Climatic forcing of nitrogen oxides through changes in tropospheric ozone and methane; global 3D model studies

A three-dimensional global chemical tracer model and a radiation transfer m odel have been used to study the role of NOx emissions for radiative forcin g of climate. Through production of tropospheric O-3,O- NOx emissions lead to positive radiative forcing and warming. But by affecting the concentrati on of OH radicals, NOx also reduces the levels of CH4, thereby giving negat ive forcing and cooling. The lifetime of NOx varies from hours to days, giv ing large spatial variations in the levels of NOx. We have selected geograp hical regions representing different chemical and physical conditions, and chemical and radiative effects of reducing NOx emissions by 20% in each reg ion are studied. Due to nonlinearities in the O-3 chemistry as well as diff erences in convective activity, there are large geographical differences in the effect of NOx on O-3 as well as variations in the annual profile of th e changes. The effect of NOx emissions on methane is also found to depend o n the localisation of the emissions. The calculated ozone and methane forci ng are of similar magnitude but of opposite sign. The methane effect acts o n a global scale with a delay of approximately a decade, while the ozone ef fect is of regional character and occurs during weeks. (C) 1999 Elsevier Sc ience Ltd. All rights reserved.