Atmospheric CO2 in situ measurements: Two examples of Crown Design flights in NE Spain

The new “Crown” aircraft sampling (CAS) approach integrates CO2 data obtained through horizontal transects and vertical profiles, sampling a specific volume of air at different atmospheric sections: 600 m, 1200 m, and 2500 m above the sea level (masl). This approach provides new tools to model fluxes in relation with meteorological conditions. It has been applied over a plain in the basin of the river called Ebre in the North East of Spain, with a triangular section defined by Linyola, Mequinensa's dam and Binèfar, taking up 1500 km2. The main human activity is agriculture, either irrigated or dry land. Horizontal transects at 600 and 1200 masl were sampled for CO2, with vertical profiles at the vertexes from 600 to 2500 masl (ground level is around 400 masl). The three‐dimensional configuration of the CAS Design enables the study of the CO2 fluxes between sections. Data have been obtained on a monthly basis since February 2006 until nowadays. In order to study the influence of forced turbulence in the atmospheric CO2 spatial distribution, two data sets are examined: the flight of 7 February 2006, performed under stable anticyclonic weather conditions; and the 6 March 2006 flight, characterized by strong northerly winds. The presence of stable/unstable layers in the February flight resulted in a stepwise vertical distribution of CO2 (with an increase from the free troposphere, 383 ppmv, to 400 ppmv close to the ground) whereas the unstable low troposphere sampled in March 2006 provided flat CO2 vertical profiles (concentrations ranging from 385 to 387 ppmv). In regard to the variability in the horizontal sections, the calculation of the Hurst exponents (H) provides a way to measure the persistence of vertical transport along the horizontal transects. In the February flight, CO2 concentration in transects shows the imprint of a spreading vertical transport from land (H > 0.90) but it is partially wiped away by high turbulence levels in the March flight (H = 0.79). From these results we expect CAS Design to provide better estimates of CO2 fluxes in the future.