|The manuscript has been improved from the last version. However, I still think it has not yet met the standard of an ACP paper. |
1. My major concern is the writing style of the paper. The authors tend to provide too much information and do not organize it well, which makes a reader difficult to follow.
An example is Line 506-534, where the authors try to explain a lot of things in a very long paragraph without a clear reasoning flow. In addition, Section 3.2.2 is 7 pages long and the topic of the section changes from O3 to PM2.5 and then to aerosol composition without indications of topic changes.
Line 408-417 is yet another example. The authors described several previous studies one by one with some unnecessary details. In my opinion, this information should be presented in a more concise way and should be put in the context of the current study.
In addition, some wording may not be right, for example, “pure contribution” (L207), “pure impact” (L211), “outside emissions” (L24, 26, 375, 390, 509, 560), and “only-Beijing emissions” (throughout the text).
2. I am also concerned about how a reader would interpret the policy implication of the study because the authors did not thoroughly discuss the matter in the text. I think the authors need some extra work on the discussion, especially on how their work adds to the debate on whether local or non-local emissions play the major contribution to the air pollution in Beijing (Guo et al., 2014; Li et al., 2015;Zhang et al., 2015) .
The paper may lead a reader to conclude that the major culprit for air pollution in Beijing is neighboring regions. However, this interpretation may not be completely right.
(1) As shown in Figure 2, the emission rate from neighboring regions is not significantly higher than Beijing. Although the total emissions (Table 1) from the neighboring regions are large, the areas of those regions are also very large. From Figure 2, the area average emission rate (numbers in Table 1 divided by areas) of Beijing is probably still highest.
(2) Consistent with (1), Figure 9 and SI-Table 1 shows that the flux is almost symmetric around 0, suggesting Beijing is likely to contribute equal amount of pollution to the neighboring regions.
(3) BTH is a polluted air basin (Zhao et al., 2009; Parrish et al., 2015). Applying FSA to any city in BTH may generate similar results as in this paper.
The paper may also lead a read to conclude that the most effective way to control air pollution in Beijing is to reduce non-Beijing emissions in BTH. The FSA method is based on simulations completely turn on/off emissions from a certain region. Because a) the method cannot provide information on the local sensitivities of air pollution to emission reduction and b) emission reduction to zero in a vast region is apparently an infeasible scenario, inference of control strategies from the results is improper. In Line 568-575, the authors briefly mentioned this but this limitation is not explicitly stated.
Line 128 “2.2 Model Configuration”=> 2.2 Pollution episode simulation.
Line 219: “2.4 Statistical Methods for Comparisons”. => Statistical metrics for observation-model comparisons
Line 230: “2.5 Pollutants Measurements” =>2.5 Pollutant Measurements
Line 379: Apparently, …
Line 419: inducing the high O3 concentrations level in Beijing
Line 424-431: “The contribution of background to O3 is obvious, which is much more different from that for NO2. In addition, the trans-boundary transport flux of NO2 is much lower than O3 (Figure 9). Given NOx lifetime in the summer is short, regional transport of NOx is not important. Furthermore, the emissions of NOx and VOCs around Beijing are much more than those in Beijing, especially in Hebei and Shandong provinces, which is subject to contribute 429
more O3 production (Table 1). Compared to the direct input of regional O3, the transport of O3 precursors probably does not play an important role in the high O3 level in Beijing.”
Too much redundant or irrelevant sentences. I would change to “Compared to the direct input of regional O3, the regional transport of NOx is unlikely a significant contributor to high O3 concentrations in Beijing, partly due to its short lifetime in the summer.”
Line 464: e.g., i.e.,
Line 483: “high atmospheric oxidation capability caused by elevated O3 concentrations during summertime”. High atmospheric oxidation capacity is not directly caused by high O3 concentrations.
Line 489-491: Replicate of line 482-483.
Line 500-501: “…which is caused by the aerosol radiative effect. It is clear that the
PBL-pollution interaction plays an important role in the pollutant accumulation in Beijing…” This effect may not be called the aerosol radiative effect. I think PBL-pollution interaction is more proper.
Section SI-3: My suggestion of a “control” case in last review was to present some results in the clean period, which the authors seemed to misunderstand. I was thinking a comparison between a clean and a polluted episode may bring some insight into the regional contribution. Section SI-3 presents a polluted episode similar to the one in the main text. The results are essentially the same. I would suggest to remove this section and relevant sentences in the main text.
Parrish DD, Stockwell WR (2015) Urbanization and air pollution: Then and now. EOS 96(1):12–15.
Zhao, C., Y. Wang, and T. Zeng (2009) East China plains: A “basin” of ozone pollution, Environ. Sci. Technol., 43, 1911–1915.
Guo S, Hu M, Zamora ML, et al. Elucidating severe urban haze formation in China. Proceedings of the National Academy of Sciences of the United States of America. 2014;111(49):17373-17378. doi:10.1073/pnas.1419604111.
Li P, Yan R, Yu S, Wang S, Liu W, Bao H. Reinstate regional transport of PM2.5 as a major cause of severe haze in Beijing. Proceedings of the National Academy of Sciences of the United States of America. 2015;112(21):E2739-E2740. doi:10.1073/pnas.1502596112.
Zhang R, Guo S, Levy Zamora M, Hu M. Reply to Li et al.: Insufficient evidence for the contribution of regional transport to severe haze formation in Beijing. Proceedings of the National Academy of Sciences of the United States of America. 2015;112(21):E2741. doi:10.1073/pnas.1503855112.