On-line monitoring of biogenic isoprene emissions using photoacoustic spectroscopy

Isoprene (C₅H₈) is one of the most important biogenic volatile organic compounds (VOCs) in the atmosphere. To calculate the impact of isoprene on atmospheric processes models have been developed that describe the isoprene release from plants. Measurements of this release require techniques for a fast, sensitive, on-line isoprene detection. Photoacoustic (PA) spectroscopy is applied here for the first time to monitor biogenic isoprene emissions. A CO overtone PA spectrometer is used for the detection, probing the C-H stretching vibrations in the 3 to 4 μm range. This allows us to detect isoprene down to a few ppb with a time resolution of one minute in a continuous gas flow. The number of laser lines can be adjusted to meet the requirements of the respective experiment in terms of time resolution and selectivity against other possibly interfering VOCs. This results in a highly versatile instrument for the isoprene detection in biological experiments. Furthermore, the infrared fingerprint offers the potential to detect different isoprene isotopomers simultaneously, thus allowing us to carry out on-line labelling experiments. The new apparatus was used to study the light dependence of isoprene emission from Eucalyptus globulus. The results demonstrate, that the detector system is a promising tool for the study of plant gas emissions. It allows the validation of existing emission models which are important for atmospheric processes. Received: 18 March 1999 / Revised version: 23 August 1999 / Published online: 30 November 1999