Inverse modeling of the global CO cycle 1. Inversion of CO mixing ratios

A three-dimensional modeling study on atmospheric carbon monoxide is presented, based on the TM2 model. A Bayesian inverse technique is applied to optimize the agreement between model and observational data, including a priori source information as regularization term. Using the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory data set for CO mixing ratios at 31 globally distributed sites, a posteriori CO budgets can be derived, which allow the model to reproduce the observations at most sites within two standard deviations of monthly mean values. Use of different spatiotemporal emission distributions for terpenes (Global Emissions Inventory Activity, similar to 80% of emissions in the tropics; Hough [1991], similar to 70% of emissions in the extratropical Northern Hemisphere) showed a large impact on calculated a posteriori source strengths and on the modeled partitioning among individual CO sources. In order to reproduce the interhemispheric gradient of observed CO mixing ratios, a ratio between total sources in the Northern Hemisphere and those in the Southern Hemisphere of similar to 1.8 is required. While it is obvious that this asymmetry is mainly due to CO emissions from technological sources, the inversion results suggest that either(1) the global technological CO source strength is higher (similar to 800 Tg CO/yr) than present inventory based estimates or (2) CO from terpenes or vegetation (or additional sources with dominant emissions in the Northern Hemisphere) have a significant impact on the northern hemispheric mixing ratios. Further sensitivity studies showed that a posteriori results slightly depend on biomass burning seasonality (shifted by 1 month), but they are virtually identical for the two different OH fields (CH4-nonmethanehydrocarbons chemistry vs. CH4-only chemistry). Inversion results, however, were sensitive to model wind fields used (based on meteorological observations of 1987 and 1986, respectively), mainly due to stations near source regions. Use of a reduced set of stations resulted in virtually identical a posteriori source strengths for both model wind fields. The analysis is extended in the companion paper which considers the additional information on the CO budget provided by measurements of the stable isotope ratios (C-13/C-12, O-18/O-16).