Chemistry and Atmospheric Physics
The oxygen atom exists in nature in several forms:
a) as a free atomic particle (O), which is very reactive and unstable
b) oxygen (O2) its most common and stable form, is a colorless and a pale blue liquid
c) ozone (O3), is a blue gas and has a molecular weight of 48, a density one and a half times that of oxygen and contains a lot of excess of energy in its molecular bonds. It has a distinctive pungent smell – usually experienced in ground based situations around electrical discharges such as lightening storms or electrical devices.
d) O4 is a very unstable, rare molecule. It is a pale blue gas which readily breaks down into two molecules of oxygen.
Ozone is present in the atmosphere in significant concentrations between 40,000 feet and 140,000 feet. It is formed when ultraviolet radiation from the sun bombards the upper atmosphere with short wavelength (200 nm) energy, splitting oxygen molecules into oxygen atoms which then recombine with oxygen molecules to form O3. The maximum ozone concentration in the atmosphere is typically about 100,000 feet and is found at 10 ppm (parts per million). It is about 4 ppm at 50,000 feet, less than .1 ppm below 40,000 feet, and typically .03 ppm at sea level. However, the amount of ozone present varies with latitude (being higher in polar regions), season and varying wind patterns. Moreover, ozone plumes sometimes extend downwards to about 25,000 feet containing concentrations of around 1 ppm (up to 4 ppm at Concorde cruise altitudes). Aircraft pass through these plumes from time to time, and ozone will then be drawn into the ventilation systems.1
Ozone reacts with tissue as it is an oxidant. At concentrations to be found in the environment, it acts as a respiratory irritant, and can also cause eye irritation. Acute exposure to 0.6- 0.8 ppm for 2 hours is sufficient to reduce lung flow volumes on measurement, and cause swelling and irritated lung linings (alveolar membrane). 10 ppm is enough to be potentially fatal from lung swelling (pulmonary edema). 0.1 ppm is the threshold value for producing symptoms in most individuals. Very high concentrations of ozone in the lab have been shown to act like X-rays on cells, but this is a theoretical concern only.2
FAA regulations require that the airplane cabin ozone concentration during flight must be shown not to exceed 0.25 parts per million by volume at any time above flight level 320, and 0.1 parts per million by volume time-weighted average during any 3-hour interval above flight level 270. (Note: time weighted average is an occupational hygiene measure meaning increasing concentrations can be traded off for reduced times – for example .2 ppm for 1.5 hours, up to a maximum ceiling of .25 ppm). JAA has also now adopted these regulations. Most of the initial concern around ozone was during the development of Concorde. Ozone is highly thermo-unstable. It decomposes completely to oxygen after exposure to 400 degrees Celsius after 0.5 seconds. During climb and cruise this aircraft generated sufficient temperatures to destroy ozone, but during prolonged descent temperatures could fall to 300 C where dissociation is only 90%, consequently some ozone could enter the cabin.2
During and shortly after passing through an ozone plume, ozone levels in the cabins of aircraft where ozone removal equipment is not fitted typically average 0.01-0.02 ppm (but sometimes up to ten times higher). As noted in supplementary material from Boeing and Airbus Industrie , ozone at the higher concentrations will be detected by cabin occupants and occasionally give rise to minor eye, nose and chest symptoms in some.1
Specifics for Bombardier CRJ series
Consequently, in order to comply (and in fact exceed compliance) with FAA/JAA ozone standards in all possible atmospheric conditions, Bombardier has mandated altitude restrictions for its RJ a/c. This should not cause alarm among air crew as the data show very low levels of exposure and no significant biological effects. The data I have been able to gain from Bombardier show that the 100/200 series compressor temperatures may, in some flight profiles, fall below the acceptable 400 deg C range. Even though the 700 series have different engine parameters, the same rules have arbitrarily been applied to this a/c as well.
- http://www.publications.parliament.uk/pa/ld199900/ldselect/ldsctech/121/12101.htm 2. Aviation Medicine; Butterworths pub; Ernsting and King
Brendan Adams MD, FCBOM