Radiative forcing from surface NO x emissions: spatial and seasonal variations

Abstract

The global three-dimensional Lagrangian chemistry-transport model STOCHEM has been used to follow changes in the tropospheric distributions of methane CH4 and ozone O3 following the emission of pulses of the oxides of nitrogen NO x . Month-long emission pulses of NO x produce deficits in CH4 mixing ratios that bring about negative radiative forcing (climate cooling) and decay away with e-folding times of 10–15 years. They also produce short-term excesses in O3 mixing ratios that bring about positive radiative forcing (climate warming) that decay over several months to produce deficits, with their attendant negative radiative forcing (climate cooling) that decays away in step with the CH4 deficits. Total time-integrated net radiative forcing is markedly influenced by cancellation between the negative CH4 and long-term O3 contributions and the positive short-term O3 contribution to leave a small negative residual. Consequently, total net radiative forcing from NO x emission pulses and the global warming potentials derived from them, show a strong dependence on the magnitudes, locations and seasons of the emissions. These dependences are illustrated using the Asian continent as an example and demonstrate that there is no simple robust relationship between continental-scale NO x emissions and globally-integrated radiative forcing. We find that the magnitude of the time-integrated radiative forcing from NO x -driven CH4 depletion tends to approach and outweigh that from ozone enhancement, leaving net time-integrated radiative forcings and global warming potentials negative (climate cooling) in contrast to the situation for aircraft NO x (climate warming). Control of man-made surface NO x emissions alone may lead to positive radiative forcing (climate warming).