by Academician Oleg FAVORSKY, and Alexander STARIK, Dr. Sc. (Phys. & Math.), deputy director, P.I. Baranov Central Institute of Aviation Instrument Engineering
It is common knowledge that our production activity has a negative effect on the air we breathe. The gas and aerosol composition of the atmosphere has notably changed as a result in these last few decades. Aviation is one of the culprits.
And yet aircraft engines discharge only a fortieth or even a fiftieth part of compounds compared with the surface sources (power engineering, transportation, industry and agriculture). Still, the problem of aviation-caused discharges is gaining in urgency, for they occur in the upper troposphere and the lower stratosphere, strata highly sensitive to any disturbances. The range of thus emitted compounds is fairly wide and depends on the type of engine and fuel. They exert a cumulative effect on the atmosphere. For instance, supersonic aircraft contribute to an increase in the concentration of nitrogen oxides in it and, conversely, cause a decrease in the content of ozone, and that intensifies the biologically harmful ultraviolet radiation just above the earth's surface. Penetrating ever deeper into the troposphere, UV radiation must increase the ozone concentration at low altitudes and thus change its temperature and radiation balance which is found to be 30 times as sensitive to discharges of nitrogen oxides from stratospheric aviation as to those coming from the surface sources.
Water vapor likewise impacts the atmosphere's radiation balance by absorbing the infrared radiation and giving rise to cirri (fleecy clouds) that enhance the hothouse effect.
The emission of vapors: of sulfuric, nitric and nitrous acids and of
Aviation effects on atmospheric processes.
water may evolve into a major factor in changing the composition of the atmosphere and in the formation of stratospheric clouds over the polar regions. In the last 20 to 25 years the concentration of aerosols in them has been going up by 5 percent a year, which correlates with the rate of increase in the amount of fuel consumed by aviation.
Even subsonic craft are responsible for a range of hydrodynamic effects, and this is all the more true of the supersonic airliners. We can single out three stages of such effects.
First, the initial flight stage. The hot gases ejected from the nozzle intermix with the ambient air, their temperature and composition change dramatically, with liquid (sulfate by and large) aerosols produced thereby. The so-called single-jet regime is realized-its time does not exceed 10 seconds, which corresponds to a distance of 1 km covered by the trail from the nozzle exit section of subsonic machines.
Simultaneously, turbulent wing wakes are formed. At some distance from the nozzle exit section these streams interact with a flow composed of combustion products and air. This is the second stage of the aircraft trail turbulence. The trail spreads largely in the vertical direction but much less horizontally; the time is not above 100 seconds, and extension-20 km. The course of the trail is far more intricate than at the previous stage.
And last comes the regime of large circular formations. That's the third stage, and it sets in after the disintegration of the condensation trail. Since at this stage the jet exhausts are carried into the atmosphere, it needs closer scrutiny.
The effect of substances formed in the engine and in the reaction on the atmosphere is all-important here. Related studies furnish information on the makeup and amount of discharges attending particular flight modes.
The combustion chamber of an aircraft engine operating on hydrocarbon fuel generates-besides carbon dioxide and monoxide, water vapor, nitrogen oxides and soot-other products too, such as nitric and nitrous acid vapors, hydrogen oxides, organic compounds, atomic oxygen, ions and sulfur-containing substances, mostly sulfurous anhydride. From 3 to 10 percent of the sulfurous anhydride is oxidized to sulfuric anhydride and sulfuric acid in a stretch from the combustion chamber exit to the nozzle exit section, which causes a drop in the concentration of oxygen atoms and OH radicals.
A jet of hot gases spurting from the nozzle of a jet-propulsion unit is cooled as it intermixes with the atmospheric air and expands. From 30 to 50 m from the nozzle, conditions are obtained for supersaturation of sulfuric acid vapors, and fine (1 nm in diameter) liquid particles of binary aerosols, H 2 O/H 2 SO 4 , are formed. More than 100 m away from the nozzle exit, these particles "coalesce" into much larger aggregations (tenfold as large).
At a distance of 25 - 100 m from the nozzle exit, these liquid aerosols are precipitated on the surface of soot particles emitted from the engine. The quite recent calculations invite important conclusions: in the nozzle exit section, 50 to 60 percent of these particles accumulate a positive charge, 10 to 20 percent-a negative charge, and about 30 percent remain neutral. Owing to electrostatic interactions, water and sulfuric acid molecules may precipitate directly on the particles of soot.
Even if sulfur is not present in the fuel, sulfuric anhydride and sulfuric acid vapors are formed during the combustion of hydrocarbons in the air containing sulfurous compounds. Minute (0.4 nm in diameter) drops of sulfate aerosol are formed in the jet to precipitate on the soot particles, which means that the H 2 O/H 2 SO 4 solution may accumulate on their surface even in this case.
