|The authors have conducted a thorough revision of their manuscript and I appreciate their efforts to evaluate OMI against IAGOS profiles, additional ozonesondes and long-term surface observations. The results are much more defensible, especially since the authors now focus on southern China and have clarified that their analysis is representative of the boundary layer rather than the surface. The new Figure 4 is also very helpful as it shows that ozone variability in the boundary layer is greater than in the mid-troposphere (in southern China), which can help to distinguish boundary layer pollution episodes. I still have a few concerns, as described below, and I could recommend the paper for publication if the authors can address these concerns.|
The paper has been revised to address the concerns of the referees that OMI does not detect surface ozone but rather has some sensitivity to ozone in the deeper boundary layer, provided that the enhancements are relatively strong compared to the mid-troposphere. However, this revised focus is incomplete as the paper still states or implies, in many places throughout the manuscript, that OMI is detecting surface ozone. The most important statement in the revision is this sentence at the beginning of section 4:
“The correlation of OMI with the MEE surface ozone data likely does not reflect a direct sensitivity of OMI to surface ozone, which is very weak, but rather a sensitivity to boundary layer ozone extending up to a certain depth and correlated with surface ozone.”
The reader should not have to wait until the middle of the paper to read this very important statement. It needs to appear in the Introduction and then the authors need to thoroughly revise their paper to make sure that the discussion and the conclusions are consistent with this statement.
For example, the authors changed the title of the paper so that it reflects the analysis in the revised manuscript, but it’s still not quite right. In their response to the comments of all three referees the authors clarified that the OMI retrievals are reflective of the boundary layer rather than the surface. But the title gives the impression that OMI has the ability to observe surface ozone pollution. At a minimum the title needs to be revised, with “surface” replaced with “boundary layer”. But even then the reader has the impression that the product can be used for all of China, whereas the product is not effective in northern China. A better title would be something like:
“An evaluation of the OMI satellite instrument’s ability to observe boundary layer ozone pollution across China: application to 2005-2017 ozone trends”
Other statements in the paper that are not consistent with the first sentence at the beginning of Section 4 are:
1) First sentence of the abstract: “Nadir-viewing satellite observations of tropospheric ozone in the UV have been shown to detect surface ozone pollution episodes but no quantitative evaluation of this ability has been done so far.” This isn’t entirely accurate. A better summary statement would be “…have been shown to have some sensitively to surface or near-surface ozone pollution episodes”
2) Second sentence of the abstract: “Here we use 2013-2017 surface ozone data from the new China Ministry of Ecology and Environment (MEE) network of ~1000 sites, together with vertical profiles from ozonesondes and aircraft, to quantify the ability of OMI tropospheric ozone retrievals to characterize surface ozone pollution in China.”
This also gives the impression that OMI can detect surface ozone, when you should be talking about the correlation between OMI and surface ozone. A better statement would be:
“…to quantify the correlation between OMI tropospheric ozone retrievals and surface ozone pollution events in China.”
3) Third sentence of the abstract:
“After subtracting the Pacific background, the 2013-2017 mean OMI ozone enhancements over eastern China can quantify the spatial distribution of mean summer afternoon surface ozone with a precision of 8.4 ppb and a spatial correlation coefficient R=0.73.” This sentence would be more accurate as:
“After subtracting the Pacific background, the correlation between observed surface ozone and the 2013-2017 mean OMI ozone enhancements over eastern China can be used to estimate the spatial distribution of mean summer afternoon surface ozone with a precision of 8.4 ppb and a spatial correlation coefficient R=0.73.”
4) Fifth sentence of the abstract:
“OMI is much more successful at capturing the day-to-day variability of surface ozone at sites in southern China <34 N (R = 0.3-0.6) than in northern China (R = 0.1-0.3) because of weaker retrieval sensitivity and larger upper tropospheric variability in the north.”
Would be better as:
“In terms of day-to-day variability, OMI has a higher correlation with observed surface ozone at sites in southern China <34 N (R = 0.3-0.6) than in northern China (R = 0.1-0.3) because of weaker retrieval sensitivity and larger upper tropospheric variability in the north.”
There are many more statements like this throughout the paper that need to be revised, following the examples above.
In Section 2 the following important statement appears:
“We focus on summer when ozone pollution in China is most severe and when OMI has the strongest sensitivity”
A similar statement needs to appear in the abstract and introduction so that the reader is aware that the evaluation has so far only focused on a region and time of year when boundary layer ozone is very high.
