very difficult to quantify accurately.
Any one of these difficulties would make calculations of the total atmospheric impact of aircraft emissions liable to error.
Combined, they present a very formidable challenge indeed for the science of atmospheric chemistry modelling. The most recent calculations indicate that the effect of aircraft NOx emissions on producing O3 in the upper troposphere / lower stratosphere is greater than the effect of sulphur and soot emissions on destroying O3, except at high latitudes Colvile et al., 2000.
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I am bitchy about this, and don't often participate in 'Green' threads. 'Nuff said. 
Sorry! That is Hawaiian slang for 'brother', one of my ex-wives had a couple of tours there, and I picked up a little. Don't get me going about Minehune! 
