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 NOxenhancement due to aircraft emissions produces a short-term O3increase with a positive RF (+17.3mW/m2) (as an average value of the two models). This is partly compensated by the CH4decrease due to the OH enhancement (-9.4mW/m2). The latter is a long-term response calculated using a surface CH4flux boundary condition (FBC), with at least 50 years needed for the atmospheric CH4to reach steady state. The radiative balance is also affected by the decreasing amount of CO2produced at the end of the CH4oxidation chain: an average CO2accumulation change of -2.2 ppbv/yr is calculated on a 50 year time horizon (-1.6mW/m2). The aviation perturbed amount of CH4induces a long-term response of tropospheric O3mostly due to less HO2and CH3O2being available for O3production, compared with the reference case where a constant CH4surface mixing ratio boundary condition is used (MBC) (-3.9mW/m2). The CH4decrease induces a long-term response of stratospheric H2O (-1.4mW/m2). The latter finally perturbs HOxand NOxin the stratosphere, with a more efficient NOxcycle for mid-stratospheric O3depletion and a decreased O3production from HO2+NO in the lower stratosphere. This produces a long-term stratospheric O3loss, with a negative RF (-1.2mW/m2), compared with the CH4MBC case. Other contributions to the net NOxRF are those due to NO2absorption of UV-A and aerosol perturbations (the latter calculated only in the ULAQ-CCM). These comprise: increasing sulfate due to more efficient oxidation of SO2, increasing inorganic and organic nitrates and the net aerosols indirect effect on warm clouds. According to these model calculations, aviation NOxemissions for 2006 produced globally a net cooling effect of -5.7mW/m2(-6.2 and -5.1mW/m2, from ULAQ and Oslo models, respectively). When the effects of aviation sulfur emissions are taken into account in the atmospheric NOxbalance (via heterogeneous chemistry), the model-average net cooling effects of aviation NOxincreases to -6.2mW/m2. Our study applies to a sustained and constant aviation NOxemission and for the given background NOy conditions. The perturbation picture, however, may look different if an increasing trend in aviation NOxemissions would be allowed.
All Science Journal Classification (ASJC) codes
- Atmospheric Science