Variability and budget of CO2 in Europe: analysis of the CAATER airborne campaigns – Part 1: Observed variability
Authors: Xueref-Remy I. , Messager C. , Filippi D. , Nedelec P. , Ramonet M. , Paris J. D. , Ciais P. 
Published: April 2010

Abstract. Atmospheric airborne measurements of CO2 are very well-suited to estimate the time varying distribution of carbon sources and sinks at the regional scale. We present here an analysis of two cross-European airborne campaigns that have been carried out on 23–26 May 2001 (CAATER 1) and 2–3 October 2002 (CAATER 2) over Western Europe. The area covered during CAATER 1 (respectively CAATER 2) was comprised between longitude 4° W to 14° E and latitude 44° N to 52° N (respectively longitude 1° E to 17° E and latitude 46° N to 52° N). High precision in-situ CO2, CO and Radon 222 measurements have been recorded. Flasks samples have been collected during both campaigns to cross-validate the in-situ data. During CAATER 1 (respectively CAATER 2), the mean CO2 concentration was 370.1±4 ppm (respectively 371.7±5 ppm). A HYSPLIT backtrajectories analysis shows that during CAATER 1, dominant winds were blowing from the north-west. In the planetary boundary layer (PBL) airmasses got contaminated over Benelux and Western Germany by pollution from these high urbanized areas, reaching about 380 ppm. Air masses passing over rural areas are depleted in CO2 because of the photosynthesis activity of the land cover vegetation, as low as 355 ppm. During CAATER 2, the backtrajectory analysis shows that airmasses were distributed among the 4 sectors. Airmasses got enriched in CO2 and CO when passing above polluted spots in Germany but also in Poland, as these countries are known to hold part of the most polluting plants based on coal consumption, the so-called "dirty thirty" from WWF. Simultaneous measurements of in-situ CO2 and CO combined to backtrajectories helped us to discriminate the role of fossil fuel emissions from over CO2 sources. The ΔCO/ΔCO2 ratios (R2=0.33 to 0.88, slopes=2.42 to 10.37), calculated for polluted airmasses originating from different countries/regions, matched quite well national inventories, showing that the airborne measurements can help to identify the role of fossil fuel sources even several days/hundreds of kms further in the PBL. CO2 observations have been compared to surrounding ground stations measurements, confirming that the stations located near the ground (ex. CBW, WES, HUN) are representative of the local scale, while those located in the free troposphere (FT) are representative of atmospheric CO2 on a regional scale of a few hundred kilometers (ex. CMN). Stations located several 100 km away measure CO2 concentrations different from a few ppm, indicating the existence of a gradient of a few ppm in the free troposphere. Observations at stations located on top of small mountains (ex. SCH, PUY) match or not the airborne data whether they sample air from the FT or air coming up from the valley. Finally, the analysis of the CO2 vertical variability conducted on the 14 profiles recorded per campaign shows that is at least 5 to 8 times higher in the PBL (4 ppm and 5.7 ppm for CAATER 1 and CAATER 2, respectively) than in the FT (0.5 ppm and 1.1 ppm for CAATER 1 and CAATER 2, respectively). The CO2 jump between the PBL and the FT equals 3.7 ppm for the first campaign and −0.3 ppm for the second campaign. A very striking zonal CO2 gradient of about 11 ppm could be observed in the mid-troposphere during CAATER 2, with higher concentrations in the West than in the East. This gradient could originate from differences in atmospheric mixing, ground emission rates or a earlier beginning of the Fall in the west. More airborne campaigns are currently under analysis in the framework of the CARBOEUROPE-IP project to better assess the role of these different hypothesis. In a companion paper (Xueref-Remy et al., 2010), a comparison of vertical profiles from observations and several modeling frameworks is conducted for both campaigns. An attempt to calculate CO2 fluxes during CAATER 1 using CO2 and Radon-222 observations and modeling tools is also carried out.