Airborne Measurements of CO2 Exchange above a Heterogeneous Northern-latitude Forest

Abstract:

Northern latitude forests represent an important global sink for carbon dioxide (CO₂). Estimating the landscape-scale exchange of CO₂ is complicated by the heterogeneity of forested areas. Airborne eddy-covariance measurements can complement continuous tower-based measurements for determining the magnitude and spatial variability of carbon uptake in forested areas, and to assess means for scaling-up. While aircraft provide accessibility, the resulting flux measurements represent a narrow time slice, and average over a comparatively large source area. The goal of this study is to improve our ability to attribute aircraft flux data to finer spatial scales. We hypothesize that this can be achieved by (i) improving the spatial scale of the sampling method, (ii) examining inter-day variability, and (iii) relating airborne eddy-covariance flux estimates to remote sensing determinations of the land cover.

For this purpose identical flight experiments were conducted on May 29 and June 1, 2014 over a 240 km² region encompassing the Harvard University EMS eddy flux tower at Harvard Forest, MA, using the Purdue University ALAR aircraft. In the early afternoon of each day, 19 flight legs, 20 km in length, were flown over the heterogeneous forest canopy. The two replicate experiments allow assessment of inter-day variability in CO₂ exchange under similar meteorological conditions. Furthermore, the experiments were coordinated with high-resolution (≤1 m) and medium-resolution (≤100 m) remote sensing retrievals of forest canopy structure and composition (NEON AOP) and soil moisture (NASA AirMOSS), respectively. This unprecedented hierarchy of observations enables evaluation of the ability of different data processing approaches to calculate finer scale CO₂ exchange with the surface.

Analyses of the flights conducted on May 29 and June 1 show a transect-averaged (± 1σ) CO₂ uptake of 13 ± 3 µmol m^-2s^-1 and 11 ± 2 µmol m^-2s^-1, respectively. In complement to the aircraft-based estimates, tower measurements averaged over the course of the flights show a CO₂ uptake of 6 ± 2 µmol m^-2s^-1 and 4 ± 1 µmol m^-2s^-1, respectively. We investigate the environmental parameters and differing source areas that may contribute to inter-day variability in flux estimates. Results and recommendations will be discussed further.