A disjunct eddy accumulation system for the measurement of BVOC fluxes: instrument characterizations and field deployment

[摘要] Biological volatile organic compounds (BVOCs), such as isoprene andmonoterpenes, are emitted in large amounts from forests. Quantification ofthe flux of BVOCs is critical in the evaluation of the impact of thesecompounds on the concentrations of atmospheric oxidants and on theproduction of secondary organic aerosol. A disjunct eddy accumulation (DEA)sampler system was constructed for the measurement of speciated BVOC fluxes.Unlike traditional eddy covariance (EC), the relatively new technique ofdisjunct sampling differs by taking short, discrete samples that allow forslower sampling frequencies. Disjunct sample airflow is directed intocartridges containing sorbent materials at sampling rates proportional tothe magnitude of the vertical wind. Compounds accumulated on the cartridgesare then quantified by thermal desorption and gas chromatography. Herein, wedescribe our initial tests to evaluate the disjunct sampler including theapplication of vertical wind measurements to create optimized samplingthresholds. Measurements of BVOC fluxes obtained from DEA during itsdeployment above a mixed hardwood forest at the University of MichiganBiological Station (Pellston, MI) during the 2009 CABINEX field campaign arereported. Daytime (09:00 a.m. to 05:00 p.m. LT) isoprene fluxes, when averaged over thefootprint of the tower, were 1.31 mg m⁻² h⁻¹ which are comparable toprevious flux measurements at this location. Speciated monoterpene fluxesare some of the first to be reported from this site. Daytime averages were26.7 μg m⁻² h⁻¹ for α-pinene and 10.6 μg m⁻² h⁻¹for β-pinene. These measured concentrations and fluxes werecompared to the output of an atmospheric chemistry model, and were found tobe consistent with our knowledge of the variables that control BVOCs fluxesat this site.

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