Jury is still out on the radiative forcing by black carbon

Abstract

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 unspecified 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. 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 nearconstant 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 dilution effects may lengthen the aging timescale in the real atmosphere compared with the static chamber measurements performed by Peng et al. It is unclear how representative these measurements are for global and annual averages, but we know that generalizations can introduce serious errors due to spatial and temporal sampling issues (9). This implies that a simple scaling of the BC RF by the absorption enhancement factor measured by Peng et al.-as performed by the authors in their table S1 and figure 4, and extended in the commentary (10)-is overly simplistic.In conclusion, although we welcome the advances made by Peng et al., their conclusion of a +0.45 [0.21-0.80] W·m −2 additional RF due to a large BC enhancement factor is premature. The jury is still out on the question of the net climate impact of BC and how much climate cobenefit will result from the necessary mitigation of BC emissions. Reducing the uncertainty on the BC forcing requires better constraining BC MAC and atmospheric lifetime in global aerosol models.

Reply to Boucher et al.: Rate and timescale of black carbon aging regulate direct radiative forcing

“…. .is consistent with recent findings" (8). Indeed, the consistency in the recent atmospheric measurements at different global locations confirms that the results by Peng et al are applicable to diverse environmental conditions (7).…”

“…. .is also an upper bound" (8). Furthermore, they notice the times of 5 (Beijing) to 18 (Houston) hours to achieve the BC absorption enhancement factor of 2.4 reported by Peng et al…”

“…Gustafsson and Ramanathan (7) comment that "Peng et al assess the implications on climate by the constrained coatingenhancement of BC absorption" and "if this were to be confirmed by forthcoming studies, also focusing on properties of aged aerosols most relevant to large-scale effects, this will close the factor of 2-3 gap between model predictions and observations on the effect of BC aerosols on climate." In contrast, Boucher et al (8) argue that "the jury is still out on the question of the net climate impact of BC and how much climate cobenefit will result from the necessary mitigation of BC emissions." We noticed that there exist several confusing views in the Letter by Boucher et al First, the authors claim that "their 'enhancement factor'.…”

Rapid Adjustments Cause Weak Surface Temperature Response to Increased Black Carbon Concentrations

“…Quantifying the magnitude of these impacts is crucial to our ability to provide accurate estimates of future climate change. However, the climate impact of aerosols in general, and BC in particular, is still associated with significant uncertainty (Bond et al, 2013;Boucher et al, 2016Boucher et al, , 2013Samset, Myhre, & Schulz, 2014).…”

Are Changes in Atmospheric Circulation Important for Black Carbon Aerosol Impacts on Clouds, Precipitation, and Radiation?

“…Due to the complex nature of such interactions and disagreement among models, the role of BC in anthropogenic climate change remains uncertain and poorly understood (Andreae & Ramanathan, 2013;Boucher et al, 2013;Wang, 2013b). The problem is compounded by difficulties in constraining the emissions, transport, and atmospheric lifetime of BC, leading to large uncertainties in the BC mass concentrations in models (Bond et al, 2013;Boucher et al, 2016). A further source of uncertainty is the high sensitivity of BC absorption to particle properties and mixing state (Jacobson, 2001;Matsui et al, 2018;Peng, 2016;Samset et al, 2018).…”