Aircraft emissions of NOx: radiative forcing from long-term stratospheric changes of H2O and O3

Two independent chemistry-transport models with troposphere-stratosphere coupling are used to quantify the different components of the radiative forcing (RF) from aircraft emissions of NOx, i.e., the University of L’Aquila climate-chemistry model (ULAQ-CCM) and the University of Oslo chemistry-transport model (Oslo-CTM3). The tropospheric NOx enhancement due to aircraft emissions produces a short-term O3 increase with a positive RF (+17.3±2.1 mW/m2) (the uncertainty range represents the spread between the two models). This is partly compensated by the CH4 decrease due to the OH enhancement (-9.4±1.0 mW/m2). The latter is a long-term response calculated using a surface CH4 flux boundary condition (FBC). This induces a long-term response of tropospheric O3 due to less HO2 being available for O3 production, compared with the reference case where a constant CH4 surface mixing ratio boundary condition is used (MBC) (-3.92±0.01 mW/m2). The CH4 decrease induces a long-term response of stratospheric H2O (-1.2±0.1 mW/m2). The latter finally perturbs HOx and NOx in the stratosphere, with a more efficient NOx cycle for mid-stratospheric O3 depletion and a decreased O3 production from HO2+NO in the lower stratosphere. This produces a long-term stratospheric O3 loss, with a negative RF (-1.4±0.1 mW/m2), compared with the CH4 MBC case. Other minor contributions to the net NOx RF are those due to NO2 absorption of UV-A (+0.5±0.1 mW/m2), increasing sulphate due to more efficient OH oxidation of SO2 (-0.2 mW/m2) and increasing nitrates formed via NO3 + BVOC or HNO3 heterogeneous reactions (-3.1 mW/m2). These aerosol contributions have been calculated only in the ULAQ-CCM. According to these model calculations, aviation NOx emissions for 2006 produced a small global net cooling effect of -1.3±0.8 mW/m2 (an average of ULAQ-CCM and Oslo-CTM3 results). The ULAQ-CCM’s calculated RF is -2.1 mW/m2 (O3 column changes of +0.32 and -0.056 DU in troposphere and stratosphere, respectively), whereas the Oslo-CTM3’s calculated RF is -0.5 mW/m2 (O3 column changes of +0.45 and -0.082 DU in troposphere and stratosphere, respectively).