Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 6 6 2006 10.5194/acpd-6-10929-2006 http://www.atmos-chem-phys-discuss.net/6/10929/2006/ http://www.atmos-chem-phys-discuss.net/6/10929/2006/acpd-6-10929-2006.html http://www.atmos-chem-phys-discuss.net/6/10929/2006/acpd-6-10929-2006.pdf 10929 10958 2006-11-02 On using radon-222 and CO₂ to calculate regional-scale CO₂ fluxes A. I. Hirsch adam.hirsch@noaa.gov Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA NOAA/ESRL Global Monitoring Division, Boulder, Colorado, USA Because of its ubiquitous release on land and well-characterized atmospheric loss, radon-222 has been very useful for deducing fluxes of greenhouse gases such as CO₂, CH₄, and N₂O. It is shown here that the radon-tracer method, used in previous studies to calculate regional-scale greenhouse gas fluxes, returns a weighted-average flux (the flux field <i>F</i> weighted by the sensitivity of the measurements to that flux field, <i>f</i>) rather than an evenly-weighted spatial average flux. A synthetic data study using a Lagrangian particle dispersion model and modeled CO₂ fluxes suggests that the discrepancy between the sensitivity-weighted average flux and evenly-weighted spatial average flux can be significant in the case of CO₂, due to covariance between <i>F</i> and <i>f</i> for biospheric CO₂ fluxes during the growing season and also for anthropogenic CO₂ fluxes in general. A technique is presented to correct the radon-tracer derived fluxes to yield an estimate of evenly-weighted spatial average CO₂ fluxes. A new method is also introduced for correcting the CO₂ flux estimates for the effects of radon-222 radioactive decay in the radon-tracer method.