The authors have made major improvements to this paper, which have addressed the concerns of myself and the other reviewers. They have added new calculations to test the sensitivity of SO2 geoengineering to altitude of injection, and added important discussions in relation to other published works. I found the paper to be more scientifically sound at this point and worth of publication in ACP. However, there are quite a number of updates needed to correct errors of English usage, and a few errors of scientific terminology, as detailed below. I would deem these minor corrections.
Add comma on line 4 of abstract
“…depends upon the injected amount of sulfur dioxide, but aerosol model studies indicate…”
Add comma on line 9 of abstract
“To understand the effects of the injection of larger amounts of SO2, we have calculated the effects of SO2 injections…”
Abstract, line 12: I’m not sure that the aerosol layer loses efficiency at high injection rates. Rather, the aerosol layer and its burden and size distribution does not grow linearly with injection rate. It is the efficiency of the given geoengineering methodology that decays exponentially. Perhaps change to:
“Our calculations show that the efficiency of such a geoengineering method, expressed as… , decays exponentially.”
Abstract line 15: Incomplete sentence here with period after “2020”. Change to:
“required to keep temperatures constant at that anticipated for 2020, while maintaining “business as usual” conditions”
Page 3, line 12
“However, the results differ in detail, in the simulated particle size distribution as in the poleward transport of stratospheric aerosol particles.” As in doesn’t make sense here. Do you mean “and in”?
Page 3, line 13: Change “on” to “the”
“These differences have implications for the estimated radiative forcing and hence the impact of the stratospheric aerosols on the climate and ozone concentration.”
Page 3, line 27-28: Revise the sentence by removing both commas, changing “if” to “of”, and adding “the” before “RCP8.5”
“This raises the question of whether or not it will be possible to counteract strong GHG forcing, like the RCP8.5 scenario, for example, down to a level anticipated for 2020.”
Also, please give the reader an idea how large the RF needs to be (in 2070, 2100?) to maintain the 2020 RF level.
Page 4, line 9
“The model is not coupled to an ocean model, nor to a full atmospheric chemistry module.”
Page 4, line 11: Remove “increasing” and “of”
“ECHAM5-HAM simulations of injection rates up to 100Tg(S) yr−1
Page 4, line 14: Replace “different protocols defining the used aerosol module” with “different model configurations”, as both mode width and vertical resolution are discussed in Section 3.2.4.
Page 4, line 19: Please indicate how many years after 2020 before -5.5 W/m2 would be needed. Obviously not in 2030 or 2040.
“which would be necessary to reduce RCP8.5 anthropogenic forcing to a level anticipated for the year 2020 in years 20xx and later.”
Page 5, line 10: Precursor species are generally thought to be source gases which are precursors to other species of interest, such as OCS is a precursor to SO2, or N2O is a precursor to NO and NO2. I think the term is used incorrectly here. In this context, OH, NO2 (or do you mean NO3?), and O3 act as radical species which react with SO2. Also, is there no surface emission of SO2? Is SO2 prescribed at the tropical tropopause? If not, your model is missing a significant fraction of the sulfate aerosol burden in the very lowest stratosphere under nonvolcanic, non-geoengineering conditions. Though that shouldn’t matter for this study.
“The gaseous precursor species (OH, NO2, and O3) are prescribed…”
Page 5, line 27: Do you mean a narrower accumulation mode?
“The volcanic setup (Volc) contains no coarse mode and a smaller accumulation mode (standard deviation σAS = 1.2)”
Page 11, lines 3-5: change “particle” to “particles” twice.
“In contrast to the distribution of precursor gases and the particles in the nucleation mode, the distribution of the coarse mode particles along the Equator in both simulations is almost equal (Fig. 3, bottom).”
Page 13, line 24: Should be “model vertical resolution” not “vertical modal resolution”.
“Additional, we discuss the impact of the model vertical resolution on the results.”
Page 14, line 21: Change “amount” to “number” and “space” to “spacing”
“Increasing the number of vertical levels and, consequently, reducing the vertical grid spacing…”
Page 16, line 7: “24 km” should be “25 km”
“… 20 - 25 km in the SO2-BROAD”
Page 22, line 10: comma needed after “SO2”
“Similar to the injection of SO2, aerosol injections could also be considered.”
Page 22, line 11: Ferraro needs a year (2011) for the citation.
Figure 2: y-axis label should be “dN/dlogr” not “N/dlogr”.
Figure 4: Suggest that the legend labels in the left panel should be the same as the labels in the figure caption, rather than Geo10-145 and AREA-145 as they are now.
Figure 5 Caption: “Geo10-g” should be “Geo10-5”. Change “extend” to “extent”. Change “direction” to “directions”.
“… for the meridionally extended regions (Geo10-5, Geo10-30).”
“…varying extent of the injection area in zonal (Geo10-lon) and meridional (Geo10-5, Geo10-30) directions
Figure 7 caption: “Broad” should more accurately be described at 20-25 km. Pierce used 20-25, English used 19.9-24.6.