Eddy covariance flux measurements of biogenic VOCs during ECHO 2003 using proton transfer reaction mass spectrometry

[摘要] Within the framework of the AFO 2000 project ECHO, two PTR-MS instrumentswere operated in combination with sonic anemometers to determine biogenicVOC fluxes from a mixed deciduous forest site in North-Western Germany. Themeasurement site was characterised by a forest of inhomogeneous composition,complex canopy structure, limited extension in certain wind directions andfrequent calm wind conditions during night time. The eddy covariance (EC)technique was applied since it represents the most direct flux measurementapproach on the canopy scale and is, therefore, least susceptible to thesenon-ideal conditions. A specific flux calculation method was used to accountfor the sequential multi-component PTR-MS measurements and allowing anindividual delay time adjustment as well as a rigorous quality control basedon cospectral analysis. The validated flux results are consistent with lightand temperature dependent emissions of isoprene and monoterpenes from thisforest, with average daytime emissions of 0.94 and 0.3µg m^-2s^-1, respectively. Emissions of methanol reached onaverage 0.087µg m^-2s^-1 during daytime, but fluxeswere too small to be detected during night time. Upward fluxes of theisoprene oxidation products methyl vinyl ketone (MVK) and methacrolein(MACR) were also found, being two orders of magnitude lower than those ofisoprene. Calculations with an analytical footprint model indicate that theobserved isoprene fluxes correlate with the fraction of oaks within thefootprints of the flux measurement.

[学科分类] 大气科学