This study provides a much needed analysis of the long-term trend at Mt. Waliguan, a very important GAW baseline monitoring site upwind of eastern China. Overall I have no major concerns about the analysis, however the paper still needs quite a bit of work in terms of improving the writing style. The authors also need to reference many new studies that I have listed below, containing results of great relevance to this study. My recommendation to the editor is that the paper needs a major revision, after which it will likely be acceptable for publication.
MAJOR COMMENTS
1) The standard of English in the manuscript is quite good, but there are still many minor grammatical errors. I recommend that the authors either find a colleague with excellent English skills to edit the grammar line-by-line, or employ the assistance of an ACP journal copy-editor.
2) Please be clear regarding what you mean when you use the term “background ozone”, which is a model construct. In contrast, “baseline ozone” is the observed ozone that flows into a region before it is impacted by local emissions. See the description and implications of these two quantities in:
Cooper, O. R., A. O. Langford, D. D. Parrish and D. W. Fahey (2015), Challenges of a lowered U.S. ozone standard, Science, 348, 1096-1097.
3) According to the ACP data policy, the underlying chemical observations used in the analysis should be publicly available, as described here:
Statement on the availability of underlying data:
http://www.atmospheric-chemistry-and-physics.net/about/data_policy.html#data_availability
“Authors are required to provide a statement on how their underlying research data can be accessed. This must be placed as the section "Data availability" at the end of the manuscript before the acknowledgements.”
This paper contains no data availability statement and the authors need to provide one. Because Mt. Waliguan is a WMO GAW site the hourly data should be available from the World Data Center for Greenhouse Gases. But when I visit their webpage I can only find monthly or daily values, not hourly:
http://ds.data.jma.go.jp/gmd/wdcgg/cgi-bin/wdcgg/download.cgi?index=WLG236N00-CMA¶m=201405120001&select=inventory
4) When reporting trends this manuscript uses ppbv per decade, but the standard units for reporting ozone trends are ppbv yr-1. To be consistent with other studies and to make comparisons as easy and simple as possible, please also use ppbv yr-1.
5) In the first paragraph of the Introduction references should be made to authoritative ozone review papers. Staehelin et al. 2001 is not a good paper for these purposes as it is now quite old.
Monks, P. S., A.T. Archibald, A. Colette, O. Cooper, M. Coyle, R. Derwent, D. Fowler, C. Granier, K.S. Law, G.E. Mills, D.S. Stevenson, O. Tarasova, V. Thouret, E. von Schneidemesser, R. Sommariva, O. Wild, and M.L. Williams (2015), Tropospheric ozone and its precursors from the urban to the global scale from air quality to short-lived climate forcer, Atmos. Chem. Phys., 15, 8889-8973, doi:10.5194/acp-15-8889-2015.
The Royal Society. 2008. Ground-level Ozone in the 21st century: Future Trends, Impacts and Policy Implications. Royal Society policy document 15/08, RS1276. Available at http://royalsociety.org/Report_WF.aspx?pageid57924&terms5groundlevel1ozone
Young PJ, et al. (2013), Pre-industrial to end 21st century projections of tropospheric ozone from the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP). Atmos. Chem. Phys 13: 2063–2090. doi:10.5194/acp-13-2063-2013
MINOR COMMENTS
If no explanation is given for a comment, please insert the suggested text into the appropriate place in the manuscript
Page 1, Line 14-15
Long-term determination of trends of baseline ozone is highly needed information for environmental and climate change assessment. So far, studies on the ….
Page 2, line 7-8
The results of this study can be used for assessments of climate and environmental change and in the validation of chemistry-climate models.
Page 2, line 17
Data on the spatiotemporal variations of ozone are greatly needed for assessing the impacts of ozone on human health, ecosystems, and climate. Since ozone is a secondary pollutant with a lifetime of 22 days [Young et al., 2013], its mixing ratios are influenced both by local photochemistry and by transport of ozone and its precursors
Page 2, line 22
There are many, many references here on STE, when you could just reference Stohl et al 2003:
Stohl A, Bonasoni P, Cristofanelli P, Collins W, Feichter J, et al. 2003. Stratosphere-troposphere exchange: A review, and what we have learned from STACCATO. J. Geophys. Res 108(D12): 8516. doi:10.1029/2002JD002490
Page 2, line 32
… to monitor atmospheric composition including …
Page 3, line 3
Cooper et al. 2010 does not address ozone at the surface, only ozone in the mid-troposphere.
Page 3, lines 6-17
This section needs a lot of work as it has errors and incorrectly attributes part of the ozone trend over western North America to STE. While these studies clearly show that STE impacts ozone at the surface and in the free troposphere over western North America they did not look for or find a trend in STE. Also, Cooper et al. 2010 did not attribute the ozone trend specifically to China, they just concluded that the ozone was coming from the broad region of South and East Asia. Finally, the most important result from Lin et al. 2015 is that 20 years of data gives a robust trend above North America with both observations and the model giving the same trend of 0.3-0.4 ppbv yr-1.
Page 4, lines 9-10
The text would sound better as:
“…downwind of the European, Central Asian and Indian outflow, representative of the baseline of Eurasia.”
I removed the word “continent” because technically, Eurasia is not defined as a single continent, as it is two continental masses joined together.
Page 4 line 15
a substantial portion of the airflow comes from
page 4 line 22
…may improve understanding of ozone changes…
Page 4, line 23
…particularly in the rural and remote regions of Eurasia.
