Aircraft emissions may perturb the global amount and the size distribution of atmospheric aerosols in two ways: (a) direct emission of ultrafine black carbon (BC) soot and sulphuric acid particles in aircraft plumes and (b) release of gas phase SO2, later dispersed on large atmospheric scales and oxidized in H2SO4 by the OH radical. Direct particle emissions are estimated to account for 4–15% of the overall aircraft emitted sulphur. These direct particle emissions do not significantly change aerosol mass and extinction but may substantially increase surface area density (SAD) in the Northern Hemisphere UTLS. Release of gas phase SO2, on the other hand, may increase the net production of H2SO4, thus enhancing the sulphate mass in the accumulation mode and consequently, the direct RF. It also may increase the gas phase contribution to SAD, in the range of 25% of the change produced by direct plume particle emission. The University of L’Aquila climate-chemistry coupled model (ULAQ-CCM) has calculated the accumulation of sulphate aerosols and BC and their globally averaged direct radiative forcing (RF) at the NCEP tropopause with temperature adjustment and in total sky conditions, i.e. -3.3 W/m2 and +1.1 W/m2, respectively (with forcing efficiencies of -140 W/g-SO4 and +2300 W/g-BC, respectively). The increase of BC in the upper troposphere due to aviation emissions may trigger formation of ice cloud particles (i.e. aviation ‘soot-cirrus’). The formation of background upper tropospheric ice particles is produced by homogeneous and heterogeneous freezing of supercooled aerosols. The ULAQ-CCM considers the basic physical processes that eventually determine the number of ice crystals Ni forming during an adiabatic ascent, including the link of Ni on temperature and updraft speed. In normal conditions the homogeneous freezing mechanisms dominates, but under significant local emissions of BC from aircraft the competition of heterogeneous and homogeneous freezing mechanisms becomes important. In the parameterization used for the formation of aviation soot-cirrus particles in a model grid-box, the change of ice crystals number concentration ∆Ni-HET is calculated as a function of ∆NBC and PHET, where ∆NBC is the change of soot particles due to aviation emissions, assuming a 1% non-hydrophobic fraction of the particles that may act as ice nuclei. PHET, in turn, is the probability that heterogeneous freezing may occur at given grid-box in the model, calculated as the probability to have ice super-saturation for a given temperature (RHICE › 100%) and taking into account water vapour transport due to subgrid vertical updraft velocity. The ULAQ-CCM calculates a +4.9 W/m2 indirect RF of BC, through formation of soot-cirrus (with forcing efficiency of +5 W/g-ice). Uncertainties in this model calculation of soot cirrus RF are however rather large. The feedback of aviation-produced sulphate aerosol SAD on heterogeneous NOx chemistry represents another significant indirect radiative impact of aviation aerosols, via O3 and CH4 changes produced by the aerosol induced NOx perturbation. Here the physical and chemical approaches and the final calculations are much more robust than for upper tropospheric ice. In this case the results of the University of Oslo models (i.e. UiO-CTM2 and UiO-CTM3) have been used together with those from the ULAQ-CCM used in CTM mode, with the following results: -0.8 ± 0.2 W/m2 and +0.5 ± 0.1 W/m2, for O3 and CH4, respectively, where the error bar is obtained from the three models dispersion. The net aviation-aerosol RF calculated in this study accounts to -2.2 W/m2 (direct) and +4.6 W/m2 (indirect, including soot-cirrus) or -0.3 W/m2 (indirect, not including soot-cirrus), that is (in total) +2.4 W/m2 (including soot-cirrus) or -2.5 W/m2 (not including soot-cirrus). Taking into account that the non-CO2 aviation RF of gas species as calculated in the ULAQ-CCM (i.e. O3 and CH4 from aviation NOx changes and stratospheric H2O) accounts to +9.0 W/m2, the net aerosol impact represents a relative correction of the radiative forcing equal to +27% (including soot-cirrus) and -28% (not including soot-cirrus) .

