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Our ability to predict future climate change relies on our understanding ofcurrent and future CO2 fluxes, particularly on a regional scale(100–1000 km). CO2 regional sources and sinks are still poorlyunderstood. Inverse transport modeling, a method often used to quantifythese fluxes, relies on atmospheric CO2 measurements. One of the mainchallenges for the transport models used in the inversions is to properlyreproduce CO2 vertical gradients between the boundary layer and thefree troposphere, as these gradients impact on the partitioning of thecalculated fluxes between the different model regions. Vertical CO2profiles are very well suited to assess the performances of the models. Inthis paper, we conduct a comparison between observed and modeled CO2profiles recorded during two CAATER campaigns that occurred in May 2001 andOctober 2002 over Western Europe, as described in a companion paper. We testdifferent combinations between a global transport model (LMDZt), a mesoscaletransport model (CHIMERE), and different sets of biospheric fluxes, allchosen with a diurnal cycle (CASA, SiB2 and ORCHIDEE). The vertical profilecomparison shows that: 1) in most cases the influence of the biospheric fluxis small but sometimes not negligible, ORCHIDEE giving the best results inthe present study; 2) LMDZt is most of the time too diffuse, as itsimulates a too high boundary layer height; 3) CHIMERE better reproducesthe observed gradients between the boundary layer and the free troposphere,but is sometimes too variable and gives rise to incoherent structures. Weconclude there is a need for more vertical profiles to conduct furtherstudies to improve the parameterization of vertical transport in the modelsused for CO2 flux inversions. Furthermore, we use a modeling method to quantify CO2 fluxes at theregional scale from a chosen observing point, coupling influence functionsfrom the transport model LMDZt (that works quite well at the synoptic scale)with information on the space-time distribution of fluxes. This modelingmethod is compared to a dual tracer method (the so-called Radon method) fora case study on 25 May 2001 during which simultaneous well-correlated in situCO2 and Radon 222 measurements have been collected. Both methods give asimilar result: a flux within the Radon 222 method uncertainty (35%),that is an atmospheric CO2 sink of −4.2 to −4.4 gC m−2 day−1.We have estimated the uncertainty of the modeling method to be at least33% on average, and even more for specific individual events. This methodallows the determination of the area that contributed to the CO2observed concentration. In our case, the observation point located at1700 m a.s.l. in the north of France, is influenced by an area of1500×700 km2 that covers the Benelux region, part of Germany and western Poland.Furthermore, this method allows deconvolution between the differentcontributing fluxes. In this case study, the biospheric sink contributes73% of the total flux, fossil fuel emissions for 27%, the oceanic fluxbeing negligible. However, the uncertainties of the influence functionmethod need to be better assessed. This could be possible by applying it toother cases where the calculated fluxes can be checked independently, forexample at tall towers where simultaneous CO2 and Radon 222measurements can be conducted. The use of optimized fluxes (from atmosphericinversions) and of mesoscale models for atmospheric transport may alsosignificantly reduce the uncertainties

atmospheric chemistry and physics

Variability and budget of CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; in Europe: analysis of the CAATER airborne campaigns – Part 2: Comparison of CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; vertical variability and fluxes between observations and a modeling framework