Effects of the physical state of tropospheric ammonium-sulfate-nitrate particles on global aerosol direct radiative forcing

[摘要] The effect of aqueous versus crystalline sulfate-nitrate-ammonium tropospheric particleson global aerosol direct radiative forcing is assessed. A global three-dimensional chemicaltransport model predicts sulfate, nitrate, and ammonium aerosol mass. An aerosolthermodynamics model is called twice, once for the upper side (US) and once for lower side (LS)of the hysteresis loop of particle phase. On the LS, the sulfate mass budget is40% solid ammonium sulfate, 12% letovicite, 11% ammonium bisulfate, and37% aqueous. The LS nitrate mass budget is 26% solid ammonium nitrate, 7% aqueous, and67% gas-phase nitric acid release due to increased volatility upon crystallization. The LS ammonium budget is45% solid ammonium sulfate, 10% letovicite, 6% ammonium bisulfate, 4% ammonium nitrate,7% ammonia release due to increased volatility, and 28% aqueous. LS aerosol water mass partitionsas 22% effloresced to the gas-phase and 78% remaining as aerosol mass. The predicted US/LSglobal fields of aerosol mass are employed in a Mie scattering model to generateglobal US/LS aerosol optical properties, including scattering efficiency, single scatteringalbedo, and asymmetry parameter. Global annual average LS optical depth and mass scattering efficiencyare, respectively, 0.023 and 10.7 m² (g SO₄^-2)^-1, which compare to US values of 0.030 and13.9 m² (g SO₄^-2)^-1. Radiative transport is computed, first for a base case having no aerosol and then for thetwo global fields corresponding to the US and LS of the hysteresis loop. Regional, global,seasonal, and annual averages of top-of-the-atmosphere aerosol radiative forcing on the LS andUS (F_L and F_U, respectively, in W m^-2) are calculated. Including both anthropogenic and naturalemissions, we obtain global annual averages of F_L=-0.750,F_U=-0.930, and DF_U,L=24% forfull sky calculations without clouds and F_L=-0.485, F_U=-0.605, andDF_U,L=25% when cloudsare included. Regionally, DF_U,L=48% over the USA,55% over Europe, and 34% over East Asia. Seasonally, DF_U,Lvaries from 18% in DJF to 75% in SON over the USA. The global annual average contribution from anthropogenic aerosol isF_L=-0.314 and F_U=-0.404, which yield normalized direct radiative forcings(G) of G_L=-205 W (g SO₄^-2)^-1and G_U=-264 W(gSO₄^-2)^-1.

[学科分类] 大气科学