Review of “Mapping the dependence of BC radiative forcing on emission region and season”.

 Summary

This paper uses a large set of simulations with the NorESM1-Happi aerosol scheme in an offline mode (i.e., aerosols do not effect climate) to quantify the BC radiative forcing (direct and snow/ice) in response to BC emissions perturbations in 192 grid boxes.  Seasonally uniform emissions, as well as emissions in each season are separately evaluated.  The authors find the BC RFs significantly vary as a function of emission location (spatially) and season.  When normalized by emissions, the direct specific BC RF varies by up to a factor of 10 (even larger for the corresponding snow/ice RF).  The authors also discuss reasons for the spatial variability (e.g., enhanced lofting in tropical convective regions, which enhances the BC lifetime and the corresponding RF).  The authors also show that for present-day BC emissions, the RF is nearly additive across regions/seasons (within ~10%); however, the indirect BC RF is not.

Overall, this is a well written paper that presents comprehensive model simulations and interesting results relevant for SLCF mitigation.  The main novelty here is the use of much smaller emission regions (5x5 grid boxes, as opposed to continental scale).   This is more relevant to BC mitigation.  This in turn leads to the conclusion that the BC specific RF (spatially/seasonally) varies more than previously recognized.   

Limitations of the present study (which are discussed in Section 6.2) include the fact the authors are not able to quantify the rapid adjustments, which have been shown to be very important for the climate impacts (e.g., Stjern et al. 2017).  See also:

Smith, C. J., Kramer, R. J., Myhre, G., Forster, P. M., Soden, B. J., Andrews, T., et al. (2018). Understanding rapid adjustments to diverse forcing agents. Geophysical Research Letters, 45, 12,023– 12,031. https://doi.org/10.1029/2018GL079826

Specific Comments

L130 “the semidirect effect of BC cannot be included”.  Shouldn’t this be more general, i.e., rapid adjustments cannot be included?  Semi-direct effects traditionally refer to clouds alone, but there are several rapid adjustments including those associated with the clouds.

Section 2.3  In the context of BC emissions, the two main sources are fossil fuel and biomass burning.  In contrast to fossil fuel BC emissions, biomass burning BC emissions are likely less easily controlled to mitigation policies.  Is there any utility in separating the two?  Probably beyond the scope of this work, but perhaps the authors could comment.

L200. In the context of convective lofting, see also:

Park, S., and Allen, R. J. (2015), Understanding influences of convective transport and removal processes on aerosol vertical distribution, Geophys. Res. Lett., 42, 10,438– 10,444, doi:10.1002/2015GL066175.

This is a useful study of the effect of emission location on BC forcing efficiency. As the authors themselves note, this is a single model study with lots of uncertainty.  However they explain the physical processes behind their results which are likely to be more generally applicable even if the absolute numbers are not. This will be useful to publish after addressing the points below.

I understand the reluctance for detailed indirRF calculations since they are not likely to be additive, but it might still be useful to see the regional variation. A version of fig 4 for indirRF could be added to the supplement. Why is the indirRF positive? Does mixing with BC reduce the nucleating ability of SO4, or does it reduce the SO4 lifetime? How is indirRF calculated? Is it a double call as in Ghan 2013?

Even though they can’t be addressed in this study, there could be a bit more mention of meteorological adjustments to BC,  for instance the increased stabilisation of the atmospheric profile, and how they might affect the conclusions. Stjern et al. is cited, but not the discussions in that paper, also there are Samset papers. These meteorological adjustments will be included implicitly in the Shindell and Faluvegi coefficients. What they term “efficacy” is really an accounting for adjustments.

Introduction: Could also cite Aamaas et al. 2016 and Bellouin et al. 2016 papers from ECLIPSE.

Line 85: This should mention the magnitude of indirRF here -  it seems to be ~25% of dirRF.

Line 130: Suggest to use “meteorological adjustments” or “rapid adjustments” rather than “semi-direct effect” to align with IPCC terminology.

Line 136: Give some explanation of how F_air is calculated. Presumably this is from a double call to the radiation scheme with zero BC in the advancing step.

Section 2.3: This might comment on how the mixing affects other species. Does mixing with SO4 affect the lifetime of BC, i.e. do BC emissions have a lower lifetime if they are emitted in a high SO4 region? Does mixing with BC affect the lifetime of SO4, i.e. is there an indirect effect of BC on SO4 RF and if so is this included in F_air?

Figure 1. I suggest splitting into COARSE-REAL which shows the impact of resolution, and then RECONST-REAL which shows the additivity. I’m not sure RECONST-REAL is that useful since it mixes these two effects.

Line 240: Why aren’t the reconstructed fields for REAL and COARSE actually identical as opposed to “virtually” identical. Similarly, in fig S1 are the emissions for RECONST and COARSE identical, and if not, why not?

Figure 3: Why doesn’t dirRF scale with column burden? I would have expected 3(a) and 3(c) to be much more similar since section 6.3 suggests little non-linearity in dirRF. A plot of dirRF/columnBC would be useful in the supplement.

Line 377: The longitudinal variation seems interesting, and very policy relevant.

Section 6.1: Suggest to also compare with ECLIPSE project, Bellouin et al. 2016

Section 6.2: Suggest to also compare with ECLIPSE project, Aamaas et al. 2016 and 2017.

Lines 389: This paragraph would be clearer if it included discussion of meteorological adjustments and ERFs. The reason dirRF for BC has less effect on climate is not because it has lower “efficacy”, it is because there are adjustments that oppose the dirRF so that the ERF is lower than dirRF (e.g. Stjern et al. 2017). Studies of BC efficacy defined in terms of ERF (E.g. Richardson et al. 2019) show an efficacy of around 1.0 when compared to CO2.

Figure 7: This figure is presumably very sensitive to the assumed factor of 3 efficacy for snowRF. What is the uncertainty in this factor of 3? Would the conclusions be qualitatively the same with a lower factor?

Line 518: Suggest to use “meteorological adjustments” or “rapid adjustments” rather than “semi-direct effect” to align with IPCC terminology.