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 laye r immediately above the forest canopy by relaxed eddy accumulation (REA) an d eddy covariance (EC) Isoprene profiles obtained from aircraft flights at higher altitudes allowed larger-scale isoprene flux estimates based on a mi xed layer gradient modeling technique. Emission results derived from the th ree methods have been compared and used to generate standard emission facto rs 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 an d oak isoprene emitting biomass density in the footprint amounted to approx imately 150 g m(-2) which, when combined with the standard flux, gives a st andard emission rate of 76 mug g(-1) h(-1) for this northern Michigan fores t. 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 m odeling approach gave isoprene fluxes that were consistent with those made at the same time at the canopy scale. The continuous coverage of isoprene f luxes by eddy covariance has provided more detailed insight into emission v ariability during the daytime period. During the midday period, canopy flux es often changed significantly from one 30 min period to the next.