A global climate‐chemistry model study of present‐day tropospheric chemistry and radiative forcing from changes in tropospheric O 3 since the preindustrial period

We present simulations of present‐day tropospheric O 3 and of changes in its concentrations and the associated radiative forcing since the industrial revolution. Numerical experiments were conducted using a global tropospheric climate‐chemistry model (GCCM) developed by incorporating the University of Oslo (UiO) photochemical module (a reduced tracer scheme that lumps nonmethane volatile organic compounds (NMVOCs) to simulate the CO‐NO x ‐HO x ‐O 3 system) into State University of New York at Albany–Community Climate Model 3 (SUNYA CCM3). The GCCM was run with emissions of pollutants corresponding to two periods, the early 1990s and the preindustrial period, and the simulated tropospheric O 3 is used for off‐line calculations of radiative forcing. The study consists of two parts: an evaluation of the GCCM‐simulated present‐day distributions of tropospheric chemical species with sensitivity experiments to test the effects of NO production by lightning and stratospheric O 3 influx on the simulated results, and the radiative forcing due to O 3 changes since the industrial revolution. The model can reproduce the temporal and spatial variations of O 3 precursors, including CO and NO x (NO + NO 2 ), in the Northern Hemisphere (NH), although there are biases such as the larger CO concentrations in the Southern Hemisphere (SH) and the smaller NO x concentrations in the tropical middle‐upper troposphere. The simulated OH concentrations are sensitive to the NO production by lightning. The simulated concentrations of tropospheric OH have a global and annual average of about 9.8 × 10 5 molecules/cm 3 (corresponding to a tropospheric CH 4 lifetime due to OH of 8.6 years), when the annual production of NO by lightning is about 6 TgN/yr. Compared with O 3 sonde climatology, the model can simulate the seasonal variations of lower to middle tropospheric O 3 within the observed interannual variations when the stratospheric O 3 influx is about 600 Tg/yr, although detailed biases exist in the middle troposphere during fall to winter and in the continental boundary layer during summer. The present‐day tropospheric O 3 burden is calculated to be about 376 Tg (or 34 DU) with a net tropospheric chemical production of about 513 Tg/yr. Using realistic stratospheric O 3 concentrations instead of a prescribed O 3 influx as the upper boundary condition enhances the model biases in the extratropical upper‐middle tropospheric O 3 concentrations. The model calculates a globally and annua