|The authors have modified the original manuscript, but several of the requested "major revisions" have not been resolved.|
I cannot recommend publication before the following issues have been adressed adequately:
1. My main concern about the study is that the authors are quite bold with there statement that Chinese NO2 levels would be higher by 30% (now 25%) without regulations.
This is quite speculative and not supported by the shown data: on the one hand, the authors claim that regulations were increasingly taken over the last years, while on the other hand the 25% reduction is just estimated based on the value in 2015. So if the authors would have had written their study one year earlier, based on data for 2005-2014, they would have had to conclude that the many regulations taken so far are useless!
So unless the authors provide a good (and quantitative) explanation on why the NOx per fossil fuel ratio kept almost constant in 2010-2014, despite regulations, they cannot claim that the oberved reduction in 2015 allows for a direct interpretation as "this is the accurately quantified benefit of regulation efforts".
I already made this point in my first review. The reply given by the authors and their modifications in the text expand the discussion of (a) the changing role of transport and shipping, and (b) the effectiveness of SCR equipment.
However, (a) is provided qualitatively (which is fine for undestanding the general pattern, but far too vague to assess the robustness of the 25% reduction estimate),
while (b) refers to a submitted paper, not accessible yet, and now suddenly claims SCR equipment playing a key role (something not mentioned at all in the first version nor in the introduction of the second version).
In addition I would like to point out that if you have to consider changes of the relative role of traffic (without having a specific number for this), the concept of the simple NOx/fuel ratio is meaningless.
So the authors have to actually explain the years 2010-2014 as well as the year 2015 with data for (ship) traffic, SCR, etc. (including some error estimate of the NOx/fuel ratio, which is affected by uncertainties in both NOx emissions and fossil fuel inventory), or have to weaken their statements on NOx reduction.
2. The topic of the paper is regulation, but the reader learns almost nothing about it.
In my first review, I have asked to "discuss the different possibilities in general, and the taken measures in detail". While the authors have addressed the second point in a new table, the general possibilities/techniques for reducing NOx emissions for cars, PPs etc. are not discussed in the introduction. This has to be added (including SCR!) with appropriate references.
3. Finally, I have asked for analysis on province level, as well as the other reviewer. While the authors now provide provincial data, they do not actually provide provincial analysis. This, however, would provide a qood opportunity to shed light on the discussion of the effectiveness of regulations. So, how does the NOx per fuel ratio look like for different provinces? Is it different for coastal provinces (with significant ship emissions)? Can you relate the temporal patterns per province to SCR equipment? Are the temporal patterns for Shanghai Expo/Olympic games/APEC matching our expectations?
Such an in-detail analysis on provincal level would reveal how robust the presented ratio actually is.
- Section 3.3: "Total NOx emissions in East China reached their peak levels in 2012, and have been decreasing since".
From Figure 5, I would rather claim that overall emissions have stayed constant for 2012-2014, and decreased in 2015.
- Table 2: Please include a column on the expected NOx reduction.