Measurement of isoprene fluxes at the PROPHET site

As part of the Program for Research on Oxidants: Photochemistry, Emissions, and Transport (PROPHET), isoprene fluxes were measured in the surface layer immediately above the forest canopy by relaxed eddy accumulation (REA) and eddy covariance (EC). Isoprene profiles obtained from aircraft flights at higher altitudes allowed larger-scale isoprene flux estimates based on a mixed layer gradient modeling technique. Emission results derived from the three methods have been compared and used to generate standard emission factors for use in biogenic emission modeling. A standard emission flux of 11.4 mg m'2 h^-1 was determined for the canopy footprint region. The aspen and oak isoprene emitting biomass density in the footprint amounted to approximately 150 g m^-2 which, when combined with the standard flux, gives a standard emission rate of 76 μg g^-1 h^-1 for this northern Michigan forest. We found good agreement between isoprene fluxes determined by REA and EC methods. The general emission pattern is the same for both methods, and in many cases, the REA and EC fluxes were nearly identical. The mixed layer modeling approach gave isoprene fluxes that were consistent with those made at the same time at the canopy scale. The continuous coverage of isoprene fluxes by eddy covariance has provided more detailed insight into emission variability during the daytime period. During the midday period, canopy fluxes often changed significantly from one 30 min period to the next.