Possible Overestimation of Black Carbon Effects in Climate Models
Black carbon (BC) in the atmosphere has a strong effect on global and regional climate. Some estimates suggest that the positive (warming) radiative forcing by BC is second only to CO2, making it an important near-term climate mitigation target. In a recent study, direct measurements of BC absorption enhancements and average mixing state for BC in the atmosphere around California are reported from two field campaigns: the 2010 CalNex study and the U.S. Department of Energy’s Carbonaceous Aerosols and Radiative Effects Study (CARES). The CalNex measurements were made onboard the R/V Atlantis, whereas the CARES measurements were made at a ground site in the Sacramento urban area. Observations indicate that the BC absorption enhancements for ambient particles around large urban centers do not vary much with photochemical aging, are significantly less than predicted from traditional theory, and are in contrast to laboratory experiments. These findings suggest that the warming by BC may be overestimated in climate models. Further, they indicate a role for absorption at short visible wavelengths by non-BC aerosol components [brown carbon (BrC) in urban environments], which are not well quantified in current measurements or models.
Cappa, C. D., T. B. Onasch, P. Massoli, D. R. Worsnop, T. S. Bates, E. S. Cross, P. Davidovits, J. Hakala, K. L. Hayden, B. T. Jobson, K. R. Kolesar, D.A. Lack, B. M. Lerner, S.-M. Li, D. Mellon, I. Nuaaman, J. S. Olfert, T. Petäjä, P. K. Quinn, C. Song, R. Subramanian, E. J. Williams, and R. A. Zaveri. 2012. “Radiative Absorption Enhancements Due to the Mixing State of Atmospheric Black Carbon,” Science 337, 1078-81. DOI: 10.1126/science.1223447.