25 WHERE IS OZONE? PART 1 IONISATION

Dr Tony Phillips of NASA maintains that “Understanding the sun-climate connection requires a breadth of expertise in fields such as plasma physics, solar activity, atmospheric chemistry and fluid dynamics, energetic particle physics, and even terrestrial history. No single researcher has the full range of knowledge required to solve the problem”  http://science.nasa.gov/science-news/science-at-nasa/2013/08jan_sunclimate/

In fact it requires the efforts of a generalist, a synthesiser, like a bird that gathers a diversity of material to make its nest, to put this story together.

THE COMPOSITION OF THE ATMOSPHERE

Nitrogen and Oxygen together represent 99% of the volume of the atmosphere.  Neither ozone at up to 30 ppm nor CO2 at 400 part per million are in the list of the top eight atmospheric gases.From Wikipedia we have:

The wave lengths emitted by the Earth are centred about the 9-10 um where ozone absorbs.  It also absorbs at 5um. Because ozone, like water vapour is not uniformly distributed it gives rise to differences in air temperature and density. We are familiar with the manner in which the release of latent heat energises tropical cyclones. Climate science is blind to the manner in which ozone energises the atmosphere despite the realisation more than 100 years ago that total column ozone maps surface pressure. Carbon dioxide is another potent absorber of long wave radiation from the Earth but it is almost uniformly distributed. As such it plays no part in generating winds. It is differences in air density in the horizontal domain that drives the winds. The strongest winds are to be  found above the tropopause due to marked differences in the ozone content of the air in the horizontal domain between the 300 hPa and the 50 hPa pressure levels.

The movement of the air is influential in determining the equator to pole temperature gradient and cloud cover. High pressure cells are relatively cloud free environments. Anything that increases surface pressure in the mid latitudes expands the relatively cloud free zone and warms the planet.

In all latitude bands surface temperature variation is greatest in the winter and the range of variation increases from the equator to the poles. This points to a polar dynamic as being  responsible for natural climate variation. In the waxing and waning of polar cyclone strength according to the ozone content of the air we have a dynamic that can produce shifts in atmospheric mass. Shifts in mass are responsible for change in the planetary winds. This alone will change surface temperature.

It is vital therefore that we have a good understanding of how ozone comes to be, its distribution and the circumstances that will change its distribution and partial pressure.

PHOTOLYSIS

Ionization, photolysis, photo-dissociation and photo-decomposition and are all terms that are used to indicate a chemical reaction where electrons are dislodged from molecules by photons. How far this process extends into the lower atmosphere is a matter of interest.

A photon is a hypothetical unit of radiant energy.  Photolysis is defined as the interaction of one or more photons with one target molecule. Any photon with sufficient energy can affect the bonds of a chemical compound.  A photon’s energy relates to its wave length. Only the shorter wave lengths have the necessary energy to decompose the smallest atmospheric molecules.

SUSCEPTIBILITY TO PHOTOLYSIS

Because larger atomic weight molecules are more susceptible to photolysis than smaller atomic weight molecules only the smaller atomic weight entities can maintain their integrity at the highest altitudes.

In order of increasing atomic weight we have:    Hydrogen = 2.016,    Helium = 4.002602, Methane = 16.044,   Steam = 18.02,  Nitrogen = 28.0134,  Nitric Oxide = 30.006,  Oxygen = 31.9988,  Ozone= 47.998

Since short wave radiation is progressively ‘used up’ in its passage though the atmosphere it might be expected that the ozone content of the air would increase as the rays that disassociate ozone were used up. The ozone content of the air would then increase all the way to the surface of the planet. Part 2 will explain why that is not the case. This chapter explains where ozone is to be found above the tropopause and why that is so. An understanding of this question is vital if we want to comprehend the movement of the air and the origins of natural climate change. More than 100 years ago it was observed that ozone maps surface pressure. Surface pressure variation is the essence of weather on all time scales.

The ultraviolet spectrum includes wavelengths shorter than 400nm. These wave lengths can account for 8% of the energy that comes from the sun but only a fraction of that under quiet sun conditions. The power in the EUV spectrum varies tenfold over the course of a solar cycle.

