Effects On Optical And Radiative Properties Of Black Carbon During Mixing And Hygroscopic Growth

Unlike other aerosols in the atmosphere,Black carbon(BC),a kind of absorbing aerosol,is the second anthropogenic global warming factor only after carbon dioxide,which has a significant impact on regional climate and radiative forcing.When BC particles are emitted into the atmosphere,it will mix with other aerosols and the mixed aerosols become hydrophilic when mixing with soluble matter,their optical and radiative properties can be significantly changed.This study numerically investigates the impact of BC aggregate structure on optical and radiative properties of aged BC,i.e.,BC coated by sulfate or organic aerosols,especially during hygroscopic growth.A more realistic BC morphology based on fractal aggregates is considered,and inhomogeneous mixtures of BC aggregates are treated more realistically(with respect to particle geometries)in the multiple sphere T-matrix method for optical property simulations.During mixing process,the absorbing ability of aged BC may be enhanced when coated by non-absorbing aerosols,and the ability increase with the scattering ability of coatings.Besides,the RF of BrC coated BC aerosols in the atmosphere is about 1.5 times compared with those sulfate/OC coated.Besides,for mixed aerosols,simply treating BC as a sphere will underestimate 12.5%it's forcing in the atmosphere.During the hygroscopic growth process,as relative humidity increases,coated BC extinction is significantly enhanced due to an increase in scattering,and the enhancement depends on the amount and hydrophilicity of the coating.The hydrophilicity of sulfate is stronger than that of non-absorbing organic carbon(OC),so the extinction enhancement of sulfate-coated BC is more obvious.The absorption exhibits less variation compared with the extinction during hygroscopic growth because the coating of aerosols already leads to BC absorption close to the maximum.Furthermore,hygroscopic growth not only results in negative radiative forcing(RF)at the top of the atmosphere(TOA)but also slightly weakens the BC absorption in the atmosphere(inducing a negative RF in the atmosphere).Compared to the more realistic model with BC as aggregates,the currently popular core-shell model reasonably approximates the TOA RF but underestimates the atmospheric RF due to hygroscopic growth by up to 40%.Furthermore,based on the configuration invested in this study,the core-shell model overestimates the RFs due to internal mixing at the surface and in the atmosphere by?10%.Since the absorption decrease of OC is stronger than the sulfate,the cooling effect of hygroscopic growth with OC coating is more obvious.