Regional surface flux of CO 2 inferred from changes in the advected CO 2 column density

Abstract

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 CO 2 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 CO 2 flux of −9.0 ± 2.4 μmol m −2 s −1 measured from the aircraft data. The average flux from anthropogenic emissions over the measurement area was 0.3 ± 0.1 μmol CO 2 m −2 s −1. Large-scale subsidence occurred during the experiment, entraining 1.0 ± 0.2 μmol CO 2 m −2 s −1 into the boundary layer. Thus, the CO 2 flux attributable to the vegetation and soils is −10.3 ± 2.4 μmol 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.