It is only the very short wave radiation in the EUV spectrum, x-rays and gamma rays that is capable of disassociating nitrogen.  EUV is wholly absorbed in photolyzing oxygen and nitrogen above 80km in elevation in the ionosphere.

A wave length shorter than 240 nm is required to disassociate oxygen.

Ozone is susceptible to ultraviolet waves shorter than 320 nm.  This includes UV-C (220-290 nm) and UV-B (290-320 nm).

Wave lengths longer than 320 nm have relatively free passage through the atmosphere.  There is insufficient ozone in the southern hemisphere to screen out wave lengths in the UVB and perhaps part of the UVC.  This has important consequences for plants and animals because this radiation penetrates deeply into the cells of an organism. Human skin containing low levels of melatonin is particularly susceptible. If ones sees blood vessels below the skin, so too does UVB.

It is change in atmospheric ozone that determines the degree of penetration of short wave radiation to the surface. Cold air from high latitudes comes with more ozone aloft producing low surface pressure. When surface pressure is lower the risk of UV exposure is also lower because of the superior ozone content of the upper air. Climate change has involved a southward movement of the high pressure belts in the southern hemisphere, reduced rainfall in southern Australia and also an increase in the UV risk factor.

The UV risk factor at the surface is time of day and time of year specific and it also depends upon cloud cover. The processes of atmospheric ionisation are similarly focused on just part of the atmosphere and the intensity of the process varies according to the time of the year and the stage of the solar cycle. The diagram below is instructive in this respect.

WHERE DOES THE IONISATION OF THE ATMOSPHERE OCCUR?

Australian researchers contribute to the global effort in the field of radio astronomy. The diagram reproduced below appeared in a presentation delivered in 2012 to a CAASTRO EoR Radio Astronomy workshop in Sydney by  Dr Mike Terkildsen of IPS Radio and Space Services as reported here: http://www.spaceacademy.net.au/env/spwx/raiono.htm

Note that the fall off in the electron concentration above 300km in elevation relates to the decline in the number of particles that are candidates for ionization.

I quote:  The ionosphere is what we term a weak plasma, as only one percent of the neutral atoms in the upper atmosphere are ionised. Traces of ionisation exist from about 80 km to 1000 km in altitude, with the peak ionisation occurring around an altitude of 300 km. The maximum ionisation can vary from about 10¹⁰ to 10¹³ electrons per cubic metre.

Ionospheric ionisation is controlled by extreme ultraviolet and soft x-ray flux emitted by the Sun. The lower regions of the ionosphere show almost exclusive solar control in that the ionisation at any time is proportional to some function of the solar zenith angle at each point as is seen below.

Mileura is a radio observatory located in the Murchison district in Western Australia at 26° south latitude where radio wave interference is light due to remoteness from centres of population. We see the dependence of VTEC (Vertical Total Electron Count) at the Mileura observatory on time of day and the state of the solar cycle. Notice the dramatic difference between daylight and dark.  The difference between the maximum in the solar cycle and the minimum is as much as between day and night. There is a very strong impact of the angle of the sun that is reflected in the VTEC for the month of June.

This diagram helps us to understand that latitude impacts the degree of ionization of the atmosphere. Accordingly, at latitudes greater than 23° north or south the winter season will see a marked reduction in the vertical total electron count. We know that ozone partial pressure peaks in the high latitudes of the winter hemisphere. The availability of building blocks in terms of free atoms of oxygen to form ozone is least in winter. Ozone is not built in high latitudes via the dissociation of the oxygen molecule by UV light. It is transported there. The increase in the ozone content of the air in high latitudes in winter is not due to transport phenomena because the act of transport can not increase the concentration of any particular constituent. That increase in winter is due to low disassociation rates.

The altitudes where ionization maxima occur are referred to as the D, E and F regions.  The D region sees strong ionization only in daylight hours.

