Aircraft-based volatile organic compounds flux measurements with relaxed eddy accumulation

Abstract

Regional estimates of fluxes of volatile organic compounds (VOCs) are required to improve our understanding of their role in the chemistry of the atmosphere. Flux measurements on such a scale can best be obtained using aircraft-based systems. These systems usually rely on the eddy covariance technique, which requires fast response gas sensors for flux measurement, but such sensors are not available for most organic compounds, therefore, the relaxed eddy-accumulation (REA) technique was selected. An aircraft-based REA sampling system was developed and used to measure isoprene emission over the boreal forest during the 1996 summer. Over a short period in July at the Boreal Ecosystem/Atmosphere Study (BOREAS) southern study area (SSA), the isoprene fluxes ranged from −0.06 to 1.79 μg m -2 s -1, with a mean of 0.59±0.34 μg m -2 s -1, while in August at the BOREAS northern study area (NSA) the isoprene fluxes ranged from 0.00 to 0.26 μg m -2 s -1, with a mean of 0.14±0.09 μg m -2 s -1. In the SSA, the isoprene fluxes over aspen ranged from 0.44 to 1.79 μg m -2 s -1, with a mean of 0.92±0.33 μg m -2 s -1, whereas over black spruce, isoprene fluxes ranged from −0.06 to 0.54 μg m -2 s -1, with a mean of 0.36±0.21 μg m -2 s -1. The isoprene fluxes were exponentially correlated with solar radiation and radiative surface temperature. High correlations between isoprene fluxes and the fluxes of CO 2 and latent heat were also observed. Carbon lost through isoprene emissions was about 0.7 and 0.8% of the CO 2 assimilation rate for aspen and black spruce, respectively. The results demonstrate that the aircraft-based relaxed eddy-accumulation technique is a promising approach for quantifying the atmosphere–surface exchange of VOCs on a regional scale.