The presence of soot particles on their surface is an important factor responsible for the formation of a condensation trail. Pure soot is hydrophobic (it does not wet) and does not condense moisture. But covered with the solution, the particles of soot moisten and aggregate to corpuscles 1 mm in the radius (at a distance of 200 m away from the nozzle exit section). The solution freezes at temperature 230 - 240 К and, should their concentration be
high enough, the particles produce a visible trail. They may also act as nuclei of cloud condensation and add to the formation of fleecy clouds.
At altitudes 15 to 35 km the atmosphere has sulfate aerosols and particles (with a concentration maximum at 20 km above earth surface) composed of supercooled solution triples H 2 O/H 2 SO 4 /HNO 3 , from 0.01 to 1 0.01 to 1 mm large. Their number much depends on volcanic activity, as seen in the example of Pinatubo's eruption Philippines in June 1991 that produced a great number of articles like that. As a result, the surface of sulfate aerosols increased fifty-fold. We are studying the evolution of their composition and phasic condition so as to gain a better understanding of the mechanisms implicated in the formation of stratospheric cloud cover over the polar regions.
Such clouds are of two kinds. At altitudes of 14 - 24 km-if the temperature is not above 195 K-particles of crystalline nitric acid trihy-drate, 0.15 to 5 mm in diameter, give rise to the first kind of clouds. The other kind is formed at lower temperatures- 188 К (over the Antarctic in wintertime), and these clouds are composed of 1 - 10 mm ice particles very close in their characteristics to those making up high-altitude fleecy clouds in the troposphere.
More of the first type clouds may come as a consequence of the emission of nitric acid and water vapors at high latitudes. The enhanced concentration of sulfate aerosols discharged by aircraft contributes to the generation of stratospheric polar clouds of both kinds. Unfortunately, this problem has been but little studied. Actually, the effect of supersonic craft on the stratospheric aerosol layer and on the formation of stratospheric clouds over the polar regions may happen to be underreported.
The problem least studied at all relates to the climatic consequences of air flights in the upper troposphere (at altitude 10 - 12 km). This is due to the complexity involved in studying the interconnected processes there, such as troposphere/stratosphere exchanges, latitudinal and meridional transfer of air masses, physicochemical conversions in the gaseous phase and on the surface of aerosols, and so on.
The additional formation of fleece clouds is one grave consequence of water vapor discharges by aircraft. It has been estimated that the cloud cover in the North Atlantic air corridor increases by 2 - 3.5 percent in January (with 0.5 percent the mean value for Europe). The impact of this phenomenon on the climate occurs through changes in the radiation balance of the atmosphere and temperature increases in near-surface layers of the air (t increases 1.2 - 1.4 К with a 10 percent increase in the cloud cover).
The tropospheric aerosol layer, too, has a significant impact on the climate. This layer contains elements of the earth crust, such as silicon, calcium, metal particles (iron, aluminum, zinc, tin) as well as soot. Most of the soot comes from aviation. Thus, 0.15 mm soot particles are detected in a subsonic aircraft trail. This size is optimal for the formation of cloud condensation nuclei and fleecy clouds.
Also, air flights in the upper troposphere are responsible for the destruction of various substances on the surface of aerosol particles and for the vigorous disintegration of ozone. Nitrogen dioxide and nitric acid vapors are thereby converted to nitrogen oxide, which likewise contributes to a fall in the ozone concentration in the upper troposphere-a region of the soot aerosol maximum-and in the lower stratosphere too, where soot aerosols may rise.
The predicted double increase of fuel to be consumed by aviation in the next 18 to 25 years can result in the doubling of the surface area of soot particles in the lower stratosphere and in the tenfold rise of their concentration in air corridors.
The present-day aviation is thought not to exert an appreciable impact on the atmosphere and climate. Yet the anticipated rise in the intensity of subsonic and, possibly, supersonic flights of passenger airliners, as it is obvious from many parameters of the models used, may cause one to think better of it.
It is important to carry on studies into the emission characteristics of aircraft engines and to model the physicochemical transformations in exhausted gases as they intermix with the atmosphere. Furthermore, we should go ahead with studying atmospheric processes most sensitive to global aviation impacts. We should develop comprehensive kinetic models relative to the formation of smaller gas components, ions, particles of soot, both neutral and charged, attending the combustion of aviation fuels. The same holds for aerosols formed in the reaction jet and for the evolution of their composition. Also, it is important to identify the mechanisms implicated in the precipitation of liquid solution on the surface of soot particles and study its freezing characteristics.
Detailed research is imperative into gaseous phase processes in the atmosphere, and also on the surface of aerosol particles in the stratosphere and troposphere; and into the formative dynamics of micro-particles in the upper troposphere and lower stratosphere. We should study heterogeneous reactions on the surface of stratospheric clouds over the polar regions, on sulfate aerosols, on triple supercooled solutions and soot tropospheric aerosols. And last, we should look into the real role of discharged aerosols in the formation of polar stratospheric clouds and of the extra cloud cover in the troposphere.
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