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.