Page 4, line 29
“The non-linear variation of ozone mixing ratios with season and many other meteorological factors…”
Here I changed climate to meteorological because changes in climate occur on the scale of decades while the fluctuations you see in the ozone observations are on time periods less than 5 years.
Page 5 line 31
With very low population
Page 6 line 1
…far from major anthropogenic sources of ozone precursors.
Page 6 line 6
You should discuss the impact of the highway that runs along Qinghai Lake west of Mt Waliguan [Wang et al. 2015].
Page 6 line 8
The mixing ratios of surface ozone have ….
Page 6, line 12
Calibrations (8 points) have been conducted
Page 7 line 12
…and is influenced by boundary layer (BL) air, while during the night, winds are downslope…
Page 7 line 15
while the free-tropospheric air may sometimes contain signals of long-range transport or STE events.
Page 7 line 17
between daytime and nighttime ozone mixing ratios in order to study the trends associated with different air masses.
Page 9, line 17
but it has not yet been used on atmospheric composition data.
Page 10, line 7
I’m not sure what you mean by “according”. Would “associated” be a better word?
Page 10, line 16
I’m not sure what you mean by “but the first one”. Do you mean “accept for the first one’?
Page 11 line 17
Figure 3
Page 12 line 2
The figure shows data at monthly intervals (12 per year) not at seasonal intervals (4 per year), therefore you should discuss the monthly variation or annual cycle rather than seasonal variation.
Page 12 line 13
If you are going to mention controversial conclusions you should briefly summarize them so that the reader understands the issue.
Page 12 line 17
Better to use monthly instead of seasonal.
Page 12 line 18
Again, the ozone mixing ratios peak in summer and are lowest during winter
Figure 5
This is a nice figure but it would be even more effective if the y-axis in each plot was exactly the same. This would allow the reader to more clearly understand the decrease in ozone in autumn and winter. If you have to make the panels a little taller to accommodate the larger ozone range, then that would be fine.
Page 14, line 6
northerly winds
Page 14, lines 15-17
Monks et al. and Vingarzan et al. aren’t very good papers when it comes to reviewing the seasonal cycle. They are both out of date and Monks et al. has a heavy reliance on coastal sites which experience major transport shifts from spring to summer, so the decrease in ozone in summer is due to transport rather than photochemistry. Vingarzen et al. just isn’t a very good paper at all, in my opinion. The peak is sometimes in spring, but often in summer as well. So I don’t think that Waliguan is unusual by having a summertime peak. In fact this observation agrees very well with the peak in satellite detected tropospheric column ozone above central Asia (see Figure 7 in Cooper et al. 2014, listed below). Better reviews of the ozone seasonal cycle are:
Parrish, D. D., K. S. Law, J. Staehelin, R. Derwent, O. R. Cooper, H. Tanimoto, A. Volz-Thomas, S. Gilge, H.-E. Scheel, M. Steinbacher and E. Chan (2013), Lower tropospheric ozone at northern mid-latitudes: Changing seasonal cycle, Geophys. Res. Lett., 40, 1631-1636, DOI: 10.1002/grl.50303
Cooper, O. R., R.-S. Gao, D. Tarasick, T. Leblanc, and C. Sweeney (2012), Long-term ozone trends at rural ozone monitoring sites across the United States, 1990–2010, J. Geophys. Res., 117, D22307, doi:10.1029/2012JD018261.
Cooper, O. R., D. D. Parrish, J. Ziemke, N. V. Balashov, M. Cupeiro, I. E. Galbally, S. Gilge, L. Horowitz, N. R. Jensen, J.-F. Lamarque, V. Naik, S. J. Oltmans, J. Schwab, D. T. Shindell, A. M. Thompson, V. Thouret, Y. Wang, R. M. Zbinden (2014), Global distribution and trends of tropospheric ozone: An observation-based review, Elementa: Science of the Anthropocene, 2, 000029, doi: 10.12952/journal.elementa.000029
Clifton, O. E., A. M. Fiore, G. Correa, L. W. Horowitz, and V. Naik (2014), Twenty-first century reversal of the surface ozone seasonal cycle over the northeastern United States, Geophys. Res. Lett., 41, 7343–7350, doi:10.1002/2014GL061378.
Page 14, line 21
…ozone-rich air mass transport from Eurasia.
Page 15, line 5
…however, by shutting off North American emissions….
Page 15 line 10
There are two ozone sites at Whiteface Mountain in the state of New York. Please specify that this is the Whiteface Mountain Summit site.
Table 2 has some errors and is missing some recent important trends.
The Table caption says these are all GAW sites, but that is not true, only some are part of GAW. For example Pinedale, Rocky Mtn and Whiteface Mtn are not part of GAW.
The correct name for Rocky is Rocky Mountain National Park. The correct name for Whiteface is Whiteface Mountain Summit.
Other mountaintop trends are available for Mt. Happo, Japan; Summit, Greenland; Arosa, Switzerland; and Lassen Volcanic National Park, California. See Cooper et al. 2014.
Also, Table 2 of Cooper et al. 2014 has seasonal trends at Lassen, Gothic, Whiteface Mtn Summit and Big Meadows.
Page 17 line 9-10
“dip-down” is not a standard scientific term and a better term needs to be found. Would “decrease” or “local minima” be adequate replacements?
Page 17 line 29
Both daytime and nighttime surface ozone have increased significantly at WLG.
Page 18, line 14
Since eastern China is downwind of WLG, our results imply that under rising baseline ozone conditions, even greater effort needs to be applied to reducing ozone precursors in eastern China in order to improve ozone air quality. |
- Printer-friendly version
- Supplement
