ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-11-1659-2011 Assessing the regional surface influence through Backward Lagrangian Dispersion Models for aircraft CO<sub>2</sub> vertical profiles observations in NE Spain Font A. 1 2 4 Morguí J.-A. 1 2 Rodó X. 1 3 Institut Català de Ciències del Clima, Dr. Trueta 203, 08005, Barcelona, Catalonia, Spain Laboratori de Recerca del Clima, Parc Científic de Barcelona, Universitat de Barcelona, Baldiri i Reixach, 2, 2a planta, 08028, Barcelona, Catalonia, Spain Institució Catalana de Recerca i Estudis Avançats, Passeig Lluís Companys, 23, 08010, Barcelona, Catalonia, Spain now at: Environmental Research Group, King's College London, 150 Stamford Street, London, SE1 9NH, UK 21 02 2011 11 4 1659 1670 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/11/1659/2011/acp-11-1659-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/11/1659/2011/acp-11-1659-2011.pdf

In this study the differences in the measured atmospheric CO<sub>2</sub> mixing ratio at three aircraft profiling sites in NE Spain separated by 60 km are analyzed in regard to the variability of the surface fluxes in the regional surface influence area. First, the Regional Potential Surface Influence (RPSI) for fifty-one days in 2006 is calculated to assess the vertical, horizontal and temporal extent of the surface influence for the three sites at the regional scale (10<sup>4</sup> km<sup>2</sup>) at different altitudes of the profile (600, 1200, 2500 and 4000 meters above the sea level, m a.s.l.). Second, three flights carried out in 2006 (7 February, 24 August and 29 November) following the Crown Atmospheric Sampling (CAS) design are presented to study the relation between the measured CO<sub>2</sub> variability and the Potential Surface Influence (PSI) and RPSI concepts. At 600 and 1200 m a.s.l. the regional signal is confined up to 50 h before the measurements whereas at higher altitudes (2500 and 4000 m a.s.l.) the regional surface influence is only recovered during spring and summer months. The RPSI from sites separated by 60 km overlap by up to 70% of the regional surface influence at 600 and 1200 m a.s.l., while the overlap decreases to 10–40% at higher altitudes (2500 and 4000 m a.s.l.). The scale of the RPSI area is suitable to understand the differences in the measured CO<sub>2</sub> concentration in the three vertices of the CAS, as CO<sub>2</sub> differences are attributed to local surrounding fluxes (February) or to the variability of regional surface influence as for the August and November flights. For these two flights, the variability in the regional scale influences the variability measured in the local scale. The CAS sampling design for aircraft measurements appears to be a suitable method to cope with the variability of a typical grid for inversion models as measurements are intensified within the PBL and the background concentration is measured every ~10<sup>2</sup> km.

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