Eddy covariance measurement of biogenic oxygenated VOC emissions from hay harvesting

Biogenic oxygenated volatile organic compound (VOC) fluxes have been directly measured by eddy covariance using the combination of a fast response, real-time VOC sensor and an acoustic anemometer. VOC detection is based on proton-transfer reaction mass spectrometry which has currently a response time of ca. 0.8 s and the system is suitable for making nearly unattended, long-term and continuous measurements of VOC fluxes. The eddy covariance system has a detection limit, for most VOCs, of less than 0.1 mgm−2 h−1. The system was field tested above a hayfield near St. Johann, Austria where cut and drying grasses released a variety of VOCs. High fluxes were observed for about 2 days after cutting and were dominated by methanol (1–8.4 mgm−2 h−1), acetaldehyde (0.5–3 mgm−2 h−1), hexenals (0.1–1.5 mgm−2 h−1) and acetone (0.1–1.5 mgm−2 h−1). The eddy covariance measurements generally agreed with flux estimates based on enclosure measurements and surface layer gradients. The sensitivity and selectivity of the system make it suitable for quantifying the fluxes of the dominant biogenic VOCs from a variety of landscapes and sources.