Paul Crutzen

The Importance of the Tropics in Atmospheric Chemistry and Climate


Abstract

Trospospheric ozone, although present in average volume mixing ratios of only about 30 nmol/mol, is of the greatest importance for atmospheric chemistry. The absorption of solar ultraviolet radiation of wavelenghts less than about 340nm by ozone gives rise to electronically excited oxygen atoms which have enough energy to react with water vapour to produce hydroxyl (OH) radicals. The OH radicals, although present in the stratosphere at an average volume mixing ratio of only 4 x 10–14 (corresponding to a molecular density of 106 molecule/cm3), is the main compound that oxidies, and thereby removes, most of the gases that are released into the atmosphere by natural processes and human activities. Hydroxyl radicals are therefore the “detergent” of the atmosphere.

High humidities, combined with a year-around minimum in the vertical ozone columns in the tropics(resulting in maximum intensity of photochemically active solar ultraviolet radiation) lead to maximal production and abundance of OH radicals in the tropics. Large fractions of a great number of trace gases are, therefore, removed from the atmosphere in this part of the world. Because rainfall and water vapour also in general maximize in the tropics, the tropics thus play an extremely important role in cleansing the atmosphere. The chemistry of the tropics is also influenced by large natural emissions of organic and other biogenic gases.

With these facts in mind, one might expert that the tropics are the cleanest part of the atmosphere. This is, however, not generally true. Some parts of the tropics and subtropics are heavily influenced by mostly anthropogenic biomass burning as well as growing fossil fuel burning. The impact on the distributions of CO, O3 and other gases, and aerosol are even clearly noted from space. In a recent international field campaign (INDOEX) large concentrations of highly sunlight absorbing aerosol, containing 10-15 % of soot, were found during the northeast, winter monsoon period in 1999 over large tracts of India and the Indian Ocean north of the Intertropical Convergence Zone (the meterological equator, leading to more than 10% loss of sunlight over the ocean and more than that over the Indian subcontinent. This carries the possibility of significant effects on marine biospheric processes, evaporation from the ocean, cloud formation, precipitation, climate, agricultural productivity, and health in the highly polluted regions, including indoor air pollution. Note that much of the present pollution is produced by poor people who have no access to clean energy sources and are forced to burn anything they find, in particular vegetation. Current very uncertain estimates are that 2-5 thousand million tonnes of biomass are burned each year in the developing nations of the tropics and subtropics, producing high levels of pollution also over rural areas. Even the southern hemisphere is strongly affected during its dry season, which lasts from about August to late October.

In future, expanding populations as well as growing agricultural and industrial activities will, even more strongly than now, affect the tropics and subtropics with possible substantial repercussions for atmospheric chemistry and climate. We emphasize the need for much enhanced research in the atmospheric chemistry in the tropics and subtropics and the involvement of local scientists, supported by scientists from the developed world.


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