Long-term visibility variation at the Ebro Observatory (1960&ndash;2020)

Curto, Juan José; Tacoronte, Nicolás

doi:https://doi.org/10.5194/acp-2021-1092

Preprints

https://doi.org/10.5194/acp-2021-1092

Preprints

22 Feb 2022

| 22 Feb 2022

Status: this preprint was under review for the journal ACP. A revision for further review has not been submitted.

Long-term visibility variation at the Ebro Observatory (1960–2020)

Juan José Curto and Nicolás Tacoronte

Abstract. This study explores the interdecadal variability and trends of surface horizontal visibility in the Baix Ebre area (Spain) from 1960 to 2020, using the historical archive of the Ebro Observatory located in the town of Roquetes, in the centre of the region. We detected a prominent improvement in visibility in the first decades of the second part of the twentieth century but this was followed by a dramatic deterioration in the last decade recorded (2010–2020). This was not accompanied by a significant trend in relative humidity over the same period. It was found that very good visibility (20–50 km) occurred at a frequency of more than 40 % in the 1980s and 90s, while this percentage dropped to just 15 % during the 2010–2020 decade.

Visibility was found to be negatively correlated with relative humidity and positively correlated with wind speed. This correlation was statistically valid for most of the time periods we looked at. Wind regime, prevailing wind direction and the corresponding origin (marine or continental) of the air mass were found to display a strong influence on the short-term visibility levels at the Ebro Observatory. However, long term visibility trends are controlled by other factors, which are probably of anthropogenic origin. The rapid increase in population in Roquetes and other neighbouring towns during recent decades could be the cause of the implicit impairment of air quality observed because of increased levels of air pollution on a regional scale. Apart from these local sources, visibility at the Ebro Observatory could also be influenced by the aerosol load from neighbouring regions (Castelló and Tarragona) which have a significant concentration of industries.

Received: 27 Dec 2021 – Discussion started: 22 Feb 2022

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.

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Juan José Curto and Nicolás Tacoronte

Status: closed (peer review stopped)

RC1: 'Comment on acp-2021-1092', Anonymous Referee #1, 01 Jul 2022

Review of Curto and Tacoronte, “Long-term visibility variation at the Ebro Observatory (1960-2020)".

This manuscript highlights human-made visibility observations and trends at a single site in Spain. The results suggest that visibility deteriorated significantly during the later periods of observations. The authors also study the role of relative humidity and wind speed and direction on visibility. The manuscript describes a long data history that would be important to have in the literature. Unfortunately, necessary details that would allow for interpretation of these data are weak in this version of the paper. I recommend the authors provide additional exploration of the measurements themselves to assure readers of potential biases in these measurements, especially with regard to existing literature regarding human-made observations of visibility (e.g., Li et al., 2016, see below).

Some of the many questions and details that need to be addressed include:

Expanding the introduction to include any additional measurements in the surrounding region of visibility or aerosol composition or behavior may help interpret these results. The authors refer to studies in Taiwan, China, the US, etc., but these do not help interpret the observations reported here.

Include elevation of study location.

Additional details need to be included regarding visibility measurements and potential biases over time due to any potential changes in methodologies over the time period assessed. Li et al. (2016) performed a global trends in visibility based on human observations and details some of the related issues (ACP, 16, 2435-2457, 2016). It is difficult to fully interpret these trends without some discussion as to whether additional analysis revealed any potential biases. Figure 2 shows the shift in visibility, especially in 1980, and again around 2015 that should be investigated in more depth to determine if it is real. Some discussion of uncertainty in these measurements in needed.

Were periods of high relative humidity flagged as corresponding to fog, precipitation, or low cloud?

It is not clear why the WMO classes were used when the data could easily be used instead?

What time periods are used to generate Figure 3?

Figure 4 might be more compelling without the use of the classes and present the data themselves, and similar results are achieved as Figure 1, especially since Figure 4 is presented as “% of observations” and not “visibility” as noted in the caption or on line 180 of page 6.

What years correspond to Figure 5?

Define “relative frequency” for observations of wind directions in Figure 6.

