Motivation

Why do we need IAGOS CARIBIC ?

Scientists have too little detailed systematic information about the composition of the atmosphere. The problem is that the atmosphere is complex and very large.

Millions tons of pollutants are injected into the atmosphere every year. This man-made pollution, but also by nature itself (plants !) has to be cleaned-up by chemical reactions. The atmosphere has a healthy self-cleansing mechanism based on energy supplied by sunlight. From summer to winter, from the cold poles to the hot tropics, from the surface to great height, day and night, the composition of the atmosphere changes. It is often unconsidered that hundreds of chemical reactions take place in the air around us. To understand this complex system many measurements are needed.

The air around us looks just like air. Yet, invisible to the eye, many chemical reactions take place. This is a natural process. For instance, ozone is produced. High up in the stratosphere ozone protects life on earth against harmful ultra violet radiation. But destruction of ozone also takes place (ozone hole).

When you let some natural gas (mainly methane gas) escape into the atmosphere, what does happen to the molecules of this gas? Nearly each methane molecule is oxidized in the atmosphere. But this takes time. A methane molecule survives for about 8 years. When you spill some petrol while filling up at the petrol station, also these bigger molecules have also to be removed. These are attacked faster, and may be destroyed up in a few days.

Is the earth's atmosphere now really sick?

A little bit, at least. The main problems as we see it now are:

1. The decrease in stratospheric ozone due to halocarbons (CFC's). This mechanism has been proven.

2. The change in climate due to the increase in greenhouse gases is highly likely.
The earth surface seems to be warming, while there is very little change in solar radiation. At the same time we do know that greenhouse gases do have a warming effect, and we do know that greenhouse gases are increasing. Because of the extraordinary high importance of climate and climate change, global warming must be taken seriously and must be studied - CARIBIC does that.

What happens principally is that we have shifted natural equilibria in the atmosphere. For instance, because of all the polluting gases that we are releasing, the atmosphere cleans itself more slowly. Greenhouse gases like methane can built up. Yet, when we reduce emissions again, the atmosphere will most certainly recover itself! This is good news for the patient, and for us of course.
CARIBIC can help to supply information. For instance detailed measurements about methane are made.
Yet, some disturbances in the atmosphere will take a long time to heal. Furthermore, we can only hope that there is no irreparable damage to the complex system of the atmosphere, oceans, soils, global vegetation etc.

Focus

The importance of the tropopause region:

Use is made of a passenger aircraft for making frequent atmospheric chemistry measurements mainly in the tropopause region. Relatively few measurements are available for this important region. The tropopause is the division between the troposphere (this is the lower well mixed part of the atmosphere where the weather as we know it takes place) and the stratosphere (this is the not so well mixed, layered part of the atmosphere). In the stratophere temperatures increase with altitude due to the presence of the ozone layer.

First of all, the cross-tropopause transport mechanisms are not yet known in sufficient detail. This makes it for instance difficult to estimate reliably how much ozone is imported from the stratosphere.

Secondly, the chemistry in the tropopause region is not well documented. Pollutants uplifted from the boundary layer, and nitrogen oxides formed by lightning create a complex chemical mix in this region.

At last, but not least, aerosol (very small particles), the role of which presently receives increasing attention because of radiative forcing, and its role in cloud formation, is extremely poorly researched in all aspects like abundance, size distribution, and chemical composition. There is a great deal of uncertainty in the complex 3 dimensional chemistry transport models that are being continuously developed, improved, and refined to reconstruct better the essential aspects of the chemistry of the atmosphere. Difficulties range from incomplete chemistry to fundamental flaws in basic aspects like transport mechanisms. Measurements remain vital for validation (this is nice for experimentalists to know).

