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 model have been used to study the role of NOx emissions for radiative forcing of climate. Through production of tropospheric O3, NOx emissions lead to positive radiative forcing and warming. But by affecting the concentration of OH radicals, NOx also reduces the levels of CH4, thereby giving negative forcing and cooling. The lifetime of NOx varies from hours to days, giving large spatial variations in the levels of NOx. We have selected geographical regions representing different chemical and physical conditions, and chemical and radiative effects of reducing NOx emissions by 20% in each region are studied. Due to nonlinearities in the O3 chemistry as well as differences in convective activity, there are large geographical differences in the effect of NOx on O3 as well as variations in the annual profile of the changes. The effect of NOx emissions on methane is also found to depend on the localisation of the emissions. The calculated ozone and methane forcing are of similar magnitude but of opposite sign. The methane effect acts on a global scale with a delay of approximately a decade, while the ozone effect is of regional character and occurs during weeks.