Open-top chambers to study air pollution impacts in South Africa. Part I: microclimate in open-top chambers

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Abstract
South Africa's large industrial growth has created an obvious need to extend, improve and evaluate air pollution impacts on plants in order to assess present-day and future impacts of air pollutants on crop yields. In order to provide guidelines for policy-making and the imposition of air quality guidelines, there is a need to quantify effects and to develop dose–response relationships for crops. Plant responses to define such exposure–response relationships could include physiological, biochemical, growth and yield parameters measured in plants grown under different pollutant levels using controlled fumigation techniques. The experimental tool that has been regarded as the most acceptable for dose–response research is open-top chambers (OTC). Open-top chambers provide a compromise between free air fumigation, which is generally regarded as expensive, and closed growth chambers, which are considered artificial. This paper reports on the design and operation of Africa's first OTC facility, which consists of 12 chambers, each having a volume of 5 m3. Evaluation of the OTC microclimate during the summer of 2007 revealed light transmission of more than 90% photosynthetically active radiation. Effective ventilation at about 2.5 air changes per minute limited the chamber temperature to a maximum temperature elevation of 4.2°C at 12:00 in summer, 1°C at night and a overall mean increases of 2.3°C. The relative humidity in the OTCs showed a 20% decrease at 12:00. The SO2 and O3 concentrations could be controlled accurately in the ppb range as is often recorded near power stations in South Africa.