Buysee et al provide an analysis of ozone data and trends in Sequoia National Park and in the upwind source area of the San Joaquin Valley represented by a site in Visalia, CA. They focus their analysis on the relationship between these two areas and on a comparison of metrics typically used for human health assessments versus metrics relevant to vegetation effects. This is a useful exploration of ozone in an area where ozone is expected to have substantial impacts on vegetation and ecosystem health especially since relatively few ambient ozone analyses in the literature focus on ecosystem impacts. The paper is generally well written and the data analysis is sound. There are some improvements that could be made in terms of background information and references. In addition, there are some places where minor clarifications/updates to the analysis and presentation of the results would be beneficial. I recommend the paper be published after the following comments are addressed.
Introduction:
1) Lines 28-30: I think this comparison is misleading for a few reasons. First, as the authors clearly understand, ozone is not highest near emissions sources but rather downwind within a metro area. While the number of days above 70 ppb in LA proper may be 76, the number of days above 70 in the LA metro area (defined as the Los Angeles Nonattainment area which includes portions of Riverside and San Bernardino) is actually in the range of 150-175. I think that is a more appropriate comparison, otherwise the authors give the false impression that SNP experiences more days above 70 ppb than the LA metro area. In addition, while determinations of the three most ozone polluted cities in the US by the ALA is likely based on the US standard of the 4th highest MD8A value, the number of days above a threshold has a lot to do with regional climate. Therefore, it would be more appropriate to compare SNP to other local urban areas (such as those listed on line 14 of page 4) than to Denver and Phoenix. Also note that the number of days above 70 ppb in Phoenix has decreased dramatically over the time-period evaluated (more than 100 days in 2001).
2) The recent Tropospheric Ozone Assessment Report (TOAR) presented analysis that is relevant to the work described here. There are two articles of particular significance: Lefohn et al (2018) describe the relationship between various ozone health and vegetation metrics and show how trends in different metrics compare across the US, EU and Asia. Mills et al (2018) specifically focus on characterizing ozone levels and trends for metrics most important to vegetation health. These articles should be discussed in the introduction. In addition, it would be worthwhile to put your results in context of those other recent findings both within California and globally.
3) EPA, 2015 would be a more appropriate reference for statements at the top of page 3 than EPA, 2016 and EPA, 2010. Also, EPA, 2010 is not listed in the references section.
4) Line 9, Page 3: Is SUM0 the same as the M12 metric from the TOAR analysis?
Methods:
1) Much of the methods description appears to be in the results section with bits included in the introduction. I think the article would be clearer if the methods were broken out into a separate section. The methods section could include: description of monitoring sites (currently lines 23-29 on page 5), description of where data were retrieved from, gap filling methodology (currently lines 10-14 on page 8), calculation of metrics (currently mentioned in various places in the introduction and results section), and description of trends calculation methodology (not currently included in the text at all).
2) Why did analysis only use data through 2012? Certified ozone data is available through 2017. I suggest that the authors extend their analysis to include more recent data.
3) What methods did the authors use to calculate trends? There are many different linear regression and other methods that could be used to determine trend magnitudes and significance. For instance, other trends analysis have used ordinary least squares regressions, Thiel-Sen regressions, Spearman rank order regression etc. Some regression methods also account for temporal variability on different time scales (inter-annual variability versus seasonal variability etc). Please specify the method used here. Also, please add p-values to the trends magnitudes reported in Table 1 and include some description of which trends were statistically significant in the results section.
4) Metrics: I suggest that the authors also include an analysis of the AOT40 metric which is commonly used in Europe and is mentioned in the introduction.
5) The authors have some limited analysis of number of days above 70 ppb (lines 20-25 on page 9) but they should also calculate trends in this metric similar to the other metrics. These trends for # of days > 70 could be included in both Table 1 and Figure 6.
6) The authors use a cutoff of 70.4 ppb for the US ozone standard, but the US EPA actually truncates rather than rounds fractional ppb values when calculating days above the standard. So, the correct threshold should actually be 70.9 ppb. See description in 40 CFR Appendix U to Part 50 from October 2015 (https://www.law.cornell.edu/cfr/text/40/appendix-U_to_part_50)
Results:
1) If the authors decide not to include a separate methods section, then I suggest the authors at least move the general description of monitoring sites (lines 23-28 on page 5) up to the general results section (section 3) instead of having it buried in section 3.1.
2) Line 2, page 6: Additionally, rush-hour NOx emissions are likely to impact the diurnal pattern of Ox described here for Visalia.
3) Section 3.2: If this analysis is important enough to warrant its own section, then the authors should include a Table or Figure displaying the results described in lines 16-27 of page 7, so that readers will easily be able to see a summary of these results.
4) Lines 16-20 on page 7: Is it also possible that SNP becomes VOC limited on hot days? Maybe due to changes in biogenic VOC emissions?
5) Lines 7-8, page 8: It might be useful to note in the description of W126 that the sigmoidal weighting function has an inflection point at around 60 ppb, so days below 60 ppb receive little weighting while days above 60 ppb are weighted heavily.
6) Lines 10-11, page 8: How many months had less than 75% complete data at these three sites?
7) Lines 26-29, page 9: The authors may also wish to note that in EPA’s 2015 review of the ozone standard, they considered potential secondary W126 ozone standard levels between 7 and 17 ppm-hrs and the Clean Air Science Advisory Committee recommended a W126 standard level between 7 and 15 ppm-hrs (See EPA, 2015). These levels are consistent with the levels discussed here from other literature sources.
8) Lines 24-25, page 11: There are more recent EPA regulations on heavy-duty onroad and nonraod emissions that could be cited here. See: https://www.epa.gov/emission-standards-reference-guide/epa-emission-standards-regulations
9) Lines 3-23, page 12: Authors may want to note that less substantial trends in the spring may also be due to a larger fraction of ozone coming from background ozone sources in the spring than in the summer. The authors discuss background ozone in the next paragraph but never explicitly state this.
10) Line 5, page 13: Also note, that satellite observations as well as Chinese emissions estimates indicate that Chinese NOx emissions have been decreasing since 2011 so the influence from Asia may have become less important since 2011.
11) Figure 6: in caption or figure headings clarify that MD8A is the seasonal average.
References:
Policy Assessment for the Review of the Ozone National Ambient Air Quality Standards, Final Report, U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, Research Triangle Park, North Carolina, EPA-452/R-14-006, August 2014, available at: https://www.epa.gov/naaqs/ozone-o3-standards-policy-assessments-review-completed-2015
Lefohn AS, Malley CS, Smith L, Wells B, Hazucha M, Simon H, et al.. Tropospheric ozone assessment report: Global ozone metrics for climate change, human health, and crop/ecosystem research. Elem Sci Anth. 2018;6(1):28. DOI: http://doi.org/10.1525/elementa.279
Mills G, Pleijel H, Malley CS, Sinha B, Cooper OR, Schultz MG, et al.. Tropospheric Ozone Assessment Report: Present-day tropospheric ozone distribution and trends relevant to vegetation. Elem Sci Anth. 2018;6(1):47. DOI: http://doi.org/10.1525/elementa.302 |
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