Define “normalized visibility” as presented in Figure 7.

Equation (1) is not necessary to include.

An increase in RH should lead to a decrease in visibility for hygroscopic particles, because those particles grow into larger sizes that are more efficient as scattering light. What time of day do the correlation coefficients correspond to, and was this limited to the same time of day for RH? Was the correlation analysis applied to daily data? Effects of RH will be most apparent on visibility if the particles causing visibility degradation are hygroscopic. Some discussion of potential aerosol sources would be helpful.

Citation: https://doi.org/10.5194/acp-2021-1092-RC1
RC2: 'Comment on acp-2021-1092', Anonymous Referee #2, 01 Jul 2022

It is nice to see a long time series of visibility data analyzed. In general, it is an interesting topic of interest. However, the manuscript lacks in various aspects listed below. his reviewer is not at all sure if can be accepted for publication in ACP even after major revision. the last comment below is the most important one.

Lines 33 – 40: Don’t forget to mention that the visibility can also be measured with a whole suite of instruments. Even if they are not applied at the Ebro Observatory, they need to mentioned. They come with big advantages (continuous measurement, no errors from human physiology).

Lines 42 – 45: This list is far from being complete or updated.

Line 57: Please provide more precise coordinates. You are offering a +/- 10 km resolution, you should rather provide 4 decimals of the full degrees of latitude and longitude.

Lines 68 – 69: “The last sixty years of these records have been digitized”. Why didn’t you digitize more data, dating further back?

Lines 109 – 111: “The criterion used consists in eliminating those values greater than 1.5 times the interquartile range below the first quartile and above the third for each hour of the day.” The criterion is not clear to this reviewer. Please elaborate.

Lines 133 – 134: Please elaborate how the correlation between visibility and wind direction (polar coordinates) was computed.

Fig. 2 - 9: Graphs should not have headlines.

lines 163 – 165: “The results shown in Figure 3 indicate that, in general, the maximum average visibility was found during the winter months and the minimum in July, August and September, except for the last period of time measured.”. No, the observation also does not apply to the period 1991 – 2005 either.

Lines 167 – 169: No good English, unclear. Please fix the wording.

Figure 4: The type of presentation is confusing. It would be much better to present stacked graphs (similar to those used in Fig. 5)

Line 197: What is the “upper part of the region” and the “lower part” (line 199)? Are authors talking about topographic altitudes?

Fig. 5. There is something wrong with the wind directions. What means “C” in the x-axes? There is no data for direction N. Direction NNW and NNE seem not to connect well to each other.

Line 206. Please say low wind speeds instead of soft wind speeds.

Figure 6: Thank you for the test. Please say “Relative frequency of” instead of “Visibility variation for”. Please take away the parentheses around the word “sector”. Again, what means “C” on the x-axis. Please take out these data points.

Figure 7: Same comment for the x-axis as for Fig. 6.

Data presented in Fig. 7 are not referred to in section 4.2 (except a general remark in line 219).

Lines 239 – 240: You mean “when its magnitude is greater than 0.1453”, correct?

Lines 243 – 246: This hypothesis seems to make sense. However, it is also highly speculative, because it is not at all based on any analysis or data presented so far. This section needs to be considerably weakened or moved further down within the manuscript.

Lines 251 – 257 and Table 2: It not clear, how the correlation war computed (see comment on lines 133, 134 above), and what exactly is shown in Table 2. This section needs extensive re-write.

Lines 267 - 268: This statement needs to be supported much better. There is an ongoing discussion about this topic in the literature, authors should not ignore that or simply comment it with a very short comment.

Line 272: “SSE-SSW” This reviewer does not agree. It should be said ESE – SSE instead, see Fig. 7. Also, WSW – N is more appropriate that W – NW. Note that the visibility data (Fig. 6) have another maximum than the wind direction data. Overall, the presented data analysis is not sufficient. Likely, it would be helpful to show visibility roses.

Lines 283 – 284: How do you arrive at this conclusion? Please refer to the data and respective discussion in the manuscript.

