|The authors made great efforts of revising the manuscript. However, the paper is still not well written, and the conclusions were not convincingly supported by the data and method. This is really an interesting scientific issue. I think there is still considerable more work necessary to get the manuscript ready for publication at ACP. My major concerns are as follows:|
(1) The whole manuscript is based on the assumption that the co-occurrence of high ozone and PM2.5 is under high HONO concentration. This assumption is highly possible to be true, but it is lack of supportive measurement data. The authors have valuable HONO measurements at three mega-cities including Beijing, Shanghai and Xi’An shown in Figure 8. Since ozone and PM2.5 are routine measurement air pollutants, I would recommend including them into the plot as well. Also, in Figure 8, since the measurement time is different, I do not think they are comparable. I recommend separating Figure 8 into three subplots by including ozone and PM2.5, and each subplot is for each city. So that the assumption should be more solid.
(2) The authors still did not state the set up of the WRF-Chem simulation, e.g. the gas-phase mechanism used in the model? The authors need to at least briefly explain why the HONO calculated by WRF-Chem is much lower than the observation. I think the model only consider the HONO source with NO+OH only right? Also, how could the authors compare one WRF-Chem modeling result to observations at three different cities during three measurement time periods? All of those statement and comparison are not rigorous. Please revise.
(3) Some conclusions and rationales are not rigorous. For example:
Line 278-279: Unless the authors show the error bars, this conclusion is not solid.
Line 281-287: see my major concern (1).
Line 289-295: If it is possible, it would be very helpful to include ozone measurement into Figure 9 as well.
(4) The literature is not cited properly:
Line 100-102: the mixed regime for ozone formation is missed in the statement.
Line 130: Shi et al. (2015) never talked about “several potential HONO sources, including surface emissions, conversion of NO 2 at the ocean surface, etc., and adding these sources can improve the calculated HONO concentrations.” These conclusions are from Zhang et al. (2016).
Line 266: see my comments above, wrong citation.
(5) The paper is not very well written and organized. There are numerous typos and grammar errors. Please carefully review the whole manuscript and revise them accordingly. I listed some as follows, but not limited to:
Line 35: only “fall”? It seems the authors mentioned both “late spring and fall” in the manuscript?
Line 56: here is “spring and fall”? Please be consistent through the whole manuscript.
Line 99: grammar error - “… are becomes ...” Please revise.
Line 121: is it just “fall” or “late spring and fall”? Please be consistent through the whole.
Line 145 and 219: two section 2? Please revise.
Line 174-176: the sentence is redundant. Consider the following:
“The heavy aerosol concentrations play important roles to reduce solar radiation, causing the reduction of O3 formation.”
Line 176: there is no Fig. 3a. Please indicate the upper panel as (a) in the plot or in the figure capital.
Line 187: now the seasons include “late spring, summer, and early fall” instead of “late spring and fall”. I am very confused. Please be consistent about the seasons through the whole manuscript.
Line 204-205: the sentence is redundant. Consider the following:
“both PM2.5 and O3 are severe air pollutants in eastern China.”
Line 207-217: Good!
Line 219 and Line 145: two section 2? Please revise.
Line 225: you mean “the surface solar radiation”, not “the surface of solar radiation” right?
Line 236-237: “It can be expressed as”
Line 297-298: the sentence is redundant. Consider the following:
“the high HONO concentrations in daytime become a significant source of OH radicals.”
Line 339: it is “P2” not “P1” right?
Line 363 and Line 380: two section 3.3.
Line 384: “Figure 10 shows the OH concentrations in September and December”? What does this mean? I thought Figure 10 shows a sensitivity study of OH production P using measured and modeled HONO. Do I understand this correctly? Please revise.
Line 412-413: “a double peak of PM2.5 and O3”? It sounds like for each pollutant, there is a double peak. You mean “a co-occurrence of high PM2.5 and O3 concentrations”?
Line 413 and 432: only “fall” season?
Line 440: Delete “Because”