Public Release: 

Smog over the Mediterranean


In the summer of 2001, the Mediterranean Intensive Oxidant Study (MINOS), made up of an international team of scientists from eight countries, under the leadership of the Max Planck Institute for Chemistry in Mainz, investigated the Mediterranean atmosphere over a six-week period. The scientists measured the chemical composition and the energy budget of the Mediterranean troposphere (the part of the atmosphere where the weather and interactions with the earth's surface take place) both from a ground-station and with the use of aircraft. The objective was to study long-range transport of air pollution and its effects on air quality and climate. As reported in Science, (25th October 2000) the outcome indicates that pollution decreases air quality on a large scale and reduces precipitation.

The operational base of MINOS was Crete, where a monitoring station was equipped to measure a large number of gaseous and particulate air constituents as well as solar radiation transfer to the earth's surface. The measurement campaign was hosted by the University of Crete in Heraklion. Two aircraft, a twinjet Falcon from the German Aerospace Center (DLR) and a King Air from Tel Aviv University, performed more than 20 research flights to sample the air over the Mediterranean Sea. The tropospheric energy budget was measured by a research group from the University of California in San Diego.

The measurements provide evidence for a remarkably high level of air pollution from the surface to the top of the troposphere at 11-15 km altitude (see Fig. 1). The greatest amount of pollution was observed in the lower 4 km, originating from both western and eastern Europe (see Fig. 2). Industrial activity, traffic, forest fires, agricultural and domestic burning are the source of this pollution. Since the Mediterranean region has relatively few clouds in summer, the solar radiation levels are high so that noxious reaction products are formed in a photochemical smog. The resulting air pollution includes ozone and microscopic particles called aerosols.

At higher altitudes, above 4 km, long-range pollution transport from North America and Asia was a major contributor to pollution. This pollution transport followed the prevailing westerly winds that are typical outside the tropics. Another pollution layer was discovered in the upper troposphere, above 8 km altitude, over the eastern Mediterranean Sea. This pollution originates from South Asia and is lifted to the upper troposphere by thunderstorms in the Indian monsoon. Subsequently it follows the easterly tropical jet stream and turns north over the eastern Mediterranean. From there it can even penetrate the lower stratosphere, thus carrying Asian pollution into the lower part of the ozone layer (see Fig. 3). The effects are yet unclear.

Near the surface the air pollution has several undesirable consequences. Firstly, the European Union eight-hourly air quality standard for ozone (110 microgram per cubic meter) is exceeded throughout the summer in most Mediterranean regions (see Fig. 4). Secondly, high ozone concentrations are harmful for ecosystems and human health. In the former they cause agricultural crop loss. In the latter the concentrations of aerosols are high and affect human health by carrying toxic products from combustion processes into the lungs.

Furthermore, aerosols affect the Mediterranean atmospheric energy budget by scattering and absorbing solar radiation. The microscopic particles, which are composed for example of sulphate and soot, reduce solar radiation absorption by the sea by approximately 10% and they alter the heating profile of the lower troposphere. As a consequence, evaporation and moisture transport, in particular to North Africa and the Middle East, are suppressed. This aerosol effect is substantial today, although sulphate from Europe has actually decreased in the past two decades through the abatement of acidification. Therefore, it is likely that the climate effects of the aerosols have been even much larger in the past. The period with highest aerosol concentrations over the Mediterranean Sea was around 1980 and contributed to the drought in the eastern Sahel. It thus appears that aerosols contribute substantially to climate and water cycle changes.


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