Evaluation of accuracy in measurements of VOC emissions with dynamic chamber system

摘要

We determined the accuracy of the dynamic chamber system for measuring VOC emissions of trees and soil in the field at SMEAR Ⅱ station, Hyytiala, Southern Finland. The study was based on creating a known source of different VOCs inside the chamber and measuring the concentration dynamics in the chamber by a proton transfer reaction mass spectrometer (PTR-MS) connected to the chamber. The chamber system underestimated the artificially generated VOC emission rates at varying degree. In isoprene, monoterpene and many oxygenated VOCs the underestimation was 5-30% but in some compounds with protonated mass >100 amu the signal was almost completely lost. The underestimation mainly originated in the chamber but the length of the sample tubing also affected the observed magnitude of the standard gas emission. The systematic error was higher at high relative humidity than in drier conditions, which suggests that the thickness of the water film adsorbed on the chamber inner surfaces contributes to the VOC loss rate in the chamber. Increasing the length of the chamber closure time slightly reduced the systematic underestimation of emission in most of the studied compounds although the underestimation persisted for at least 10 min. A dynamic chamber setup enables measurement of VOC emissions with good accuracy and precision while minimising the disturbance of the chamber to the object being measured.