The transformation of black carbon aerosols from hydrophobic to hydrophilic and the global distribution

The transformation time for black carbon (BC) aerosols from hydrophobic to hydrophilic is about 10 times shorter in urban than in remote areas. Condensation of H2SO4 vapor is found to be the most important mechanism in the transformation processes. The representative transformation time for this mechanism ranges from about 30 hours to 100 hours in urban and urban-influenced rural areas, and the transformation time in remote areas is in between. The homogeneous nucleation of H2SO4 vapor and water vapor reduces the aging time in polluted areas in different degrees, depending on the concentration of H2SO4 vapor. The coagulation of hydrophobic BC aerosols with pre-existing water-soluble aerosols is important only in urban and urban influenced rural areas, while the coagulation with cloud droplets is relatively more important in remote areas. Sensitivity tests on several important microphysical parameters, such as the accommodation coefficient and the nucleation factor in the condensation and nucleation of H2SO4 vapor show that the transformation time is affected very weakly by variations of these parameters. The variation of temperature, humidity, and pressure does not affect the transformation time. The soluble mass fraction on BC aerosols at the conversion point from hydrophobic to hydrophilic depends on BC aerosol size and the supersaturation Sc. The lower limit of the soluble mass fraction corresponding to Sc of 0.8% is ~53% for hydrophobic BC aerosols with diameters of 30nm, which is several times higher than the values assumed in several other BC aerosol modeling researches. Since BC aerosols are removed from the atmosphere much more quickly near the emission sources, the importance of emission is reduced compared to using a uniform transformation time. And since the condensation of H2SO4 vapor plays a critical role in the transformation process, one would expect that BC aerosol modeling be improved significantly if simulated together with sulfate chemistry. A global chemistry model, MOZART II, is applied in this purpose. There are ozone, sulfate, ammonia, and nitrate chemistry in the version of MOZART II for this study. The results are compared with simulations by using a global uniform transformation time.