Evaluation of the tropospheric chemistry general circulation model ECHAM5–MOZ and its application to the analysis of the chemical composition of the troposphere with an emphasis on the late RETRO period 1990–2000

The Tropospheric Chemistry General Circulation model ECHAM5-MOZ was developed between 2001 and 2005 and was used to investigate the variability and trends of ozone, CO and NOx in the second half of the 20th century in the framework of the RETRO project. The multi–decadal simulation of the period of 1960 to 2000 was one of the first of that kind. The model captures many features of the seasonal cycle and vertical gradients of trace gas concentrations measured on the ground or from balloons, aircraft or satellite. We diagnose a significant high bias in the simulated ozone concentrations in the 1990s, which can in part be attributed to an overestimated stratosphere troposphere exchange and possibly underestimated dry deposition of ozone. Wintertime CO concentrations in the northern hemisphere are underestimated by up to 30%. The observed interannual variability of the tropospheric NO2 column, surface CO concentrations and ozone is generally captured by the simulation, but the model fails to capture the surface ozone increase observed at several stations around the world during the 1980s and 1990s. The increase in the tropospheric ozone column between the 1960s and 1990s is consistent with model simulations of preindustrial conditions. The global ozone burden and chemical formation and loss are continuously rising during the entire 41-year simulation period. The dry deposition flux increases until the early 1980s and shows a more irregular behavior afterwards. Until around 1980 regionally averaged precursor emissions correlate well with surface ozone changes. Thereafter, the emission trend in Europe and North America is reversed, while ozone levels remain high. Asian emissions and ozone concentrations continue to rise, but the slope of the correlation changes.