A global-scale study of the mixing state of black carbon and its effects on direct radiative forcing (formerly paper number P1.21)

Whether anthropogenic aerosols cool or warm climate hinges significantly on the mixing state of black carbon (BC). All estimates of aerosol climate effects to date have treated BC as externally or internally-mixed in one size distribution. In reality, the atmosphere contains multiple distributions, most containing mixtures of BC, organic matter, sulfate, sea spray, and/or soil. Here, global simulations, in which 16 size distributions were treated, were run to estimate the mixing state and radiative effects of BC. It was found that internal mixing due to coagulation, growth, and chemistry was rapid, increasing the mass of 90 percent of pure BC particles by >20 percent within two days. Coagulation alone caused 40 percent of this increase, demonstrating that it plays a crucial role in the global-scale internal mixing of aerosols. The steady-state BC direct forcing from multiple distributions was closer to that obtained when BC was a core in a single distribution than to that obtained when BC was externally-mixed. Thus, the lower bound of anthropogenic aerosol direct forcing should be closer to zero than all previous radiative forcing studies have predicted.