Optimizing the NBL budget technique for trace gas flux measurement

The desire for continuous measurements of CO2 exchange between the surface and the atmosphere is hampered by the absence or intermittent nature of the turbulence within the commonly stable conditions present at night. Under such calm, clear nights, radiative temperature inversions develop and act to inhibit the upward migration of gases emitted at the surface. Since 1996, researchers have explored the utility of budget techniques for measurement of atmospheric trace gas exchange within the nocturnal stable boundary layer. We will present work which provides enhancements on the applicability of the NBL budget technique. A tethersonde and light-weight IRGA were used to probe the NBL depth. A sodar provided continuous profiles of vertical wind characteristics. While the application of the NBL budget relies on the ability to determine the height of the NBL, in practice, successive profiles may not converge or this height may be ambiguous. Our observational evidence suggests that the common presence of a nocturnal low-level jet acts as a barrier to trace gas flux movement. The best conditions for using the NBL budget occur when wind direction is consistent between successive profiles, a LLJ is present and is decoupled from the ground surface. In this case, the trace gas concentration diffuses upwards and is mixed within the shear layer on the underside of the jet. Above the jet, the measured CO2 concentration coincides with the latest turbulent concentration level of the previous day. The height of the jet's stable nose thus represents the best estimate of the practical NBL height. This observed height (by tethersonde) compares well with NBL height estimated from sodar signal returns. Future work will indicate the broader applicability of the NBL budget in ecosystem scale studies.