COSMIC RAYS AND A POSSIBLE IMPACT ON CLIMATE AT SOLAR MINIMUM

Some researchers refer to a lower C layer created by galactic cosmic radiation, a force that is independently capable of ionising the atmosphere that is particularly active over the poles. This activity can be monitored as a muon count. Precipitating muons penetrate to the surface and to deep underground, their incidence increasing with the temperature of the polar atmosphere.  It follows that the muon count creates a proxy record of the incidence of stratospheric warmings. Stratospheric warmings and in general the variability of the temperature of the stratosphere over the pole occur in winter where they build on a low base temperature established due to the descent of cold mesospheric air. The stratosphere warms from this low base as the tongue of very cold mesospheric air either withdraws or is displaced by ozone rich warmer air that circulates on the margins of the tongue outside what is referred to as ‘the polar vortex’. The vortex, is a rapidly circulating cone of air energised by the conjunction of cold dense air inside the vortex and ozone rich low density air outside the vortex.

Paradoxically, in the world of climate science the term ‘strong vortex’ relates to the situation where the flow of mesospheric air towards the surface is weak due to low surface pressure in the polar regions. In the Arctic, weak atmospheric pressure ensures that cold air is retained at high latitudes.  This is the positive phase of the ‘Arctic Oscillation’.

In climate science a ‘weak vortex’ refers to the situation where the AO index is negative, polar surface atmospheric pressure is high, the downdraught of mesospheric air is strong and cold air migrates into the mid latitudes. In this situation the jet stream that marks the edge of the polar vortex that in turn relates to the position of a chain of intense polar cyclones, wanders equator-wards taking with it very cold air. Is it any wonder that there is confusion about matters polar?

The notion of strong and weak vortex as described above is at odds with the circulation of the air in the stratosphere. In the stratosphere a faster zonal wind corresponds with deeper penetration of mesospheric air and weaker polar cyclone activity due to the  erosion of ozone. The result is a return of atmospheric mass to the pole from the mid latitudes and an accelerated flow of  of cold polar air to the mid latitudes. So a strong stratospheric flow is associated with coldness, not warmness. At the root of this problem is the notion that the vortex is some sort of impenetrable wall across which little mixing can occur. The reverse is actually the case because between the surface and 50 hPa polar cyclones violently mix very different atmospheric constituents from both sides of the ‘vortex’. The problem is a lack of appreciation of the motive force behind this circulation and a complete misinterpretation of its geometry. Behind that problem is the  notion that the circulation of the atmosphere is just problem in fluid dynamics where the energy to drive the system is assumed to be heating at the equator. In all other respects  it is assumed that the system is closed to external influences. Primitive thought patterns. Well, in fact that is not the case at all. All change begins in the Antarctic stratosphere. It is no accident that the entire southern hemisphere is something of an ‘ozone hole’.

Recent research (abstract below) suggests that ionisation due to cosmic rays in polar latitudes may be a pathway for the generation of ozone down to jet stream altitudes. If this is the case stratospheric warmings will be associated with the generation of ozone and the intensification of polar cyclone activity that lowers surface pressure across the entire polar cap impeding the flow of mesospheric air into the ozonosphere and, via the impact of enhanced ozone in columns of descending air in the mid latitudes, evaporating cloud and warming the surface of the planet. However, the solar wind conditions the ionosphere in such a way as to inhibit the flux of cosmic rays that reach the upper atmosphere. According to this construct the response to cosmic rays will tend to be greater at the low point of the solar cycle as the fluctuations in the solar wind are diminished at this time. There is in fact evidence in the incidence of the El Nino Southern Oscillation phenomenon that the climate system is particularly variable in terms of the distribution of atmospheric mass during solar minimums and it could be the cosmic ray mechanism that is responsible.

At this point it is important to note that the cosmic ray effect is dependent upon warming of the stratosphere that is in turn dependent on surface pressure over the polar cap. It is high surface pressure in winter that drives the zonal wind in the upper stratosphere bringing that tongue of cold mesospheric air into the polar stratosphere. A change in surface pressure results in an immediate change in the temperature of the air over the polar cap conditioning the process of ionisation by cosmic rays.

