|The authors have done a good job responding to reviewer comments. They have brought in a lot more material from their companion paper on the model description and evaluation, which address reviewer concerns in those regards. I have a few remaining comments, described below, which amount to only minor changes to manuscript text and this minor revisions, after which point the paper will be suitable for publication in ACP. |
Presentation of the main equations still comes across as a bit folksy. The authors refer to it as “perturbation approach” — their quotes, not mine, sometimes double and sometimes single — but more rigorously I think as scientists they can more specifically refer to this as a first order approximation that includes the first (linear) term of a Taylor expansion of PM as a function of emissions. Without the remaining higher order terms the expression is approximate.
Further, the authors state “So the equal sign is correct, although this equation represents an approximation”, which is an oxymoron. The authors confuse discussion of a computational equation implemented in their model (which may well be approximate) and noting whether or not that equation is exact (with an equals sign) or an expression of an approximation (with an approximation sign). In this case it is the latter, and the equation on paper needs to be fixed to show this.
That being said, I appreciate the additional discussion added to the main text and the SI regarding the equations used for estimating PM responses owing to emissions perturbations, which have indeed helped make the manuscript stronger and more complete.
The additional content on model accuracy, again drawing from the companion paper, is now more detailed, which is appreciated.
The response regarding other sources of uncertainty — I appreciate the added discussion regarding model errors from Solazzo 2018. However my comments were with regards to uncertainties in the concentration-response functions, which are typically the only ones considered. Further, the response of the authors in this regard could still be much stronger. The title of this paper includes “uncertainty analysis”. However, the abstract only notes that the uncertainty analysis for health impacts was performed (last sentence), and does not even state the results. This is a big loss for this work — the authors should do a better job of capturing these quite interesting results (up to 1 million premature deaths uncertainty associated with emissions uncertainties?) in the abstract and conclusions, specifically in comparison to the level of uncertainty normally associated with these types of studies. The only other statement regarding uncertainty in the abstract (second to last sentence) is rather obvious and could be omitted, unless it is going to be quantified.
Regarding SOA, given the pace at which our understanding of how SOA forms has evolved, I’m not sure a 2010 paper (Farina) is “recent” anymore. But still, the discussion here is appreciated.
In response to my question about previous line 12.10 - 15 (“why doe these regions have large extra-regional contributions”), the response (health impacts are large because pollution is large) is a bit lacking. Why is it larger here, say, than other parts of the world? Is the long-range transport here particularly strong or efficient? I also wonder if the answer may have to do with underlying baseline mortalities being higher in some regions.