Lines 285 – 294: In lines 79 – 81, authors mentioned that the Ebro observatory “has never … experienced significant changes in its surroundings from 1910 onward”. Interestingly, this is supported by a reference dated 1987. Here, authors claim “that from the year 2000 onward there has been a large increase in the population of the town of Roquetes where the Ebro Observatory is located”. Note that this is highly contradictory. If man-made pollution is claimed to be one of the drivers of visibility trends, man-made pollution data measured at a site within or nearby the observatory need to be shown. Data shown in Figure 9 are by far not sufficient. Once the data is presented, a novel data analysis will have to be made to test the hypotheses presented here. The regional representativity of the Ebro visibility data will also have to be carefully discussed then.

Citation: https://doi.org/10.5194/acp-2021-1092-RC2

Status: closed (peer review stopped)

RC1: 'Comment on acp-2021-1092', Anonymous Referee #1, 01 Jul 2022

Review of Curto and Tacoronte, “Long-term visibility variation at the Ebro Observatory (1960-2020)".

This manuscript highlights human-made visibility observations and trends at a single site in Spain. The results suggest that visibility deteriorated significantly during the later periods of observations. The authors also study the role of relative humidity and wind speed and direction on visibility. The manuscript describes a long data history that would be important to have in the literature. Unfortunately, necessary details that would allow for interpretation of these data are weak in this version of the paper. I recommend the authors provide additional exploration of the measurements themselves to assure readers of potential biases in these measurements, especially with regard to existing literature regarding human-made observations of visibility (e.g., Li et al., 2016, see below).

Some of the many questions and details that need to be addressed include:

Expanding the introduction to include any additional measurements in the surrounding region of visibility or aerosol composition or behavior may help interpret these results. The authors refer to studies in Taiwan, China, the US, etc., but these do not help interpret the observations reported here.

Include elevation of study location.

Additional details need to be included regarding visibility measurements and potential biases over time due to any potential changes in methodologies over the time period assessed. Li et al. (2016) performed a global trends in visibility based on human observations and details some of the related issues (ACP, 16, 2435-2457, 2016). It is difficult to fully interpret these trends without some discussion as to whether additional analysis revealed any potential biases. Figure 2 shows the shift in visibility, especially in 1980, and again around 2015 that should be investigated in more depth to determine if it is real. Some discussion of uncertainty in these measurements in needed.

Were periods of high relative humidity flagged as corresponding to fog, precipitation, or low cloud?

It is not clear why the WMO classes were used when the data could easily be used instead?

What time periods are used to generate Figure 3?

Figure 4 might be more compelling without the use of the classes and present the data themselves, and similar results are achieved as Figure 1, especially since Figure 4 is presented as “% of observations” and not “visibility” as noted in the caption or on line 180 of page 6.

What years correspond to Figure 5?

Define “relative frequency” for observations of wind directions in Figure 6.

Define “normalized visibility” as presented in Figure 7.

Equation (1) is not necessary to include.

An increase in RH should lead to a decrease in visibility for hygroscopic particles, because those particles grow into larger sizes that are more efficient as scattering light. What time of day do the correlation coefficients correspond to, and was this limited to the same time of day for RH? Was the correlation analysis applied to daily data? Effects of RH will be most apparent on visibility if the particles causing visibility degradation are hygroscopic. Some discussion of potential aerosol sources would be helpful.

Citation: https://doi.org/10.5194/acp-2021-1092-RC1
RC2: 'Comment on acp-2021-1092', Anonymous Referee #2, 01 Jul 2022

It is nice to see a long time series of visibility data analyzed. In general, it is an interesting topic of interest. However, the manuscript lacks in various aspects listed below. his reviewer is not at all sure if can be accepted for publication in ACP even after major revision. the last comment below is the most important one.

Lines 33 – 40: Don’t forget to mention that the visibility can also be measured with a whole suite of instruments. Even if they are not applied at the Ebro Observatory, they need to mentioned. They come with big advantages (continuous measurement, no errors from human physiology).

Lines 42 – 45: This list is far from being complete or updated.