THE ENERGY INVOLVED IN IONIZATION PROCESSES

The wave lengths that are capable of ionising atmospheric gases represent a tiny part of the electromagnetic spectrum emitted by the sun. The EUV itself contributes an insignificant amount to TSI, only a few mW m−2 , as compared to 1360 W m−2 , or a few parts in a million. Inevitably these very short wave lengths are exhausted in the process and largely so above 80km in elevation. But these wave lengths vary tenfold in terms of their power over the solar cycle. It follows that the state of inflation of the ionised region is a direct function of solar activity within the eleven year cycle and over the longer 100 and 200 year intervals between individual solar cycles of very low strength of the sort that the Earth is currently experiencing.

During the satellite age we have seen a marked reduction in the incidence of EUV radiation in line with reduced sunspot activity. In consequence the elevation that is required to reduce atmospheric drag on satellites is reduced and satellite life has been extended well beyond design expectations. This is a direct consequence of a reduction in the output of EUV by the sun. Over this period the concentration of ozone in the stratosphere shows no such variation. It is plain that the ozone content of the stratosphere is independent of the output of short wave radiation from the sun that is responsible for the inflation of the ionosphere.

The diagram below is included to give a sense of scale. We see that the temperature of the upper atmosphere peaks at the 1 hPa level (50 km) with 99.9% of the atmosphere below. This is just below the level where the D region of the ionosphere manifests during daylight hours (60- 75km).

The temperature of the upper air from about 7km in elevation at the poles and 15km at the equator, is conditioned by the presence of ozone that absorbs in the infrared spectrum emitted by the Earth and its atmosphere.  The decline in the temperature of the air in the mesosphere that lies between 45 and 80 kilometres in altitude relates to the declining partial pressure of ozone. The increase in the temperature of the air beyond the mesosphere relates to energy gain in the process of ionisation. But remember that only one percent of the neutral atoms in the upper atmosphere are ionised. That is 1% of the 0.01% that is present above 1 hPa. It does not take a lot of atmosphere to exhaust the incident EUV wave lengths.

OZONE  CREATION IN LOW LATITUDES

Given that the ionic population in the D region exists in the main above 50 km in elevation we can infer that ozone is created in the main in the mesosphere that represents the transient tail end of the ionosphere.  Below the mesopause the population of ions is adequate to support chance encounters between atoms and molecules of oxygen to enable the synthesis of ozone, at least in daylight hours. Here the intensity of destructive radiation is so diminished (particularly at night and at low sun angles) as to allow the large ozone molecule a life. It is then diffused or carried to lower elevations in areas of descent. It follows that the ozone content of the atmosphere below the levels where ionisation is possible is a function of atmospheric dynamics, day length, chemical interactions and the seasonal existence of relatively ‘safe zones’ in high latitudes where the atmospheric path is long and the wave lengths in the UVB and UVC spectrum are so eroded that the atmosphere offers a safe haven for ozone.

The upshot is that the stratosphere in general represents a relatively ‘safe zone’ for ozone, and particularly so in the winter hemisphere. This interpretation is consistent with the observation that the ozone content of the atmosphere varies little across the solar cycle even though EUV varies tenfold. In trying to understand the Earth system one must always remember that the Earth is an orb that rotates about the sun taking 365.25 days and spins on an axis that is inclined 23.5° off a vertical that is at right angle off the plane of its orbit. At the top of the atmosphere irradiance varies by 6% across the year due to the elliptical nature of this orbit and is greatest in January when the Earth as a whole is coolest due to increased cloud cover. This is very different situation to a plane surface that is uniformly lit from vertically above.

Between 1 hPa and the upper limits of the mesosphere at about 80 km in elevation, the temperature of the air and its ozone content descends to a minimum. This minimum is called the mesopause. Beyond the mesopause, atmospheric temperature increases in line with the excitation of the atmospheric constituents by extreme ultraviolet radiation.

