Review Hernandez Paniagua et al.

I appreciate that the authors have responded to the comments of the referees, but I have some major concerns that need to be addressed prior to publication. My primary issue is the paper lacks focus and, after reading, I am not sure what the authors were trying to communicate scientifically about Ox trends in the urban areas in Mexico studied. Instead, the paper is largely descriptive, presenting many trends, but without clear purpose. It is insufficient for publication in ACP to describe a wide array of measurements and then speculate on the causes of the observed patterns.

While there are few studies on Ox in Mexico, too much is known about urban Ox chemistry generally for pages of text to describe Ox seasonal trends and diurnal patterns. If the Ox AVd metric is being interpreted as a proxy for O3 production, then say this explicitly. Trends in O3 AVd may simply indicate changes in NO2. Second, the Introduction states that the authors are concerned with Ox trends based on health-based metrics (page 4, line 135). It is not obvious to me then that a discussion of wintertime (and possibly springtime) trends is even warranted. Additionally, annual average assessments obscure summertime differences, which, if concerned with high O3 (health-based metrics), should be the focus. To this aim, I see the presentation of trends in annual average Ox and AVd to be a distraction, as they may track wintertime changes (generally always VOC-limited chemistry), rather than changes in summertime O3 production (potentially VOC or NOx limited). Consideration of the 95th percentile O3 trends is likely a de facto summertime trend, but this is never stated nor are the data analyzed in this context.

I recommend the authors provide the reader with a focused statement of purpose and then only present observations relevant to this task. For me, an inter-annual high-Ox trend analysis would be an incredibly valuable contribution to the literature. I also recommend the authors present separate Results and Discussion sections. This would ensure the paper escapes the trap of presenting an observation and then immediately speculating on the cause. For example, this is done three times in the paragraph on page 9, lines 321–330, but is ubiquitous throughout the manuscript.

I encourage the authors to revise.

Minor comments:

Abstract, line 15. “In developed countries, long-term trends in O3 have been studied extensively.” I would not say this is true.

Page 2, line 45. “being VOC-limited” should say “being called VOC-limited.”

Page 2, line 55. The reaction between NO and O3 is not an O3 loss process, as NO2 immediately photolyzes to yield O3. At night NO2 simply stores O3.

Page 2, line 64–75. Discussion of trends in background and annual O3 is irrelevant to this analysis. Remove from Introduction.

Page 3, lines 81–82. Are you quoting trends in emission inventories or in actual trends? Make this clear. Should also say that there is variability between cities.

Page 3, line 96. A decrease of 33% in what?

Page 3, line 96. Delete “By contrast.”

Page 3, line 97. Delete “relatively.”

Page 3, lines 100–101. Not clear what this means: “has breached the 1-h average standard of 110 ppb O3 by up to 80 %.”

Page 3, lines 104–106. Critical without point. Remove or rephrase.

Page 3, line 109. What is meant by “data?”

Page 4, lines 137–140. This statement is problematic: “The data sets contain features representative of industrial, urban-background and urban monitoring sites, which allow assessment of O3 trends and dynamics, pollutant emissions and their contribution to the atmospheric composition depending on local meteorology and air mass transport.” Really the data allow you to - assess ozone trends in locations with mixed sources and variable meteorology.

Page 4, lines 141–142. Change “oxidants” to “odd oxygen.” A clearer way to say - Ox include O3 stored as NO2.

Page 4, lines 142–143. Sentence is not meaningful.

Page 6, line 207. This subtitle should not be “Analysis of data,” which implies scientific consideration of the data.

Page 9, lines 307–309. Irrelevant, delete: “A study conducted among asthmatic children resident in the MCMA revealed an increase in coughing and wheezing rates, associated with cumulative exposure to high 1-h averages mixing ratios of O3 and NO (Escamilla-Nuñez et al., 2008).”

Page 9, 313–314. Speculation at best, “likely influenced by the significant (p<0.05) annual growth of 1.90 ppb y-1 in NO in levels as shown in Fig. 4.” Best to have a separate Results and Discussion section.

Page 9, 322–323. Speculation - “which arise either from an increment in NOx or O3 levels as shown in Fig. 4.” Either give evidence for this statement or delete.

Page 9, 324–325. Speculative: - “Ox trend is likely due to the decreasing levels of NOx.” Give evidence or delete.

Page 9, 328–330. Speculation - “This could be due to the arrival at OBI and at STA of chemically processed air masses with decreased VOC/NOx ratios, compared with those arriving at SNN loaded with fresh emissions from the nearby industrial area.”

Page 14, line 519. Need to verify there is also no weekend effect in Ox.

Dear authors,

Please address the following comments prior to publication.

Best,
Sally

The abstract is not well focused. It should be streamlined, conveying just the key summary information.

Line 56: chemical loss, not photochemical loss.

Line 136: In sentence - “We show that air mass origin influences strongly the O3 annual growth rates.” It is not clear what “growth rates” means.

Line 285: Revise - “photochemical season.”

Line 306: Delete sentence - “Diurnal variations in O3 arise from the balance between its net production and destruction.”

Line 320: Delete sentence - “O3 and OX levels depend strongly on the photochemical processing of NOX and VOCs emissions.”

AVd Ox may be a proxy for PO3, but not AVd O3.

The authors are not convincing that “seasonal amplitude values (AVs) provide insight into inter-annual variations in the net O3 production in response to changes in precursor emissions and meteorology.” Low O3 in winter may be simply O3 loss to NO, not low PO3. This would be consistent with the later discussion of substantial NOx emission reductions in 1994-1996.

Line 413: “Stephens et al. (2008) suggested that the most plausible explanation for the lack of weekend O3 effect 413 at MCMA during 1987-2007, is that weekday O3 production is limited by VOCs and inhibited by NOx.” As described, this would cause weekday-weekend O3 differences.

Figure 9: Colors for Ox and O3 in MCMA and MMA are difficult to distinguish.

Lines 610-618: Delete. No need to compare to cities globally, keep the focus on cities in Mexico.

Lines 643-646: Delete - not relevant.

Lines 648-649: Delete.

Line 659: Replace “It is clear that” with “It has been shown that” and add citations.

Can the authors recommend NOx versus VOC control? Or do the authors advocate for both and why?