### Multi-model estimate of direct and indirect radiative impact of aviation aerosols

#####
*PITARI, Giovanni;*

##### 2013-01-01

#### Abstract

Aircraft emissions may perturb the global amount and the size distribution of atmospheric aerosols in two ways: (a) direct emission of ultrafine black carbon (BC) soot and sulphuric acid particles in aircraft plumes and (b) release of gas phase SO2, later dispersed on large atmospheric scales and oxidized in H2SO4 by the OH radical. Direct particle emissions are estimated to account for 4–15% of the overall aircraft emitted sulphur. These direct particle emissions do not significantly change aerosol mass and extinction but may substantially increase surface area density (SAD) in the Northern Hemisphere UTLS. Release of gas phase SO2, on the other hand, may increase the net production of H2SO4, thus enhancing the sulphate mass in the accumulation mode and consequently, the direct RF. It also may increase the gas phase contribution to SAD, in the range of 25% of the change produced by direct plume particle emission. The University of L’Aquila climate-chemistry coupled model (ULAQ-CCM) has calculated the accumulation of sulphate aerosols and BC and their globally averaged direct radiative forcing (RF) at the NCEP tropopause with temperature adjustment and in total sky conditions, i.e. -3.3 W/m2 and +1.1 W/m2, respectively (with forcing efficiencies of -140 W/g-SO4 and +2300 W/g-BC, respectively). The increase of BC in the upper troposphere due to aviation emissions may trigger formation of ice cloud particles (i.e. aviation ‘soot-cirrus’). The formation of background upper tropospheric ice particles is produced by homogeneous and heterogeneous freezing of supercooled aerosols. The ULAQ-CCM considers the basic physical processes that eventually determine the number of ice crystals Ni forming during an adiabatic ascent, including the link of Ni on temperature and updraft speed. In normal conditions the homogeneous freezing mechanisms dominates, but under significant local emissions of BC from aircraft the competition of heterogeneous and homogeneous freezing mechanisms becomes important. In the parameterization used for the formation of aviation soot-cirrus particles in a model grid-box, the change of ice crystals number concentration ∆Ni-HET is calculated as a function of ∆NBC and PHET, where ∆NBC is the change of soot particles due to aviation emissions, assuming a 1% non-hydrophobic fraction of the particles that may act as ice nuclei. PHET, in turn, is the probability that heterogeneous freezing may occur at given grid-box in the model, calculated as the probability to have ice super-saturation for a given temperature (RHICE › 100%) and taking into account water vapour transport due to subgrid vertical updraft velocity. The ULAQ-CCM calculates a +4.9 W/m2 indirect RF of BC, through formation of soot-cirrus (with forcing efficiency of +5 W/g-ice). Uncertainties in this model calculation of soot cirrus RF are however rather large. The feedback of aviation-produced sulphate aerosol SAD on heterogeneous NOx chemistry represents another significant indirect radiative impact of aviation aerosols, via O3 and CH4 changes produced by the aerosol induced NOx perturbation. Here the physical and chemical approaches and the final calculations are much more robust than for upper tropospheric ice. In this case the results of the University of Oslo models (i.e. UiO-CTM2 and UiO-CTM3) have been used together with those from the ULAQ-CCM used in CTM mode, with the following results: -0.8 ± 0.2 W/m2 and +0.5 ± 0.1 W/m2, for O3 and CH4, respectively, where the error bar is obtained from the three models dispersion. The net aviation-aerosol RF calculated in this study accounts to -2.2 W/m2 (direct) and +4.6 W/m2 (indirect, including soot-cirrus) or -0.3 W/m2 (indirect, not including soot-cirrus), that is (in total) +2.4 W/m2 (including soot-cirrus) or -2.5 W/m2 (not including soot-cirrus). Taking into account that the non-CO2 aviation RF of gas species as calculated in the ULAQ-CCM (i.e. O3 and CH4 from aviation NOx changes and stratospheric H2O) accounts to +9.0 W/m2, the net aerosol impact represents a relative correction of the radiative forcing equal to +27% (including soot-cirrus) and -28% (not including soot-cirrus) .I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.