Physical description of transport processes inside an open top chamber in relation to field conditions.

The impact of ozone, nitrogen oxides and other air pollutants on plants in natural vegetations cannot be described by simple exposure-yield response relationships. Outdoor experiments in which plants are exposed to ambient air pollutant levels yield conflicting results from year to year because the interacting environmental effects are complex and not constant. The best way to study the effects of air pollutants on plants is still one of the issues scientists are dealing with. The experimental design plays an important role in extrapolating the experimental results to real field conditions. To determine the impact of air pollutants on a crop requires growing conditions which are as natural as possible and the simultaneous control of ambient pollutant levels. The Open Top Chamber is one of the experimental set-ups to study the effect of air pollutants on plants. The original objective of the OTC design was to create an environment closely resembling field conditions while, at the same time, allowing control of air quality. The U.S. NCLAN programme was the first to use OTC's on a large scale. Nowadays the use of OTC's shifts from air pollutant exposure experiments to carbon dioxide fumigation experiments. OTC's have been used by KEMA to study the effects of air pollution on natural vegetation, in particular on the biomass production of agricultural crops in The Netherlands. The chamber consists of a cylindrical construction (diameter: 3 m; height: 3 m) with an open top through which filtered or non-filtered air is blown by a fan. Gaseous air pollutants can be added simultaneously to the incoming air. Experimental results have shown differences in plant growth and development between non-filtered OTC's and ambient plots. It is obvious that the use of OTC's introduces a degree of artificiality which makes it impossible to duplicate field conditions exactly. The major aims of the present study are: - to give a physical description of the transport processes (mass, momentum and energy) inside a KEMA-type OTC in relation to field conditions, - to determine the influence of the ventilation rate and the atmospheric boundary layer on the turbulent transport processes inside an OTC. The study is composed of a theoretical as well as an experimental part. An OTC identical to the KEMA-type OTC was situated at the weather station of the Agricultural University's Meteorological Department. Broad beans were grown in the chamber. Wind velocity, temperature, humidity, radiation and concentrations of gaseous air pollutants were continuously measured at various positions inside and outside the OTC. Turbulence parameters were measured occasionally (wind velocity, temperature fluctuations). The experimental results have been used to derive the prior conditions needed for the two models used and served as a tool to verify the simulation results. The first model simulates the turbulent exchange processes inside and outside the OTC. The model has been used to determine the influence of the OTC environment, ventilation rate and plants on mean air flow and turbulence inside the chamber. The second model is a resistance model which has been developed primarily to calculate the mass and energy fluxes to or from various parts of the OTC system. The model yields information about the temperature, humidity and ozone concentration inside the chamber in relation to field conditions. A special module calculates the bulk exchange processes between soil-vegetation and atmosphere inside as well as outside an OTC. We conclude that typical OTC effects on the chamber microclimate such as - filter effects, - a 10-20% reduction of global and PAR radiation load, an increase of long-wave radiation intensity - a mean temperature increase of the order of 1 K, - a mean vapour pressure deficit increase of the order of 1 mbar, cannot be prevented without creating a kind of laboratory chamber. Furthermore, the typical non-natural flow pattern cannot be changed without removing the whole OTC- configuration. The exposure- response curves derived under OTC conditions cannot be used directly to estimate a quantitative effect under field conditions. An extrapolation of the exposure-response curves derived under OTC conditions to field conditions taking into account the differences in microclimate between OTC and field conditions can improve the quality of the experimental results.