3.1. CO2 mixing ratios in the Ebre watershed
Vertical profiles of atmospheric CO2 mixing ratios were sampled on 26 and 27 June 2007 within the ICARO-2 (Reduction of Uncertainties in the peninsular CARbon balance by means of Oscillating tropospheric transects) project in Spain at La Muela (LMU), NE Spain, within the central part of the Ebre watershed basin. Topographically, the central region of the Ebre basin is flat, with an average altitude of 250 masl. Vegetation in the region is characterized by complex agricultural patches, dominated by permanent crops and non-irrigated lands except for those situated along the irrigated channels derived from the Ebre River, where intensive irrigated arable lands persist. The dominant permanent crops comprise vineyards and olive groves. In the intensive irrigated areas along the irrigated canals, extensive herbaceous crops dominate (alfalfa, maize, cereals, etc.) and fruit growing and horticulture shape the land. The Moncayo mountain range is situated 100 km west of LMU, and natural forests of holm oak and kermes oak dominate the lowest levels of the mountain range, while oaks, beechwood and birch dominate the highest and wetter regions. The Pyrenees mountain range is 150 km north of the site and is covered by alpine forests.
The campaign was undertaken during the Northern Hemisphere growing season. Local vegetation was fairly active as indicated by the NDVI 10-day composition map (Fig. 1a). Vegetation around LMU was photosynthetically not very active (values ∼0.2) except for the one situated along the irrigation channels, reaching NDVI values of ∼0.5. The average NDVI value for the region situated around LMU (∼60 km) during the campaign was 0.41, slightly above the annual mean NDVI value (0.36). For 2007, the vegetation in the region was more active photosynthetically during late April and early May, with NDVI values that reached 0.55 as seen in Fig. 1(b). On late June, vegetation was still above the annual mean value (0.37) but the photosynthetic activity started decreasing. The time when vegetation was less active was registered in mid-September and December (∼0.27). The 1998–2006 time series of the fPAR index (Gobron et al., 2006) at LMU (Fig. 1c) shows that April and May are the most active months in LMU, with a decreasing activity in June. Generally, July and August are the most inactive seasons linked to the dry weather conditions occurring during summer time in the central part of the Ebre watershed. Regarding the synoptic weather situation, the field campaign was carried out under fairly anticyclonic conditions with light NW surface winds (∼4–5 ms−1) that prevailed during the daytime in the central part of the Ebre watershed.
Figure 1. (a) Ten-day composition NDVI map starting on 21 June 2007 from SPOT10 products (http://www.vgt.vito.be). Black dot shows the LMU site. (b) 2007-LMU-NDVI time series. The red dot shows the NDVI value for the period when the survey was done; dotted line marks the annual NDVI mean. (c) 2000–2006 LMU-fPAR time series (EC-JRC-2010; http://fapar.jrc.ec.europa.eu/). Red curve denotes the mean seasonal behaviour of fPAR through years. Blue line denotes the average fPAR value for the time encompassed.
Download figure to PowerPoint
Vertical profiles of atmospheric CO2 mixing ratios were sampled with the portable non-dispersive infrared instrument developed by AOS Inc. The mean precision of the system in laboratory conditions is ±0.11 ppmv whereas under flight conditions the mean precision is estimated at σ± 0.23 ppmv. Further details about the instrumentation may be found in Font et al. (2008). Four vertical profiles were sampled at different times during the survey (Fig. 2a). The first profile sampled on 26 June started around 8 UT, from now on abridged P1. CO2 mixing ratios at the lowest levels were ∼386 ppmv, decreasing in height and reaching ∼384 ppmv at 2000 masl. Conversely, the profile sampled later in the day (starting at 17.50 UT, P2) shows quasi-homogeneous concentrations in height, with values ∼382 ppmv until 2000 masl, and tending to slightly increase up to 2500 masl. The profiles on 27 June 2007 were sampled around noon, starting 30 min apart. They comprised the entire air column over LMU from 1000 to 4000 masl. Ascent and descent profiles are referred as P3 and P4, respectively. The lowermost part of P3 and P4 beared strong similarities, with concentrations ranging from ∼386 ppmv at ∼1000 masl down to ∼383 ppmv at 2000 masl. From 2000 to 4500 masl, large variations and discrepancies between P3 and P4 appeared. Between 2000 and 3000 masl, a rich CO2-mixing ratio layer was detected in P3 (∼384 ppmv), while P4 registered quasi-homogeneous values (∼382 ppmv). Between 3000 and 3500 masl, P3 measured lower CO2 concentrations (∼381.5 ppmv). Instead, P4 sampled a rich-CO2-content layer (∼384 ppmv). From P3 and P4 profiles it is stated that CO2 changes within the lowermost part of profiles are slower than the ones occurring higher in the altitude.
