Abstract
Abstract Airborne flux measurements of isoprene were performed over the Californian oak belts surrounding the Central Valley. The authors demonstrate for the first time 1) the feasibility of airborne eddy covariance measurements of reactive biogenic volatile organic compounds; 2) the effect of chemistry on the vertical transport of reactive species, such as isoprene; and 3) the applicability of wavelet analysis to estimate regional fluxes of biogenic volatile organic compounds. These flux measurements demonstrate that instrumentation operating at slower response times (e.g., 1–5 s) can still be used to determine eddy covariance fluxes in the mixed layer above land, where typical length scales of 0.5–3 km were observed. Flux divergence of isoprene measured in the planetary boundary layer (PBL) is indicative of OH densities in the range of 4–7 × 106 molecules per cubic centimeter and allows extrapolation of airborne fluxes to the surface with Damköhler numbers (ratio between the mixing time scale and the chemical time scale) in the range of 0.3–0.9. Most of the isoprene is oxidized in the PBL with entrainment fluxes of about 10% compared to the corresponding surface fluxes. Entrainment velocities of 1–10 cm s−1 were measured. The authors present implications for parameterizing PBL schemes of reactive species in regional and global models.
Highlights
A number of direct flux measurements of conserved tracers have demonstrated their usefulness to investigate the horizontal and vertical structure of the atmosphere utilizing aircraft (e.g., Lenschow et al 1980; Desjardins et al 1997; Metzger et al 2012)
Determined Damko€hler numbers were in the range of 0.3–0.9
Concentration gradients throughout the planetary boundary layer (PBL) are modified as a result of chemical reactions leading to locally smaller diffusion coefficients
Summary
A number of direct flux measurements of conserved tracers (e.g., heat, water) have demonstrated their usefulness to investigate the horizontal and vertical structure of the atmosphere utilizing aircraft (e.g., Lenschow et al 1980; Desjardins et al 1997; Metzger et al 2012). Only few direct flux measurements of reactive tracers exist to date. Faloona et al (2005), for example, used a combination of ozone and dimethyl sulfide eddy covariance measurements to estimate accurate
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