Research aircraft flights can provide targeted highly specialized or extensive chemical and physical surveys. Such projects are very expensive, and cannot cover larger parts of the global atmosphere at a desired frequency and affordable cost. Also true is that passenger aircraft based research cannot compete with the extensive complex capacity of research aircraft. But they clearly can help to obtain a vast amount of data as is demonstrated in a convincing way by the project MOZAIC, which entails the collection of ozone and water vapor data using 5 Airbus aircraft (Alain Marenco http://www.obs-mip.fr/services-observation/soa/MOZAIC-IAGOS).

CARIBIC is somewhere between research aircraft and MOZAIC. We deploy several automated analyzers and an air sampler in an air freight container. Another airliner atmospheric chemistry project we are aware of is: JAL (CONTRAIL, continuing), a Japanese Boeing 747 with an air sampling system.

The great strength of the CARIBIC container concept is that the instrument package is large, flexible and extendable. To some degree CARIBIC is a flying laboratory that analyses the air in great detail.

Topics

Main scientific research topics are:

a) To diagnose physical and chemical links between upper troposphere and lowermost stratosphere. Most air traffic takes place in this important region of the atmosphere. However, there are few measurements for this complex boundary layer area.

b) To quantify the frequency and influence of vertical transport (e.g. convection) from the ground to the upper troposphere. In particular in the tropics can polluted surface air be transported rapidly to high altitudes (vacuum cleaner effect ! Note that this concept was introduced by Chatfield and Crutzen already in 1984, Journal of Geophysical Research (JGR), vol. 89, pp 7111 - 7132).

c) To accumulate a comprehensive data set of trace gas variations. Because we take air samples that are analyzed in the laboratory, very precise measurements for the greenhouse gases can be made. Up to date, such data were only available for ground based stations, where pollution is a problem and the data are often not representative for large parts of the atmosphere.

A view on CARIBIC

August 2006:

1. Elemental analyses of particles

We will highlight different aspects of CARIBIC in a series of brief notes, explaining certain measurements and their relevance. We start with the work done in Sweden by Prof. Bengt Martinson and co-workers at the University of Lund.

The importance of CARIBIC lies in its capability to enable systematic atmospheric chemistry research over important parts of the globe. The atmosphere is a complex system and such information becomes increasingly relevant (cf. www.ipcc.ch).

Our work with CARIBIC is related to climate change. Although the proof of climate change, global warming, has become stronger and stronger, and the role of greenhouse gases is not contested, research on such complex issues should not weaken. For mankind, very much is at stake here.

Whereas greenhouse gases warm the surface of the earth, particles (the aerosol which is officially "solid or liquid particles suspended in a gas") often have a cooling function. The study of particles is complex. How many are there? How big are they? What surface properties do they have (black, brown, white reflective ?) How are they formed? What is in them? How do gases react with them?

Bengt Martinsson uses an ultra sensitive method to analyse the chemical elements present in particles he collects with his aerosol collection apparatus on board of CARIBIC (see instrumentation-impactor on this website). Each flight 16 samples are collected. The collection of one sample takes about 100 minutes. Despite his ultra sensitive method, this time is needed to collect enough particles. For a description of the aerosol sampler apparatus please see the detailed technical paper by Dr. Hung N. Nguyen.

When the CARIBIC aircraft has landed, back in the laboratory we carefully take the small ultra clean foils from his apparatus in a clean-room working area and send then to Lund. In Lund, Nuclear Physics department the samples are analysed using PIXE analysis, in which a particle beam (protons) from a nuclear particle accelerator impinges on the aerosol material on the foil in a vacuum chamber. The elements (Sulfur, Potassium, Iron,..) that are in the chemical compounds of the particles emit characteristic X-rays, which are then detected. The strength of the X-ray response informs us about the amount. The frequency ("color") of the X-ray, which element we are dealing with.

To give an idea: On a single foil is typically 50 nanogram aerosol material collected. This is hit by 120 nano Amperes protons for 15 minutes. Bengt Martinsson and his colleagues can detect in this way as little as 0.3 nanogram sulfur. But he also can detect for instance Calcium which is an indicator for particles that carry material from rocks and soils.

