Quantification of fossil fuel CO.sub.2 from combined CO, [delta].sup.13CO.sub.2 and Î.sup.14CO.sub.2 observations

We present a new method for partitioning observed CO.sub.2 enhancements (CO.sub.2 xs) into fossil and biospheric fractions (C.sub.ff and C.sub.bio) based on measurements of CO and [delta].sup.13 CO.sub.2, complemented by flask-based Î.sup.14 CO.sub.2 measurements. This method additionally partitions the fossil fraction into natural gas and petroleum fractions (when coal combustion is insignificant). Although here we apply the method only to discrete flask air measurements, the advantage of this method (CO- and [delta].sup.13 CO.sub.2 -based method) is that CO.sub.2 xs partitioning can be applied at high frequency when continuous measurements of CO and [delta].sup.13 CO.sub.2 are available. High-frequency partitioning of CO.sub.2 xs into C.sub.ff and C.sub.bio has already been demonstrated using continuous measurements of CO (CO-based method) and Î.sup.14 CO.sub.2 measurements from flask air samples. We find that the uncertainty in C.sub.ff estimated from the CO- and [delta].sup.13 CO.sub.2 -based method averages 3.2 ppm (23 % of the mean C.sub.ff of 14.2 ppm estimated directly from Î.sup.14 CO.sub.2 ), which is significantly less than the CO-based method which has an average uncertainty of 4.8 ppm (34 % of the mean C.sub.ff). Using measurements of CO, [delta].sup.13 CO.sub.2 and Î.sup.14 CO.sub.2 from flask air samples at three sites in the greater Los Angeles (LA) region, we find large contributions of biogenic sources that vary by season. On a monthly average, the biogenic signal accounts for -14 to +25 % of CO.sub.2 xs with larger and positive contributions in winter and smaller and negative contributions in summer due to net respiration and net photosynthesis, respectively. Partitioning C.sub.ff into petroleum and natural gas combustion fractions reveals that the largest contribution of natural gas combustion generally occurs in summer, which is likely related to increased electricity generation in LA power plants for air-conditioning.