Figure 2. CO2 measurements at LMU (NE Spain) during 26 and 27 June 2007. (a) Vertical profiles (circles) and the mean CO2 concentration from the tall tower site recorded from the three available altitudes during the aircraft survey (squares); altitude of the convective boundary layer extracted from NCEP-GFS meteorological model, interpolated at the time of the flight (dotted lines); and schematic representation of the mixing rations applied in eq. (3) and summarized in Table 1. ASC refers to the ascent profile; DES, to the descent one. (b) Continuous CO2 measurements at LMU tower on 26 and 27 June 2007. Coloured rectangles show CO2 measurements when profiles were sampled.
Download figure to PowerPoint
An 80-m-height tower situated in a plateau at 570 masl in the middle of the Ebre basin at LMU (250 masl) is equipped with a LiCor-7000, providing a continuous measuring of atmospheric CO2 mixing ratios at three different altitudes [41, 57 and 79 m above the ground level (magl)]. A target gas is measured every 200 min to control drifts. Whenever the mean precision is above 0.05 ppmv the instrument is restarted and re-calibrated (Morguí J.-A., 2008, personal communication). Hourly means at 41 and 79 m are shown in Fig. 2(b). Coloured rectangles mark CO2 measurements exactly when profiles were carried out. Continuous measurements showed a marked diurnal cycle, with high atmospheric CO2 values during night and low values during central hours of the day. The diurnal amplitude (maximum minus minimum CO2 concentrations) was ∼5.25 ppmv on 26 June, whereas it was larger on 27 June, of around ∼8 ppmv. The maximum concentration was registered between 2 and 3 UT on 26 June; and between 5 and 6 UT on 27 June, with values of ∼386.4 and ∼386.7 ppmv, respectively. The minimum concentration on the 26 June took place between 17 and 20 hours (∼381.4 ppmv), whereas the minimum on the 27 June occurred between 21 and 23 h, with values of ∼379.0 ppmv. Ground concentrations were calculated at LMU at the time when profiles were sampled, taken as the weighted average concentration from the measurements at the three altitudes of the tower. At the time P1 was sampled, ground CO2 concentration was 384.9 ppmv; 381.5 ppmv for P2 and 384.7 ppmv for P3. Measurements from the LMU tower are therefore consistent with those from the aircraft sampling (Fig. 2a) and provide a continuity of CO2 measurements from the lowest aircraft altitude down to the surface.
Additionally to the vertical profiles and tall tower continuous measurements, horizontal transects at ∼2000 and ∼2500 masl were flown on 26 June, following the 41.60°N parallel, 100 km west from LMU at ∼8:00 and ∼17:30 UT (Fig. 3). The mean CO2 concentrations (±1σ) of stacks were 383.33 ± 1.03 and 381.47 ± 0.62 ppmv, with a range difference of 4.45 and 3.11 ppmv, respectively. It is seen that there was a decrease of the mean CO2 concentration in the horizontal legs of ∼2 ppmv between the two sampling times. The horizontal transect measured in the early morning on 26 June 2007 showed large CO2 differences between regions. Large CO2 mixing ratios were measured near LMU (∼385 ppmv) whereas further west, lower mixing ratios were registered (∼380 ppmv). The afternoon stack showed less variability. However, differences still showed up: close to LMU higher concentrations prevailed (∼382 ppmv) compared to the lower concentrations 100 km west (∼379 ppmv).
Figure 3. Aircraft measurements along a 100-m-track west from LMU at (a) ∼8:00 UT and (b) ∼18:00 UT on 26 June 2007. Black line denotes the topographic profile, extracted from GTOPO30 global digital elevation model (DEM) from the U.S. Geological Survey's EROS Data Center in Sioux Falls, South Dakota; data and documentation available at http://edc.usgs.gov/.
Download figure to PowerPoint