Assessment of long tubing in measuring atmospheric trace gases: applications on tall towers

Abstract

Perfluoroalkoxy alkane (PFA) Teflon tubing has been widely used to draw air samples for analyzing atmospheric trace gases. However, impacts of long tubing on measurements of atmospheric trace gases have rarely been reported so far, especially for various organic trace gases. In this study, interactions between long tubing and various trace gases were assessed using a combination of laboratory tests, field experiments, and modeling techniques. A tower-based observation system of trace gases was also established using long tubes. The results show that measured concentrations of organic compounds required varying amounts of time (e.g., 10–474 s for a 400 m-long tubing at a flow rate of 13 standard liters per minute) to stabilize after traversing the tubing. Tubing delays of organic compounds were highly dependent on their saturation concentrations and the residence time in the tubing. In laboratory tests, there are no detectable losses of the targeted chemical species after traversing the 400 m-long tubing. In real applications, concentrations of nitric oxide (NO) cannot be well measured through long tubes in the daytime due to its low ambient concentrations and rapid consumption by ozone. Negligible losses were observed for most of the other targeted species when measured using long tubes. Measurements of various trace gases made by the vertical observation system can well characterize their concentrations and vertical distributions. However, the measurements of a trace gas made by the vertical observation system at each altitude cannot be used to interpret its temporal variability at time scales lower than its tubing delay. In addition to their usage on tall towers, long PFA Teflon tubes can be also used on mobile platforms (e.g., tethered balloons) for vertical measurements of atmospheric trace gases or to surveil emissions of targeted chemical species at multiple sites.