Variability and budget of CO2 in Europe: analysis of the CAATER airborne campaigns – Part 1: Observed variability
Résumé
Atmospheric airborne measurements of CO 2 are very well suited for estimating the time-varying distribution of carbon sources and sinks at the regional scale due to the large geographical area covered over a short time. We present here an analysis of two cross-European airborne campaigns carried out on 23-26 May 2001 (CAATER-1) and 2-3 Oc-tober 2002 (CAATER-2) over Western Europe. The area covered during CAATER-1 and CAATER-2 was 4 • W to 14 • E long; 44 • N to 52 • N lat and 1 • E to 17 • E long; 46 • N to 52 • N lat respectively. High precision in situ CO 2 , CO and Radon 222 measurements were recorded. Flask samples were collected during both campaigns to cross-validate the in situ data. During CAATER-1 and CAATER-2, the mean CO 2 concentration was 370.1 ± 4.0 (1-σ standard deviation) ppm and 371.7 ± 5.0 (1-σ) ppm respectively. A HYS-PLIT back-trajectories analysis shows that during CAATER 1, northwesterly winds prevailed. In the planetary boundary layer (PBL) air masses became contaminated over Benelux and Western Germany by emissions from these highly urbanized areas, reaching about 380 ppm. Air masses passing over rural areas were depleted in CO 2 because of the photosyn-thesis activity of the vegetation, with observations as low as 355 ppm. During CAATER-2, the back-trajectory analysis showed that air masses were distributed among the 4 sectors. Air masses were enriched in CO 2 and CO over anthropogenic emission spots in Germany but also in Poland, as these countries have part of the most CO 2-emitting coal-based plants Correspondence to: I. Xueref-Remy (irene.xueref@lsce.ipsl.fr) in Europe. Simultaneous measurements of in situ CO 2 and CO combined with back-trajectories helped us to distinguish between fossil fuel emissions and other CO 2 sources. The CO/ CO 2 ratios (R 2 = 0.33 to 0.88, slopes = 2.42 to 10.37), calculated for anthropogenic-influenced air masses over different countries/regions matched national inventories quite well, showing that airborne measurements can help to identify the origin of fossil fuel emissions in the PBL even when distanced by several days/hundreds of kms from their sources. We have compared airborne CO 2 observations to nearby ground station measurements and thereby, confirmed that measurements taken in the lower few meters of the PBL (low-level ground stations) are representative of the local scale, while those located in the free tro-posphere (FT) (moutain stations) are representative of atmospheric CO 2 regionally on a scale of a few hundred kilometers. Stations located several 100 km away from each other differ from a few ppm in their measurements indicating the existence of a gradient within the free troposphere. Observations at stations located on top of small mountains may match the airborne data if the sampled air comes from the FT rather than coming up from the valley. Finally, the analysis of the CO 2 vertical variability conducted on the 14 profiles recorded in each campaign shows a variability at least 5 to 8 times higher in the PBL (the 1-σ standard deviation associated to the CO 2 mean of all profiles within the PBL is 4.0 ppm and 5.7 ppm for CAATER-1 and CAATER-2, respectively) than in the FT (within the FT, 1-σ is 0.5 ppm and 1.
Origine : Publication financée par une institution
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