Sunday, August 11, 2013

USGS and NOAA's wholy inadequate and incomplete descriptions for the causes of aviation contrails and their persistence

The United States Geological Survey (USGS) webpage has an interesting but wholly inadequate description for the causes of aviation contrails [1]; linked to an equally inadequate description from a NOAA page [2].

These descriptions totally ignore the role of other aircraft emission pollutants such as sulfur dioxide SO2 and soot and their cloud nucleating properties which may induce the 'persistence' of contrails and eventual formation of cirrus clouds even under generally unfavorable conditions  ie. to-low atmospheric relative humidity and or to-high temperature [3] [4] with climate change - global warming implications.

[1] Contrails: Man-made clouds

"You've seen the cloud-like trails that high-flying airplanes leave behind and you probably know they are called contrails. Maybe you didn't know they were called that because they are actually condensation trails and, in fact, are not much different than natural clouds. If the exhaust from the airplane contains water vapor, and if the air is very cold (which it often is at high altitudes), then the water vapor in the exhaust will condense out into what is essentially a cirrus cloud."

Full article:
USGS: The Water Cycle: Condensation

[2] What is a contrail and how does it form?

"To answer this question, lets first identify what a contrail is. A contrail is the condensation trail that is left behind by a passing jet plane. Contrails form when hot humid air from jet exhaust mixes with environmental air of low vapor pressure and low temperature. Vapor pressure is just a fancy term for the amount of pressure that is exerted by water vapor itself (as opposed to atmospheric, or barometric, pressure which is due to the weight of the entire atmosphere above you). The mixing occurs directly behind the plane due to the turbulence generated by the engine. If condensation (conversion from a gas to a liquid) occurs, then a contrail becomes visible. Since air temperatures at these high atmospheric levels are very cold (generally colder than -40 F), only a small amount of liquid is necessary for condensation to occur. Water is a normal byproduct of combustion in engines."

"Persistence of contrails is neither an indication that they contain some kind of chemical, nor that it is some kind of spray"

Full article:
NOAA: What is a contrail and how does it form?

[3] Effects of aircraft on aerosol abundance in the upper troposphere 
G.V.Ferry • , R.F.Pueschel •õ , A.W. Strawa • , Y.Kondo 2, S.D.Howard 3, S.Verma 4, M.J.Mahoney 5, T.P.Bui •, J.R. Hannan 6, H.E.Fuelberg 6. 


A significant increase in H2SO4 aerosol concentration coincidental with an enhancement in NO was detected above 10 km pressure altitude during a cross-corridor flight out of Shannon on October 23, 1997.
The source of this aerosol is ascribed to commercial aircraft operations in flight corridors above 10 km, because (1) a stable atmosphere prevented vertical air mass ex- changes and thus eliminated surface sources, (2) air mass back trajectories documented the absence of remote continental sources, and (3) temperature profiler data showed the tropopause at least one kilometer above flight altitude throughout the flight, thus excluding stratospheric sources.
Particle volatility identified (60-80)% H2SO4, •20% (NH4)2SO 4 and •10% nonvolatile aero- sol in the proximity of flight corridors, and (10-30)% H2SO4, up to 50% (NH4)2S04, and (40-60)% nonvolatile aerosols in air that was not affected by aircraft operations below 10 km.
The newly formed H2SO 4 particles did not measurably affect surface area and volume of the background aerosol due to their small size, hence did not influence radiative transfer directly. 


Aircraft jet engines directly emit aerosols and condensable gases, such as water vapor (H20) and sulfur dioxide (SO2) which lead to the formation of new liquid (volatile) sulfuric acid (H2SO4), particles in the early plume by gas-to-particle conver- sion (nucleation) processes.
Soot aerosol formed during incom- plete fuel combustion constitutes part of the nonvolatile particle fraction.
The newly formed particles grow by condensation and coagulation amongsthemselves and with the background aero- sol. 

Aviation-produced particles may influence the state of the atmosphere in various ways.
For example, they provide surfaces for heterogeneous chemical reactions. Heterogeneous reaction rates depend on the chemical characteristics of aerosols.
For ex- ample, the reaction probabilities of N205 on NH4HSO4, formed by reaction of H2SO4 from aircraft with ammonia in background air, show a strong dependence on relative humidity and tempera- ture, decreasing with lowering relative humidity and increasing temperature (Mozurkewich and Calvert, 1988).
In contrast, meas- urements of N205 hydrolysis on H2SO4 particles show only a weak dependence on temperature and relative humidity (Hanso and Ravishankara, 1991).
Therefore, it is important to determine the enhancement of H2804 aerosol by aircraft in relation to the fraction of those particles that are neutralized to (NH4)2SO 4 in or- der to assess the atmospheric chemical modification potential of aircraft-produced aerosol.
Aviation-generated aerosols also form a sink for condensable gases, they absorb or scatter radiation di- rectly, and they change cloud properties which may affect radia- tion indirectly. In order to assess those effects, aviation-produced aerosol characteristics have to be determined in relation to back- ground aerosol properties.

