MOZART, a global chemical transport model for ozone and related chemical tracers 2. Model results and evaluation

Abstract

In this second of two companion papers, we present results from a new global three-dimensional chemical transport model, called MOZART (model for ozone and related chemical tracers). MOZART is developed in the framework of the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM) and includes a detailed representation of tropospheric chemistry. The model provides the distribution of 56 chemical species at a spatial resolution of 2.8° in both latitude and longitude, with 25 levels in the vertical (from the surface to level of 3 mbar) and a time step of 20 min. The meteorological information is supplied from a 2-year run of the NCAR Community Climate Model. The simulated distributions of ozone (O3) and its precursors are evaluated by comparison with observational data. The distribution of methane, nonmethane hydrocarbons (NMHCs), and CO are generally well simulated by the model. The model evaluation in the tropics stresses the need for a better representation of biomass burning emissions in order to evaluate the budget of carbon monoxide, nitrogen species, and ozone with more accuracy in these regions. MOZART reproduces the NO observations in most parts of the troposphere. Nitric acid, however, is overestimated over the Pacific by up to a factor of 10 and over continental regions by a factor of 2-3. Discrepancies are also found in the simulation of PAN in the upper troposphere and in biomass burning regions. These results highlight shortcomings in our understanding of the nitrogen budget in the troposphere. The seasonal cycle of ozone in the troposphere is generally well reproduced by the model in comparison with ozone soundings. MOZART tends, however, to underestimate O3 at higher latitudes, and specifically above 300 mbar. The global photochemical production and destruction of ozone in the troposphere are 3018 Tg/yr and 2511 Tg/yr, respectively (net ozone production of 507 Tg/yr). The stratospheric influx of O3 is estimated to be 391 Tg/yr and the surface dry deposition 898 Tg/yr. The calculated global lifetime of methane is 9.9 years in the annual average. Copyright 1998 by the American Geophysical Union.