Line 57: Please provide more precise coordinates. You are offering a +/- 10 km resolution, you should rather provide 4 decimals of the full degrees of latitude and longitude.

Lines 68 – 69: “The last sixty years of these records have been digitized”. Why didn’t you digitize more data, dating further back?

Lines 109 – 111: “The criterion used consists in eliminating those values greater than 1.5 times the interquartile range below the first quartile and above the third for each hour of the day.” The criterion is not clear to this reviewer. Please elaborate.

Lines 133 – 134: Please elaborate how the correlation between visibility and wind direction (polar coordinates) was computed.

Fig. 2 - 9: Graphs should not have headlines.

lines 163 – 165: “The results shown in Figure 3 indicate that, in general, the maximum average visibility was found during the winter months and the minimum in July, August and September, except for the last period of time measured.”. No, the observation also does not apply to the period 1991 – 2005 either.

Lines 167 – 169: No good English, unclear. Please fix the wording.

Figure 4: The type of presentation is confusing. It would be much better to present stacked graphs (similar to those used in Fig. 5)

Line 197: What is the “upper part of the region” and the “lower part” (line 199)? Are authors talking about topographic altitudes?

Fig. 5. There is something wrong with the wind directions. What means “C” in the x-axes? There is no data for direction N. Direction NNW and NNE seem not to connect well to each other.

Line 206. Please say low wind speeds instead of soft wind speeds.

Figure 6: Thank you for the test. Please say “Relative frequency of” instead of “Visibility variation for”. Please take away the parentheses around the word “sector”. Again, what means “C” on the x-axis. Please take out these data points.

Figure 7: Same comment for the x-axis as for Fig. 6.

Data presented in Fig. 7 are not referred to in section 4.2 (except a general remark in line 219).

Lines 239 – 240: You mean “when its magnitude is greater than 0.1453”, correct?

Lines 243 – 246: This hypothesis seems to make sense. However, it is also highly speculative, because it is not at all based on any analysis or data presented so far. This section needs to be considerably weakened or moved further down within the manuscript.

Lines 251 – 257 and Table 2: It not clear, how the correlation war computed (see comment on lines 133, 134 above), and what exactly is shown in Table 2. This section needs extensive re-write.

Lines 267 - 268: This statement needs to be supported much better. There is an ongoing discussion about this topic in the literature, authors should not ignore that or simply comment it with a very short comment.

Line 272: “SSE-SSW” This reviewer does not agree. It should be said ESE – SSE instead, see Fig. 7. Also, WSW – N is more appropriate that W – NW. Note that the visibility data (Fig. 6) have another maximum than the wind direction data. Overall, the presented data analysis is not sufficient. Likely, it would be helpful to show visibility roses.

Lines 283 – 284: How do you arrive at this conclusion? Please refer to the data and respective discussion in the manuscript.

Lines 285 – 294: In lines 79 – 81, authors mentioned that the Ebro observatory “has never … experienced significant changes in its surroundings from 1910 onward”. Interestingly, this is supported by a reference dated 1987. Here, authors claim “that from the year 2000 onward there has been a large increase in the population of the town of Roquetes where the Ebro Observatory is located”. Note that this is highly contradictory. If man-made pollution is claimed to be one of the drivers of visibility trends, man-made pollution data measured at a site within or nearby the observatory need to be shown. Data shown in Figure 9 are by far not sufficient. Once the data is presented, a novel data analysis will have to be made to test the hypotheses presented here. The regional representativity of the Ebro visibility data will also have to be carefully discussed then.

Citation: https://doi.org/10.5194/acp-2021-1092-RC2

Juan José Curto and Nicolás Tacoronte

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Short summary

Long term variation of surface horizontal visibility is a clear indicator of atmospheric quality and it does not depend on any measurement device susceptible of lack of calibration. In this paper we present visibility series measured at Ebro Observatory (Spain), in the western part of Mediterranean area. Although these observations being performed in a priori “clean” location, no natural causes can explain the visibility trends and the anthropogenic activity is the most probable cause.


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