Eddy Correlation Measurements of the Resistance to Vertical Transport of Ozone.

Abstract

The major objective of this research project was to measure the resistance to ozone uptake over a variety of surfaces and under a variety of conditions. These resistance data along with new data on reaction rate coefficients and the concentration of trace species can be incorporated into increasingly sophisticated models to provide a better underst and ing of the tropospheric ozone budget. The ozone flux F was determined by computing the covariance between the vertical wind speed w and the ozone concentration c using both analog and digital methods, by means of F = -c'w', where the overbar represents a temporal average and the prime indicates deviation from the average value of the respective quantities. The vertical wind speed was measured with a vertical propeller anemometer. The ozone concentration was measured with a chemiluminescent detector in which ozone in the ambient air was reacted with an excess of nitric oxide NO. Data were adjusted to compensate for the inadequate response of the sensors to high frequency fluctuations, delay times caused by physical separation of the sensors and ozone transit through its sampling tube, and air density effects. The total resistance to vertical transport of ozone was computed for sampling periods up to one half-hour long by dividing the average ozone concentration by the vertical flux. Measurements made over a grass field showed a gradual decrease in the total resistance over an eight-week period in the spring. The data collected under overcast skies and before the grass had begun to turn green, indicated ozone destruction by non-transpiring surface material. The average resistance for the eight-week period was 2.3 s/cm. Eddy correlation measurements made over a mature maize crop showed a strong diurnal trend with minimum resistance occurring at midday. The average total resistance, 2.4 s/cm, was nearly the same as that measured over grass; however, the flux was about twice as large as that measured over grass since ozone concentrations were two times higher. Data collected over soybeans showed that the effective bulk surface resistance was highly correlated to the canopy resistance to water vapor transport, indicating the important role stomata play in controlling ozone uptake.