Model calculations of aircraft emissions indicate that (6-10)% of SO2 is oxidized to SO3 and subsequently to H2SO 4 to form new small aerosols (Miake-Lye, 1994; Karcher, 1996; Brown et al., 1996, 1997). Measurements in aircraft wakes, on the other hand, have shown that up to 37% of SO2 can be oxidized to H2804 aerosols (Fahey et al., 1995a,b; Miake-Lye et al., 1998; Pueschel et al., 1998; Anderson et al., 1998; Paladino et al., 1998).
These measurements have also shown that the number density of volatile particles in aircraft wakes is proportional to the S-content of the fuel burned, implying that condensable S compounds are responsible for the formation of aerosol in aircraft wakes. 

In this paper we show that increases in H2SO 4 aerosol are also detectable in the vicinity of commercial aircraft flight corridors away from dedicated aircraft plumes.
The measurements were made during the Subsonic Assessment-Ozone and Nitrogen Ox- ide Experiment (SONEX). A stable atmosphere on 23 October 1997 separated air masses containing flight corridors above 10 km from air masses below 10 km that were not affected by air traffic.
Temperature profiler data determined a height of the tro- popause that was above DC-8 flight levels by at least 1 km throughout the flight, thereby eliminating stratospheric intrusions. 
Furthermore, air mass back trajectories established the absence of long range transport of pollutants from the North American con- tinent.
Thus, SONEX Flight No. 7 across commercial airline flight corridors on that day provided an opportunity to determine aerosol characteristics in both aircraft-affected and non-affected air masses in the upper troposphere. Our results document that subsonic aircraft are a source of detectable sulfuric acid aerosol. Even though the particles are too small to measurably affect sur- face area and volume densities, this finding is an affirmative an- swer to one of the key questions addressed by SONEX: Can air- craft corridors be detected? 

Full article:
Effects of aircraft on aerosol abundance in the upper troposphere 
G.V.Ferry et al.

[4] An overview of geoengineering of climate using stratospheric sulphate aerosols
Philip J Rasch1*, Simone Tilmes1, Richard P Turco2, Alan Robock3, Luke Oman4, Chih-Chieh (Jack) Chen1, Georgiy L Stenchikov3 and Rolando R Garcia1

(iv) Aerosol injection in aircraft jet exhaust
The effects of high-altitude aircraft on the upper troposphere and lower stratosphere have been extensively studied, beginning with the supersonic transport programmes of the 1970s and extending to recent subsonic aircraft impact assessments (under various names) in the USA and Europe (e.g. NASA-AEAP 1997). These projects have characterized aircraft emissions and jet plume dynamics, and developed corresponding models to treat the various chemical, microphysical and dynamical processes.

Enhancing aircraft fuel with added sulphur compounds (H2S, Sn) could increase the particle mass in a jet wake. It is well established that ultrafine sulphate particles are generated copiously in jet exhaust streams during flight (e.g. Fahey et al. 1995). The particles appear to be nucleated by sulphuric acid on ions (hereafter chemiions, e.g. Yu & Turco (1997, 1998b)) formed in the combustion process of jet engines by radical reactions. Sulphuric acid is a by-product of sulphur residues in the fuel (typically less than 1% sulphur by weight); most of this fuel sulphur is emitted as SO2. The fraction emitted as H2SO4 decreases as the fuel sulphur content increases, and accounts for roughly 2 per cent of the total sulphur as the fuel sulphur content approaches approximately 1 per cent.

The concentrations of chemiions in jet emissions are strongly limited by ion–ion recombination along the engine train to approximately 1×109 cm−3 at the exit plane (e.g. Arnold et al. 2000). Considering a variety of direct measurements of particles in jet wakes, Kärcher et al. (2000) showed that chemiion nucleation is consistent with the observed relative constancy of the ultrafine volatile (non-soot) particle emission factor, Ep∼1–2×1017 kg−1fuel (where it should be noted that the concentrations of soot particles are typically less than 1 per cent of the total number of particles emitted). Ep is quite insensitive to the fuel sulphur content, a fact that is also consistent with a chemiion nucleation source. While vapour trails formed in jet wakes can significantly modify the injected particle properties (e.g. Yu & Turco 1998a), condensation trails are extremely rare under normally dry stratospheric conditions.

Full article:
An overview of geoengineering of climate using stratospheric sulphate aeroso
Philip J Rasch1*, Simone Tilmes1, Richard P Turco2, Alan Robock3, Luke Oman4, Chih-Chieh (Jack) Chen1, Georgiy L Stenchikov3 and Rolando R Garcia1

Also in:
Chapter 12
Geo-Engineering Climate Change: Environmental Necessity or Pandora's Box?
by Brian Launder (Editor), J. Michael T. Thompson
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