Global modeling of tropospheric chemistry with assimilated meteorology: Model description and evaluation

We present a first description and evaluation of GEOS-CHEM, a global three- dimensional (3-D) model of tropospheric chemistry driven by assimilated met eorological observations from the Goddard Earth Observing System (GEOS) of the NASA Data Assimilation Office (DAO). The model is applied to a 1-year s imulation of tropospheric ozone-NOx-hydrocarbon chemistry for 1994, and is evaluated with observations both for 1994 and for other years. It reproduce s usually to within 10 ppb the concentrations of ozone observed from the wo rldwide ozonesonde data network. It simulates correctly the seasonal phases and amplitudes of ozone concentrations for different regions and altitudes , but tends to underestimate the seasonal amplitude at northern midlatitude s. Observed concentrations of NO and peroxyacetylnitrate (PAN) observed in aircraft campaigns are generally reproduced to within a factor of 2 and oft en much better. Concentrations of HNO3 in the remote troposphere are overes timated typically by a factor of 2-3, a common problem in global models tha t may reflect a combination of insufficient precipitation scavenging and ga s-aerosol partitioning not resolved by the model. The model yields an atmos pheric lifetime of methylchloroform (proxy for global OH) of 5.1 years, as compared to a best estimate from observations of 5.5 +/- 0.8 years, and sim ulates H2O2 concentrations observed from aircraft with significant regional disagreements but no global bias. The OH concentrations are similar to 20% higher than in our previous global 3-D model which included an UV-absorbin g aerosol. Concentrations of CO tend to be underestimated by the model, oft en by 10-30 ppb, which could reflect a combination of excessive OH (a 20% d ecrease in model OH could be accommodated by the methylchloroform constrain t) and an underestimate of CO sources (particularly biogenic). The model un derestimates observed acetone concentrations over the South Pacific in fall by a factor of 3; a missing source from the ocean may be implicated.