|Ozone seasonal evolutionand photochemical production regimein polluted troposphere in eastern China derived from high resolution FTS observations |
By Y. Sun and collegues
Following the referee’s remarks, the revised version has been strongly reorganized as compared to the initial version. As the authors state, (1) they have updated all retrievals with new Sa deduced from standard deviation of a dedicated WACCM run from 1980 to 2020, (2) they have organized the paper’s structure, with focus on new results, and retaining only the most pertinent figures, (3) they omitted comparisons with the correlative data, i.e., OMI, GEOS-Chem and WRF-Chem data, (4) the reoriented the papers focus on photochemical ozone regime. In my view, this responded to the major referee remarks, and makes the paper rather different from the initial one. I think, the paper shows now interesting results on data at a specific site and an interesting discussion of the ozone formation regime that can be deduced from the data. However, there is a major difficulty, because from reading the paper I think that the O3 product is not specific for PBL O3, which would be needed for the current analysis. So I would think the paper is in principle worthwhile for publication in ACP, but only if this major issue is resolved or correctly addressed. Also figures should be better explained and a general rereading by a native speaker would be needed.
In Section 3, it is not clear to me, what exactly are the altitude ranges of the partial columns that are the basis for the paper, and whether they are for the case of ozone pertinent, that is focusing on lower tropospheric ozone.
For instance, it is stated in section 3 : “In this study, we have chosen the same upper limit for the tropospheric columns for all gases, which is about 3 km lower than the mean value of the tropopause (~15.1 km).” If I understand right, this means that 0 – 12 km partial columns are considered. But looking at figures S2 and S3, such columns would be sensitive to O3 values up to 15 km. Following figure S3, wouldn’t it be wiser to analyse a 0 – 8km partial column being most sensitive between 0 and 6-8 km ? This would allow being more sensitive to lower tropospheric ozone.
Anyway, it seems that O3 partial columns used here are heavily influenced, if not dominated by free tropospheric (FT) ozone located between, say, 2 – 8 km height. Note that it would be useful to dispose of boundary layer height values allowing to state where the FT begins. This important, if not dominant, FT part for ozone needs to be clearly stated in the paper, while in the current version, it is suggested that the analysed product is representative for PBL O3. If authors think that their product has not only some fractional, but dominant information from the PBL, then please explain and prove it much better. Note that this applies for ozone, for NO2 and HCHO, the PBL sensitivity is stronger because both compounds are concentrated there.
If this suspected FT sensitivity is real, then the analysis needs to be taken into account in the following analysis. While PBL ozone can be strongly regionally controlled, FT ozone is prone to intercontinental or even hemispheric transport, including stratosphere – troposphere exchange. This needs to be addressed, and may be , the analysis completed or redirected. This point is really important to be addressed before publication.
Minor remarks :
“Briefly, VOCs first react with the hydroxyl radical (OH) to form a peroxy radical (HO2+ RO2) which increases the rate of catalytic cycling of NO to NO2. O3 is then produced by subsequent reactions between HO2or RO2 and NO that lead to radical propagation (via subsequent reformation of OH). «
Please reformulate :
“Briefly, VOCs first react with the hydroxyl radical (OH) to form a peroxy radical (HO2+ RO2) which increases the rate of catalytic cycling of NO to NO2. O3 is then produced by photolysis of NO2. Subsequent reactions between HO2or RO2 and NO lead to radical propagation (via subsequent reformation of OH). «
Section 4.1 :
“While it failed to determine the secular trend of tropospheric O3 column probably because the time series is much shorter than those in Gardiner et al. (2008),”
This sentence suggests that the O3 trend needs to be positive, but for later years, this is not necessarily the case, especially in the free troposphere.
“The direction of east origin air masses shifts from the southeast to northeast of Jiangsu Province, and that of local origin air masses shifts from the south to the northwest of Anhui province.”
Please reformulate :
“The direction of air masses originating in the eastern sector shifts from the southeast to northeast of Jiangsu Province, and that of local air masses shifts from the south to the northwest of Anhui province.”
“In contrast, trajectories of local origin air masses in SON/DJF are 20.2% larger than the MAM/JJA ones, indicating a more significant contribution of the air pollution inside Anhui province in SON/DJF.”
You mean, they are more frequent (instead of larger)?
5.1 Meteorological dependency
“The city downtown locates in eastern of the observation site and the majority of the Chinese population lives in the eastern part of China, easterly winds (direction less than 180˚) could generally transport more pollutants to the observe area than westerly winds (direction larger than 180˚), resulting in a higher O3 level.”
This again supposes, that the O3 product is mostly sensitive top PBL O3, which is clearly not proven. This also long range transport, not necessarily only from easterly regions, because trajectories can change directions, should contribute to enhanced O3 columns.
Section 5.2 :
“Pronounced tropospheric CO and NO2 variations were observed but the seasonal cycles are not evident probably because of air pollution which is not constant over season or season dependent.”
This is not clear. For NO2 a winter maximum is found. One reason is that time series are not complete enough especially in the later years.
“Since the sensitivity of PO3to VOCs and NOx is different under different limitation regimes, the relative weaker overall correlations to HCHO (Figure 6 (b)) and NO2 (Figure 6 (c)) indicates that the O3 pollution in Hefei can neither be fully attributed to NOx pollution nor VOCs pollution.”
If O3columns are representative for PBL, then weaker correlation of O3 with NO2 and HCHO are also explained by different lifetimes, hours to 1 day in summer for NO2 and HCHO, several days to weeks for O3. So older O3 enhanced air masses easily loose trace of NO2 or HCHO. Please add this explanation. If they are dominated by FT O3, then a correlation can not be expected anyway. This seems to be the case.
In figure 1, how is the trend calculated ? how are the points named ‘resampled bootstrap” obtained. Please explain in the text.
Figure 3 :
Monthly average wind speed of 0 does not make sense, as already stated by another referee. It is understood that the average of absolute wind speed is meant. If you cannot calculate it, please withdraw this figure. Also the figure on wind direction on wind direction is not easy to interprete. May be a solution is to calculate frequency distributions.
If minute average FTS measurements are used, then there should be much more points? Or do you use time averages ?
Figure 5 and Figure 6 : again, what is the measurement period one point corresponds to ?
“FigureS2. Averaging kernels(ppmv/ppmv)of O3, CO, and HCHO (color fine lines), and their area scaled by a factor of 0.2 (black bold line).They are deduced from the spectra recorded in Hefei on March 15, 2016 with a measured ILS.”
Please clarify several points in the figures legend. Each curve is representative for 1 km ? What does the area curve exactly indicate ? what does “ILS” stand for ?
“FigureS3. Partial column averaging kernels(PAVK)(ppmv/ppmv) for O3, CO, and HCHO retrievals. For all gases, large PAVKs in certain altitude range”
Are the PAVK’s obtained by summing up the km wise AVK’s ?