It should be borne in mind that the temperature of the atmosphere that contains ozone (between the mesopause and the surface of the planet) is in part a function of the energy absorbed by ozone in the infra-red and secondly due to the energy released by the disassociation of the ozone molecule as it is ionised. However there is in practice a more  influential factor at work. The temperature of the air in the stratosphere is mostly a function of the origin of the air as it moves vertically and laterally within the stratosphere. On a spherical surface that is not uniformly lit the temperature of the air very much depends upon its origin.

The notion that the stratosphere is a relatively safe place for ozone is supported by the following observations:

It appears that 40km in elevation over India is the point at which the atmospheric profile changes. Above 40km the night time partial pressure of ozone is greater than the day time  as one would expect if the pressure of ionization during daylight hours actively depleted ozone faster than it forms up. Below 40km in elevation, daytime values are higher than night time values indicating a relatively safe environment so far as ionisation is concerned.

CONVECTION IN THE STRATOSPHERE

In the diagram below we see that at the 1 hPa pressure level there is a cyclical accumulation and dissipation of ozone over centres where surface pressure tends to be low in winter (the oceans). This convective phenomenon occurs in the lee of   the continents and in particular, in 2016, over New Zealand in winter.  This particular cycle comes and goes in the space of 9-11 days and is convective in origin.  It is erroneously attributed to ‘Planetary waves’. In fact, the annular ring of high ozone values that surrounds the pole, strengthening in winter represents air of low density that is ascending to the top of the atmosphere, or at least to a level where 99.9% of the atmospheric mass is beneath.

In the northern hemisphere the Pacific Ocean tends to be the zone where low surface pressure promotes the accumulation and ascent of ozone rich air. The distribution of ozone at 1 hPa is seen below, across a similar cycle of convection in the northern hemisphere.

CONCLUSION

It is suggested that the existence and persistence of ozone in the stratosphere is in the main a response to the reduced pressure of ionisation below an elevation of about 40 kilometres over the equator. In the winter hemisphere ionisation via short wave radiation from the sun is not a factor of importance allowing ozone partial pressure to build. The influence of cosmic rays may be to build ozone levels at high latitudes and particularly so during stratospheric warmings. The distribution of ozone responds also to convective processes. The temperature of the air in the stratosphere will depend in the main on its response to radiation from the Earth itself rather than the process of ionisation. Air  from the mesosphere is cooler regardless of its ozone content.  It is well observed that air moving from low to high latitudes at the 100 hPa pressure level is cooler due to its lower ozone content. The stratosphere is warmer at the poles than at the equator due to enhanced ozone content even though the amount of infrared radiation that is available to energise ozone is much reduced. This tells us that the amount of radiation available to  energise ozone is never limiting, even at night. The air at the tropical tropopause, markedly deficient in ozone, is at a similar temperature to the air in the mesosphere, about minus 85°C.

It is the exhaustion of ionising radiation above the mesopause that allows ozone partial pressure to build at lower elevations. The partial pressure of ozone can only build when the ozone molecule is free from disassociation via wave lengths that are longer than the EUV wave lengths responsible for the ionosphere. In low latitudes this may be the case at about forty kilometres in elevation and it will be higher in mid and high latitudes. The atmospheric path is long enough to filter out the wave lengths that can disassociate ozone when the sun is low in the sky. During the polar night the atmospheric path is …….. somewhere else.

Due to the minute partial pressure of ozone that rarely exceeds 30 ppm, and only in very protective environments near the poles,  the surface of the planet is never completely free of radiation at the wave lengths that can disassociate ozone. It is the paucity of ozone in the southern hemisphere that is responsible for the pressure of damaging short wave radiation at the surface. The Andes Mountains experience particularly large amounts of energetic ultraviolet radiation due to their elevation.

The dilution of ozone via the descent of mesospheric air pre-conditions the entire southern hemisphere to an ozone deficit and is responsible for the weathered, leathery, ‘Australian skin’ and by contrast the extreme levels of melatonin in the skin of Australia’s very well adapted native peoples.

Part 2 describes the forces responsible for the erosion of ozone near the surface of the planet, the highly variable height of the tropopause and its lack of clear definition when observed on short time scales. It is seen that ozone partial pressure is greatest where ozone is free from erosive influences emanating from the surface of the planet.