In Section 3 you provide the following equation and state that “With such a precision, OMI can
provide useful information on mean summer afternoon levels of surface ozone in polluted regions.”:
[O3] = 6.9 ΔΩ + 24.6 ± 8.4
This presentation (and in the abstract as well) gives the reader the impression that they can take this equation and an OMI retrieval above a given location on a given day, and estimate the afternoon ozone on that day. For example, an OMI enhancement of 5 DU would indicate a surface ozone mixing ratio of 59.1 ± 8.4 ppbv, or a range of 50.7 to 67.5 ppbv. This sounds quite good, but when I look at Figure 1e, I can see that the full range of surface ozone values for an enhancement of 5 DU is 35-80 ppbv. And of course, this equation is not derived for estimating ozone on a given day, it is derived from data averaged over 5 years. Therefore the text needs to clearly state that the equation can only provide an estimate of multi-year average ozone. Accordingly, the text in the abstract also needs to be modified to more accurately represent the limitations of the equation. The abstract should provide a statement similar to:
“After subtracting the Pacific background, the correlation between observed surface ozone and the 2013-2017 mean OMI ozone enhancements over eastern China can be used to estimate the broad multi-year spatial distribution of mean summer afternoon surface ozone with a precision of 8 ppb and a spatial correlation coefficient R=0.73.” Note that I emphasized “multi-year”, and changed 8.4 to 8 (8.4 is far too precise for such a rough estimate).
Here you state: “The Hong Kong ozonesonde data thus indicate that OMI can quantify the frequency of high-ozone episodes in the boundary layer even if it may not be reliable for individual events.”
This is a good summary of OMI’s capability, and it’s this type of statement that your paper should focus on, rather than emphasizing the equation in Section 3.
In the last paragraph of the Introduction you state:
“However, no quantitative comparison of the satellite data to surface observations has so far been done.” The implication of this statement is that no one has ever bothered to compare satellite retrievals to surface ozone, when it could be done very easily. But the reason this has not been done before is because the available tropospheric column ozone products don’t have a true sensitivity to surface ozone. A better statement would be:
“While the available tropospheric column ozone products don’t have a true sensitivity to surface ozone, here we demonstrate that the strong ozone enhancements in the summertime boundary layer of southern China can be used to derive a relationship between OMI tropospheric ozone and observed surface ozone ”
Also, the above statement discounts the important advances made by the scientists who are combining IASI and GOME-2 to produce retrievals of tropospheric ozone between the surface and 3 km. Figure 8 and Figure 9 of Gaudel et al.  show tropospheric column ozone between the surface and 3 km above China and Europe, based on retrievals from the IASI and GOME-2 instruments. These retrievals compared well to IAGOS ozone profiles above China (see Figure S8 in the supplement to Gaudel et al. 2018). (Please also see Dufour et al  who look at ozone across China from the surface to 6 km using IASI). These papers should be referenced in the Introduction.
The following sentence appears in the Introduction:
“Retrieval of tropospheric ozone (only ~10% of the column) from these instruments has mostly been done in the past by subtracting independent satellite measurements of stratospheric ozone (Fishman et al., 1987; Ziemke et al., 2011).”
Another common method that should be mentioned is the cloud-slicing technique. This technique was recently used in an excellent paper by Ziemke et al.  who show 40 years of ozone increases above southern Asia from the TOMS and OMI instruments.
The IASI+GOME2 results for China [Gaudel et al., 2018] show that high ozone in spring and summer can extend over the North Pacific Ocean. Why do you not show OMI data above the ocean in Figure 6 and Figure S5? However, you do show ozone above Taiwan in Figure 6 and compare OMI to the TOAR observations from Taiwan, even though you don’t use Taiwanese surface observations in your equation 8.
I don’t understand Figure 5b. Figure 5a shows some clear differences between the observed and predicted high ozone episodes for a threshold of 82 ppbv. Yet Figure 5b with higher thresholds shows nearly perfect agreement between the observed and predicted. Is this for all sites averaged together? What is the scatter for the individual sites?
Dufour, G., Eremenko, M., Beekmann, M., Cuesta, J., Foret, G., Fortems-Cheiney, A., Lachâtre, M., Lin, W., Liu, Y., Xu, X., and Zhang, Y.: Lower tropospheric ozone over the North China Plain: variability and trends revealed by IASI satellite observations for 2008–2016, Atmos. Chem. Phys., 18, 16439-16459, https://doi.org/10.5194/acp-18-16439-2018, 2018.
Gaudel, A., et al. (2018), Tropospheric Ozone Assessment Report: Present-day distribution and trends of tropospheric ozone relevant to climate and global atmospheric chemistry model evaluation, Elem. Sci. Anth., 6(1):39, DOI: https://doi.org/10.1525/elementa.291
Ziemke, J. R., Oman, L. D., Strode, S. A., Douglass, A. R., Olsen, M. A., McPeters, R. D., Bhartia, P. K., Froidevaux, L., Labow, G. J., Witte, J. C., Thompson, A. M., Haffner, D. P., Kramarova, N. A., Frith, S. M., Huang, L.-K., Jaross, G. R., Seftor, C. J., Deland, M. T., and Taylor, S. L.: Trends in global tropospheric ozone inferred from a composite record of TOMS/OMI/MLS/OMPS satellite measurements and the MERRA-2 GMI simulation , Atmos. Chem. Phys., 19, 3257-3269, https://doi.org/10.5194/acp-19-3257-2019, 2019.