Transects of atmospheric CO, CH4, and their isotopic composition across the Pacific:: Shipboard measurements and validation of inverse models

Measurements of carbon monoxide, methane, and their isotopic composition are presented for samples collected on six ship voyages across the Pacific between New Zealand and the United States between 1996 and 1998. The data cover a latitude range from approximately 40 degreesS to 40 degreesN and clearly define significant latitudinal gradients in mixing ratios and isotopic composition and their seasonal variations. Observational data are compared with recent three-dimensional inverse modeling studies of the global CO and CH4 distributions (Bergamaschi et al., 2000a, b, c) constrained by CO and CH4 mixing ratio data from the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory network and additional isotope measurements from a set of five globally distributed stations. Thus the shipboard measurements represent a unique opportunity for independent validation of the inverse models. In general, we see a very good agreement between measurements and model results, both for CO and CH4 mixing ratios and their stable isotopic composition (delta (CO)-C-13, deltaC(18)O, delta (CH4)-C-13). This is in particular true for the simulation of mean latitudinal gradients of these quantities. For some voyages, however, significant deviations in the "fine structure" of the latitudinal profiles are apparent and are attributed to synoptic scale variations that are not captured by the model (for which a standard meteorology is applied). In most cases, deviations between observations and model results exhibit similar patterns for CH4 and CO, and the deviations in mixing ratios are linked with corresponding deviations in the isotopic signature. Furthermore, a clear correlation between CO and ethane mixing ratios is visible. Observations of (CO)-C-14 show a general minimum around the equator and cycles of opposite phase in each hemisphere. Virtually identical (CO)-C-14 minima were observed in both hemispheres in 1997, indicating similar OH levels. We also show model results for delta CH3D and the mass-independent isotope fraction in (CO)-O-17 (despite the absence of observational data for the ship voyages) in our discussion to illustrate the potential of these isotopic signatures for the understanding of the global cycles of CH4 and CO, respectively.