Sulfur is especially interesting, because sulfur containing particles (it is in the form of sulphuric acid and sulfates!!) have a cooling effect, by reflecting radiation back into space. There is a natural layer of sulfate particles in the stratosphere called "Junge Layer". By analysing sulfur for the CARIBIC flights, Bengt can make a budget for sulfur in the region of the atmosphere where CARIBIC flies. This is a new and very useful information. Is there much sulfate ? Well, the last major volcanic eruption was the in the Philipines, Mount Pinatubo, in June 1991. For years after the eruption, that reached into the stratosphere, the earth´s atmosphere was considerably disturbed, and the scattering of the sulfate particles caused beautiful red sunsets, but also changes in ozone It is only a matter of time that another explosive type of volcanic eruption takes place. This once again will affect processes in the atmosphere, and we scientist ought to be prepared to measure what goes on and to test this against the understanding we have of the complex atmospheric system.

Yet, today sulfate levels in the stratosphere are low. Also in the troposphere levels are not affected by volcanic emissions, but rather by natural emissions of sulphurous gases from the oceans, and by emissions from the burning of fossil fuels. When sulfur is not removed from oil, and coal or from the waste gases of power plants for instance, we humans keep loading the troposphere with sulfate. It is important to find out how much sulfate there is and in particular to find out "How much of this sulfate is due to human activities, i.e. traffic, industry, heating, power generation", and how much is really natural, for instance due to the break down of sulfurous gases emitted by seawater

The cooling effect of sulfate particles is well known, and in a recent, but already hotly debated article, Paul Crutzen (Nobel prize 1995) points out that the property of sulfate particles could be used to cool our planet . . . Whatever this debate may bring, the adventure of mankind should be guided by knowledge. Science is totally reliant on its empirical basis. In plain words, we have to measure and understand.

This is exactly what we do with CARIBIC, with the impressively sensitive, careful analyses of Bengt Martinsson and co-workers helping to unravel the budget of sulfur, and other elements in particles in the atmosphere.

Scientific Arguments

CARIBIC uses a passenger jet airliner, which means that the altitude of our research flights (apart from ascending and descending) is about 10 to 12 km. This is the altitude at which modern jet aircraft function optimally.
Interestingly just at this altitude, we cruise in the UTLS region (Upper Troposphere-Lower Stratosphere region) for most of the aicraft movements. In the tropics we clearly fly in the free tropical troposphere well below the tropopause.

Didier Hauglustaine has clearly summarized the importance of the UTLS region in a brief article in the IGAC newsletter Issue number 26, June 2002. . . . The upper-troposphere / lower-stratosphere (UTLS) is still considered as a key region of the atmosphere as far as composition and climate interactions are concerned for the following reasons:
 

• Radiative forcing by greenhouse gases such as water vapor and ozone remains especially sensitive in the UTLS to concentration changes, due to large temperature contrast with the surface.

• The UTLS is the layer in which stratospheric and tropospheric air are mixed. The exchange between these atmospheric domains controls the influx of tracers into the stratosphere (including water vapor and long-lived greenhouse gases), and the O₃ and NO_x flux from the stratospheric reservoir down into the troposphere.

• Chemistry in the lower stratosphere is very sensitive to temperature changes and, at high latitudes, to the presence of Polar Stratospheric Clouds (PSCs). Stratospheric ozone concentration and temperature changes are directly coupled providing an important climate-chemistry interaction. The UTLS also coincides with the height of transition between positive and negative trends as detected by ozone sondes.

• Due to the influence of rapid convection within cloud structures and the large-scale vertical transport associated with convergence, the imprints of lower tropospheric events such as biomass burning and forest fires and several regional air pollution episodes are also imposed on the upper troposphere.

• In this relatively dry region, species like peroxides and oxygenated hydrocarbons play an important role in generating HO_x radicals. Heterogeneous chemical reactions on cirrus clouds may also affect the budget of ozone and other species at these altitudes.