Air pollution-derived trichloroacetic acid contributes to degradation of vegetation in South Africa

During the past five decades, the expansion of deserts has accelerated in many regions of the earth including sub-Saharan Africa. Both climatic changes and the over-exploitation of natural resources in these regions have been considered to be responsible for this phenomenon. Recent data, however, revealed an additional factor that may contribute to desertification through degradation of the vegetation in the areas concerned. This factor is the organic compound trichloroacetic acid (TCA; CCl 3 COOH), that has its origin in an enhanced presence of several C 2 -chlorohydrocarbons (C 2 -CHCs) in the atmosphere. In the present study the burden on the vegetation resulting from C 2 -CHCs pollutants was assessed by sampling the concentration of TCA in pine needles along a 600-km air pollution gradient, ranging from highly industrialized areas in Gauteng to rural areas in the eastern part of South Africa. Parallel measurement of the TCA content of pine needles and their vitality over an air pollution gradient ranging from Potchefstroom eastwards towards Sasolburg and further south in the direction of Heilbron, revealed a decline in the TCA content of the needles with increasing distance from Sasolburg in the Vaal Triangle, a 'hot spot' of anthropogenic air pollution in South Africa. An inverse correlation was found between TCA content and the photosynthetic functioning of the pine needles. Studies in South Africa and Russia have shown that large vegetation fires lead to a substantial increase in the C 2 -chlorohydrocarbon content of needles of pine trees growing in the lee of the fire zone.We also discuss the distinctive air transport patterns over southern Africa of pollutants such as perchlorethene and their phytotoxic decomposition product TCA, and their possible role as an additional stress factor in the degradation of vegetation, so enhancing desertification.