Measurements of biogenic VOC emissions: sampling, analysis and calibration

We describe an experimental sq stem and techniques for sampling and analyzi ng biogenic emissions of volatile organic compounds (VOC). The system uses a Teflon chamber to enclose a single branch of a tree. Temperature, photosy nthetic active radiation (PAR), relative humidity and carbon dioxide concen tration are continuously monitored with a time resolution of five minutes. VOCs are sampled on tubes containing solid adsorbents (Tenax TA and Carbotr ap) with a time resolution of 1 h. Composition and concentration of VOC emi ssions are measured with a gas chromatographic system equipped with a flame ionization detector (FID) for quantitative and a mass spectrometer (MS) fo r qualitative analysis. To calibrate the system, a diffusion source was bui lt to produce standard mixtures of up to 36 different compounds with mixing ratios at low concentrations and high accuracy. The diffusion rates were m onitored over 17 months and shelved variations between 0.2 and 7.6% for mon oterpenes (expect for alpha -phellandrene, alpha -terpinene and gamma -terp inene) and between 10.6 and 22.6% for sesquiterpenes. FID response factors calculated from calibration measurements were corrected using correction fa ctors based on the effective carbon number concept. The individual response factors of 23 compounds were combined to a mean response factor (RFm) with a value of 23,100 muV s ng(-1) and a standard deviation of 9%. The system described here was used to measure VOC emission rates of Scots pine (Pinus sylvestris) in 1998 and 1999. (C) 2001 Elsevier Science Ltd. All rights res erved.