Jury is still out on the radiative forcing by black carbon

• Published in: Proceedings of the National Academy of Sciences
• Main Authors: Boucher, Olivier, Balkanski, Yves, Hodnebrog, Øivind, Lund Myhre, Cathrine, Myhre, Gunnar, Quaas, Johannes, Hallvard Samset, Bjørn, Schutgens, Nick, Stier, Philip, Wang, Rong
• Other Authors: Institut Pierre-Simon-Laplace (IPSL (FR_636)), École normale supérieure - Paris (ENS Paris), Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Institut national des sciences de l'Univers (INSU - CNRS)-École polytechnique (X)-Centre National d'Études Spatiales Toulouse (CNES)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Université de Paris (UP), Laboratoire des Sciences du Climat et de l'Environnement Gif-sur-Yvette (LSCE), Institut national des sciences de l'Univers (INSU - CNRS)-Université Paris-Saclay-Centre National de la Recherche Scientifique (CNRS)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ), Modelling the Earth Response to Multiple Anthropogenic Interactions and Dynamics (MERMAID), Institut national des sciences de l'Univers (INSU - CNRS)-Université Paris-Saclay-Centre National de la Recherche Scientifique (CNRS)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Paris-Saclay-Centre National de la Recherche Scientifique (CNRS)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ), Center for International Climate and Environmental Research Oslo (CICERO), University of Oslo (UiO), Fudan University Shanghai
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2016
• Subjects: envir; geo; Arctic; Indian; black carbon
• Online Access: https://doi.org/10.1073/pnas.1607005113; https://hal.archives-ouvertes.fr/hal-02875486/file/PNAS-2016-Boucher-1607005113.pdf; https://hal.archives-ouvertes.fr/hal-02875486

International audience Peng et al. (1) conclude that a fast increase in the mass absorption cross-section (MAC) of black carbon (BC) in urban environments leads to significantly increased estimates of the BC radiative forcing (RF). Their chamber measurements are highly valuable and complement observations performed in ambient conditions, but their "enhancement factor" relative to an unspec-ified baseline may not be directly comparable to values used or simulated in global aerosol models. MAC, a key parameter in our understanding of the net BC climate impact, is indeed a more relevant quantity to examine. A fast MAC enhancement in polluted environments as the BC gets coated with organic and inorganic species is consistent with recent findings (2, 3). Global models used in AeroCom [table S1 in Peng et al. (1), ref. 4] have an average MAC of ∼8 m 2 ·g −1 at 550 nm. This value is reflecting reported measurements, although there is a large spatial and seasonal variability in ambient MAC for aged particles, with values of ∼10 m 2 ·g −1 at a rural Northern Chinese site (2) (at 678 nm); 6-14 m 2 ·g −1 at rural, urban, and high-altitude Indian locations (5) (at 678 nm); and ∼6 m 2 ·g −1 at an Arctic site (6) (at 522 nm). Coating of BC by soluble species not only enhances absorption of solar radiation but also reduces the BC atmospheric lifetime (7). Fig. 1 shows an offset between the increase in average MAC value with faster BC aging and an overall shorter BC lifetime, resulting in a near-constant BC aerosol absorption optical depth and RF with aging time. Furthermore, current global aerosol models frequently have a too long BC lifetime and consequently overestimate BC concentrations downwind from source regions (8). According to Peng et al. (1), their BC absorption enhancement factor of 2.4 is also an upper bound, only reached after 5 (Beijing) to 18 (Houston) h, and possibly longer in cleaner environments. Such timescales are not small compared with the BC atmospheric lifetime of 3-5 d, especially considering that ...