Regional surface flux of CO2 inferred from changes in the advected CO2 column density

A Lagrangian experiment was conducted over Iowa during the daytime (9:00-17:30 LT) on June 19,2007 as part of the North American Carbon Program's Mid-Continent Intensive using a light-weight and operationally flexible aircraft to measure a net drawdown of CO2 concentration within the boundary layer. The drawdown can be related to net ecosystem exchange when anthropogenic emissions are estimated using a combination of the Vulcan fossil fuel emissions inventory coupled with a source contribution analysis using HYSPLIT. Results show a temporally and spatially averaged net CO2 flux Of -9.0 +/- 2.4 mu mol m(-2) s(-1) measured from the aircraft data. The average flux from anthropogenic emissions over the measurement area was 0.3 +/- 0.1 mu mol CO2 m(-2) s(-1). Large-scale subsidence occurred during the experiment, entraining 1.0 +/- 0.2 mu mol CO2 m(-2) s(-1) into the boundary layer. Thus, the CO2 flux attributable to the vegetation and soils is -10.3 +/- 2.4 mu mol m(-2) s(-1). The magnitude of the calculated daytime biospheric flux is consistent with tower-based eddy covariance fluxes over corn and soybeans given existing land-use estimates for this agricultural region. Flux values are relatively insensitive to the choice of integration height above the boundary layer and emission footprint area. Flux uncertainties are relatively small compared to the biospheric fluxes, though the measurements were conducted at the height of the growing season. (C) 2009 Elsevier B.V. All rights reserved.