A technique for measuring CO2 and water vapor profiles within and above plant canopies over short periods

A multiport air sampling apparatus was developed to measure vertical profiles of CO2 and water vapor concentrations within and above the plant canopy over time intervals of as short as 5 min. Air is drawn continuously from six heights through tubings of equal length and diameter, each connected to a three-way solenoid valve. Flow through five of the six intake lines is driven by a large purge pump while the air from the selected sampling height is drawn by a small sample pump and pushed through an infrared gas analyzer (IRGA) for the simultaneous measurements of CO2 and water vapor concentrations. A data logger controls the solenoid valves and switches the air from each height sequentially through the IRGA. The internal volume of the apparatus is minimized to ensure a fast response in TRGA output when the sample air switches from one height to another. The response was shown to be essentially complete within 2 s or less. By delaying signal recording for 2 s after switching, one cycle of sequential measurements over the six heights is completed in 1 min, and 5 min mean profiles are obtained as the mean of five 8 s measurement periods spaced 1 min apart for each height. The short interval between each measurement period is essential when the scalar concentrations fluctuate or change rapidly. We compared the water vapor pressure within the upper part of a cotton canopy measured with our apparatus, with that measured by a precision, shielded and ventilated psychrometer made of platinum resistance thermometers. The results showed excellent agreement over the daily cycle between the two instruments for the 30-min mean data, and very good agreement for the 5-min mean data, showing coincidence of nearly all the 5-min peaks and troughs between the two methods. Sample diurnal patterns measured by the apparatus over a maize field spanning two consecutive days with contrasting wind speeds are given. The delay of nearly 2 s in response is mostly due to the internal volume of the apparatus. Taking the time required to clear this volume upon switching to a new port into account, it is possible to make CO2 measurements of slightly reduced accuracy at a rate of 1 s per port. The potential use of the apparatus for measuring at several heights over very short time intervals (in terms of seconds) is discussed in the context of the requirements for studying coherent turbulence structures and calculating the storage term in CO2 flux determinations. (C) 1999 Published by Elsevier Science B.V. All rights reserved.