EVALUATING THE EFFECTS OF ELEVATED LEVELS OF ATMOSPHERIC TRACE GASES ON HERBS AND SHRUBS - A PROTOTYPE DUAL ARRAY FIELD EXPOSURE SYSTEM

In the context of global climate change, an understanding of the long-term effects of increasing concentrations of atmospheric trace gases (carbon dioxide, CO2, ozone, O-3, oxides of nitrogen, NOx etc.) on both cultivated and native vegetation is of utmost importance. Over the years, under field conditions, various trace gas-vegetation exposure methodologies with differing advantages and disadvantages have been used. Because of these variable criteria, with elevated O-3 or CO2 levels, at the present time the approach of free-air experimental-release of the gas into study plots is attracting much attention. However, in the case of CO2, this approach (using 15 m diameter study plot with a single circular array of vent pipes) has proven to be cost prohibitive (about $59000-98000/year/replicate) due to the consumption of significant quantities of the gas to perform the experiment (CO2 level elevated to 400 ppm above the ambient). Therefore, in this paper, we present a new approach consisting of a dual concentric exposure array of vertical risers or vent pipes. The purpose of the outer array (17 m diameter) is to vent ambient air outward and toward the incoming wind thus providing an air curtain to reduce the velocity of that incoming wind to simulate the mode or the most frequently occurring wind speed at the study site. The inner array (15 m diameter) vents the required elevated levels of trace gases (CO2, O-3, etc.) into the study plot. This dual array system is designed to provide spatial homogeneity (shown through diffusion modeling) of the desired trace-gas levels within the study plot and to also reduce its consumption. As an example, while in the single-array free-air CO2-release system the consumption of CO2 to elevate its ambient concentration by 400 ppm is calculated to be about 980 tons/year/replicate, it is estimated that in the dual array system it would be approximately 590 tons/year/replicate. Thus, the dual array system may provide substantial cost savings ($24000-39000/year/replicate) in the CO2 consumption ($60-100/ton of CO2) alone. Similarly, benefits in the requirements of other trace gases (O-3, NOx, etc.) are expected, in future multivariate studies on global climate change.