Assessment of fossil fuel carbon dioxide and other anthropogenic trace gas emissions from airborne measurements over Sacramento, California in spring 2009

摘要

Direct quantification of fossil fuel CO (COff) in atmosphericsamples can be used to examine several carbon cycle and air qualityquestions. We collected in situ CO, CO, and CH measurements andflask samples in the boundary layer and free troposphere over Sacramento,California, USA, during two aircraft flights over and downwind of this urbanarea during spring of 2009. The flask samples were analyzed for ΔCO and CO to determine the recently added COffmole fraction. A suite of greenhouse and other trace gases, includinghydrocarbons and halocarbons, were measured in the same samples. Strongcorrelations were observed between COff and numerous trace gasesassociated with urban emissions. From these correlations we estimateemission ratios between COff and these species, and compare these withbottom-up inventory-derived estimates. Recent county level inventoryestimates for carbon monoxide (CO) and benzene from the California AirResources Board CEPAM database are in good agreement with our measuredemission ratios, whereas older emissions inventories appear to overestimateemissions of these gases by a factor of two. For most other trace species,there are substantial differences (200–500%) between our measuredemission ratios and those derived from available emission inventories. Forthe first flight, we combine in situ CO measurements with the measuredCO:COff emission ratio of 14 ± 2 ppbCO/ppmCO to derive anestimate of COff mole fraction throughout this flight, and alsoestimate the biospheric CO mixing ratio (CObio) from thedifference of total and fossil CO. The resulting CObio variesdramatically from up to 8 ± 2 ppm in the urban plume to −6 ± 1 ppm inthe surrounding boundary layer air. Finally, we use the in situ estimates ofCOff mole fraction to infer total fossil fuel CO emissions fromthe Sacramento region, using a mass balance approach. The resultingemissions are uncertain to within a factor of two due to uncertainties inwind speed and boundary layer height. Nevertheless, this first attempt toestimate urban-scale COff from atmospheric radiocarbon measurementsshows that COff can be used to verify and improve emission inventoriesfor many poorly known anthropogenic species, separate biospheric CO,and indicates the potential to constrain COff emissions if transportuncertainties are reduced.