Use of a mixed-layer model to estimate dimethylsulfide flux and application to other trace gas fluxes

摘要

We discuss two techniques based on mixed-layer scaling for estimating trace gas surface emission fluxes from aircraft using instruments that do not have sufficient frequency response for direct eddy correlation measurements. The first is the mixed-layer gradient technique, which requires accurate measurements of mean concentrations at several heights in the clear convective planetary boundary layer (CBL) to resolve gradients from even strong surface sources of short-lived trace gases. The flux-gradient relationship is obtained from large-eddy numerical simulations of the CBL. We show that this technique is limited to trace gases with lifetimes of about a day or less. An example is dimethylsulfide (DMS), which is emitted from the ocean and has a lifetime of about a day. Surface DMS flux was estimated from data collected from the NASA P-3B research aircraft during the Pacific Exploratory Mission-Tropics (PEM-Tropics) flight 7 (August 24, 1996), when the aircraft flew a sequence of constant altitude circles about 50 km in diameter at different heights in and above the boundary layer, following the boundary layer air trajectory. The flight took place between 0530 and 1330 local solar time, providing a good opportunity to observe diurnal changes within a Lagrangian framework under nearly clear-sky conditions. The resulting DMS flux of 2.5 +- 0.8 pptv m s~(-1) (6.1 +- 1.9) * 10~(13) molecules m~(-2) s~(-1), or 8.8 +- 2.8 #mu#mols m~(-2) d~(-1)) is on the high end of previous measurements in this milieu. The second technique is the mixed-layer variance technique, which uses measurements of the variance at several heights in the CBL to estimate the surface flux. A major problem with the variance technique is accounting for the contribution of mesoscale variability to the measured variance. Several sources of mesoscale variability were identified: clusters of small cumulus clouds rising through the top of the boundary layer, mesoscale variations in the horizontal wind leading to inaccurate tracking of the air mass and, to a lesser extent, the presence of horizontal roll vortices in some areas of the boundary layer. We show that the variance technique should be applicable to estimating surface fluxes of short-lived trace gases in cumulus-free boundary layers that are horizontally uniform, if sample-collection times of about 10 s or less are used. We also show that it may be possible to utilize mesoscale variance measurements to estimate surface fluxes and lifetimes of species which have lifetimes of perhaps a week or more.