A new disjunct eddy-covariance system for BVOC flux measurements –validation on CO2 and H2O fluxes

[摘要] The disjunct eddy covariance (DEC) method is an interesting alternative tothe conventional eddy covariance (EC) method because it allows theestimation of turbulent fluxes of species for which fast sensors are notavailable. We have developed and validated a new disjunct sampling system(called MEDEE). This system is built with chemically inert materials. Airsamples are taken quickly and alternately in two cylindrical reservoirs,the internal pressures of which are regulated by a moving piston. The MEDEE system was designedto be operated either on the ground or aboard an aircraft.It is also compatible with most analysers since ittransfers the air samples at a regulated pressure. To validate thesystem, DEC and EC measurements of CO₂ and latent heat fluxes wereperformed concurrently during a field campaign. EC fluxes were firstcompared to simulated DEC (SDEC) fluxes and then to actual DEC fluxes.

Both the simulated and actual DEC fluxes showed a good agreement with ECfluxes in terms of correlation. The determination coefficients (R²) were0.93 and 0.91 for DEC and SDEC latent heat fluxes, respectively. For DEC andSDEC CO₂ fluxes R² was 0.69 in both cases. The conditions of lowfluxes experienced during the campaign impaired the comparison of thedifferent techniques especially for CO₂ flux measurements. Linearregression analysis showed an 14% underestimation of DEC fluxes for bothCO₂ and latent heat compared to EC fluxes.

A first field campaign, focusing on biogenic volatile organic compound (BVOC)emissions, was carried out to measure isoprene fluxes above a downy oak(Quercus Pubescens) forest in the south-east of France. The measuredstandard emission rate was in the lower range of reported values in earlierstudies. Further analysis will be conducted through ground-based and airbornecampaigns in the coming years.

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