Mass budget in the ABL: CO2 regional flux estimates
Ecosystem carbon exchanges on spatial scales of approximately one km2 can be well documented using the eddy-covariance (EC) technique. However, little data exists on the carbon budget at the regional scale, ranging from a few tenths to few hundreds of km2. At these scales, the carbon fluxes can be estimated either by upscaling pointwise flux measurements using for instance airborne flux transects, biophysical models and remote sensing informations or by inverting atmospheric concentration measurements using a fine scale mesoscale transport model. This latter method however is still under development and requires very dense atmospheric observation datasets. The goal of this paper is to infer surface CO2 fluxes at larger scale than EC measurements by estimating the processes that control the CO2 concentration variations in the ABL.
Gibert et al. 2007b paper deals with a boundary-layer budgeting method which makes use of observations from various in-situ and remote sensing instruments to infer regional average net ecosystem exchange (NEE) of CO2. Measurements of CO2 within and above the ABL by in-situ sensors, in conjunction with a precise knowledge of the change in ABL height by Lidar and soundings enable to infer diurnal and seasonal NEE variations. (Fig. 4a) Near-ground in-situ CO measurements are used to discriminate natural (grey line) and anthropogenic contributions of CO2 (black line) diurnal variations in the ABL (Fig. 4b). The method yields mean NEE that amount to 5 µmol.m2.s-1 during the night and -20 µmol.m2.s-1 in the middle of the day between May and July.
Fig.4: 25 and 26 May, 2004: (a) Backscatter Lidar signal. Colour plot is for backscatter signal at 0.532 µm in arbitrary unit (red is for higher return signal). Nocturnal Boundary Layer (NBL) and Mixed Layer (ML) height are estimated with the inflexion point method (IPM). The MLheight is displayed using backscatter lidar measurements (fine black solid line). Potential temperature profiles from soundings (blue solid lines) at LMD are used to calculate the mean NBL height.
A good agreement is found with the expected NEE accounting for a mixed wheat fields and forests area during winter season, representative of the meso-scale ecosytems in the Paris area according to the trajectory of an air column crossing the landscape. Day time NEE is seen to follow the vegetation growth and the change in the ratio diffuse/direct radiation. The CO2 vertical mixing flux during the rise of the atmospheric boundary layer is also estimated and seems to be the main cause of the large decrease of CO2 mixing ratio in the morning. The outcomes on CO2 flux estimate are compared to eddy-covariance measurements on a barley field. The importance of various sources of error and uncertainty on the retrieval is discussed. These errors are estimated to be less than 15 %, the main error resulted from anthropogenic emissions.
The ANR project CO2-MEGAPARIS (2008-2012) relies for one part on the ABL mass budget method developped in Gibert et al. 2007b.
Can CO2 turbulent flux be measured by lidar?
The vertical profiling of CO2 turbulent fluxes in the atmospheric boundary layer (ABL) is investigated using a Coherent Differential Absorption Lidar (CDIAL) operated nearby a tall tower in Wisconsin, USA, during June 2007.
Fig. 5: Experimental site. Park Falls, WI, USA. The NASA Langley RC 2-µm coherent DIAL operated close to the 400-m WLEF tall tower. From left to right: Fabien Gibert, Syed Ismail, Jeffrey Beyon and Grady Koch.
A CDIAL can perform simultaneousrange resolved CO2 DIAL and velocity measurements. The aims of the study are i) an assessment of performance and current limitation of available CDIAL for CO2 turbulent fluxes; and ii) the derivation of instrument specifications to build a future CDIAL to perform accurate range resolved CO2 fluxes. The synoptic conditions, as monitored by the 2-µm coherent DIAL, from June 14, 2007 to June 16, 2007 are displayed in Fig. 6.
Fig. 6: (a) Off-line Carrier to Noise Ratio (CNR) (b) Vertical velocity (w) and horizontal wind (c) speed (V) and (d) direction (dirV) as a function of the local time. The ABL height is indicated with a black solid line.
The lidar eddy-covariance technique relies on the correlation between CO2 mixing ratio and vertical velocity fluctuations. The CO2 mixing ratio are estimated with the DIAL technique with 40-s time and 75-m space resolution. Unlike in-situ sensors, the lidar measurements are averaged over time and space (which have to be lower than time and space integrale scales of turbulence) to decrease instrumental errors.
Experimental lidar CO2 mixing ratio and vertical velocity profiles were successfully compared with in situ sensors measurements. Time and space integral scales of turbulence in the ABL were addressed in Gibert et al. (2010) to limit time averaging and range accumulation of DIAL measurements. A first attempt to infer CO2 fluxes using an eddy-covariance technique with currently available 2-µm CDIAL data set is reported (Fig. 7).
Fig. 7: (a) PAR (Photosynthetically Active Radiation) W/m2. (b) CO2 mixing ratio profiles up to 396 m provided by the 6 instrumented levels of the WLEF tower. (c) In-situ CO2 eddy-covariance flux measurements at 396 m (grey solid line), averaged over 6 h (black crosses) and lidar mean CO2 eddy-covariance flux estimates using CO2 DIAL measurements with 80 s - 150 m (green markers and errors bars) and 160 s - 300 m (red stars and errors bars) time and space resolutions. The lidar turbulent flux time and space resolutions are 6 h and 1.5 km. From Gibert et al. 2010