Understanding Spatial Variability of Air Quality in Sydney: Part 1—A Suburban Balcony Case Study

Round 1

Reviewer 1 Report

I have no specific comments. The paper is well written and could be published as received.

Author Response

Thank you

Reviewer 2 Report

This paper analyses the concentration trends on a balcony site in Sydney and compare it with the measurement of surrounding monitoring stations. Although the level of novelty of this paper is not high, the document brings useful information to the definition of spatial variability of pollutant concentrations. The manuscript is precise and well-structured. I recommend some minor adjustment:

·         Figure 3. I think it could be useful to add some information about the distribution of wind direction, e.g. providing wind roses divided by different periods of the day.

·         I think that some more consideration on secondary particulate should be added in the dissertation and discussion of the results (e.g. at this scale of analysis, could secondary particulate formation explain some differences in the measurements?)

·         Page 1, Line 35, please define NSW at it first appearance in the text;

·         Page 4, Line 5, please define NEPM;

·         Page 10, substitute “Figure 5” with “Figure 6”

Author Response

-Figure 3. I think it could be useful to add some information about the distribution of wind direction, e.g. providing wind roses divided by different periods of the day.

Temporal variations in wind direction were also examined. Measurements were binned into four periods (00:00-06:00, 06:00-12:00, 12:00-18:00, 18:00-00:00) for each site and plotted as wind roses (Appendix, Figure A 1). Winds in the until 12:00 at all sites were dominated by SW winds at all sites, with a northerly flow evident at Prospect. Winds appeared more variable in the afternoon and evening with a greater spread of measurement evident. Variability observed between sites is unsurprising given the sensitivity of local surface winds to the environment immediately surrounding the measurement site.

Seasonality is observed in wind direction measurements at all sites. Sites specific wind roses binned by season are presented in the Appendix, Figure A 2. Spring and summer winds are the most variable at all sites, with noted northerly airflows observed at Chullora, and easterly flows at Liverpool during summer. Southerly and westerly flows dominate winter winds at Auburn, Chullora and Liverpool, with a distinctive NW flow evident during winter at Prospect.

The above text has been added at Page 7, Line 5. Time-of-day and seasonal site-specific wind roses have been added as Appendices A, Figures A 1 and A 2 respectively.

-I think that some more consideration on secondary particulate should be added in the dissertation and discussion of the results (e.g. at this scale of analysis, could secondary particulate formation explain some differences in the measurements?)

Secondary particulate formation is driven by regional scale processes within the Sydney basin, since the precursors are dominated by biogenic sources from the surrounding forested regions. For this reason, photochemically driven particle formation processes are not expected to contribute significantly to differences between PM_2.5 concentrations at the different sites.

 We have added the two sentences above Page 11, Line 38. 

-Page 1, Line 35, please define NSW at it first appearance in the text;

Edited as suggested.

-Page 4, Line 5, please define NEPM;

The NEPM is defined earlier in the manuscript: Page 3, Line 23.

-Page 10, substitute “Figure 5” with “Figure 6”

Edited as suggested.

Reviewer 3 Report

General comments:

In the submitted manuscript, results from an air quality 18-month measurement campaign at a suburban “balcony” site are presented and compared with data from three regulatory air quality monitoring stations. Air quality is analyzed in terms of CO, NOx, O3 and PM2.5.  

- Why is PM10 not included in the analysis?

- Authors should justify the selection of the Auburn “balcony” site.

- Measurements at the Auburn site are available from 26 May 2016 until 18 September 2017 (section 2.2, l.44). This correspond to 16-month of data. Please correct throughout the manuscript, or clarify.

- Exceedances of the national air quality standards are presented in Table 1 and discussed in Section 4 (page 13, l.6-14). Exceedances are calculated only for ozone (one hour and four hour limit values) and PM2.5 (daily limit value only). Why is the annual limit value for PM2.5 not included? Why are CO and NO2 limit values not included? Even if there are not exceedances to some of these pollutants (e.g., for CO), this should be acknowledged. Moreover, in the discussion, exceedances for different pollutants / different averaging periods are summed all together. In the reviwer's opinion, this is not correct. The exceedances analysis must be clarified (or removed).  

Minor comments:

- Abstract: PM2.5 appears as 'PM2.5' at l.20, and as 'particulate matter less then 2.5 µm diameter' in l.25. Authors should consider reordering those terms.

- Introduction: Air quality in Sidney is introduced as good when compared to other cities, and O3 (mean concentrations) and PM2.5 concentration values are presented. However, O3 mean concentrations are usually not a problem in urban areas (e.g., close to pollutant sources), due to the O3 degradation by NO. Authors should add a comment on ozone episodes in the study region. In addition, the reviewer suggests to include here a reference on the NOx concentrations, if available.

- Introduction (l.35): Please define NSW (New South Wales).

- Section 2.2: Which instrument did you use to measure the PM2.5 concentrations you present in this study? Did you compare results from TEOM and nephelometer?

- Section 2.3: Title must be formatted.

- Section 2.3: Are the instruments used in the three regulatory air quality monitoring stations the same as the ones from the MAQ station?

- Section 3: Authors must format time values consistently (see for example differences in Section 3.1, l.31, Section 3.2.2, l.30 and Section 3.2.3, l.6).

- Section 3: There are two figures numbered as Figure 5.

- Section 3: Ozone concentrations are analysed in Section 3.2.3 (traffic as a major source of CO and NOx). Authors should move this analysis to another section / rethink Section 3 subsections.

- Section 3: What is the % of valid data in the measurement campaign (page 10, l.14-15)? Authors should consider including this information in the Methodology.

- Section 3: Calibration of the ozone instrument occurred at 14:00 or 15:00 each day (page 10, l.16). The reviewer suggests to highlight calibration period in Fig. 4E (e.g., by including text in the figure, instead of the continuous line).

- Section 3: Legend of Figure 7 mentions 95% confidence intervals in the mean. However, these are not presented in the figure. Please correct.

- Section 4: The authors say that the fact that Prospect station recorded only one exceedance is likely the result of less localised pollution sources. However, since ozone is included in this analysis, and due to the regional nature of ozone pollution and due to the ozone titration by NO, the review does not agree with this reasoning.  

- Section 5: The reviewer suggests to include “outdoor” in the last sentence: “... indication of personal exposure to outdoor air quality pollutants at a simulated balcony site”.

- Reference number 24 should be corrected (typo in the manuscript title).

- Reference number 40 should be completed (pages and doi are missing).

Author Response

- Why is PM10 not included in the analysis?

We focus on PM2.5 and O3 because they are the pollutants of most concern in Sydney. CO and NOx are included because they are good indicators of the pollution sources that are either also direct emitters of PM2.5 or precursors of O3. For this reason, PM10 was initially excluded from analysis. Diurnal and seasonal cycles of PM10 have been included in the Appendix, and the follow text added at Page 12, Line 4:

Diurnal and seasonal cycles of PM₁₀ were also plotted. The diurnal cycle, provided in the Appendix, Figure A 3B), is very similar to that of PM_2.5­ at all sites. The seasonal cycle (Appendix, Figure A 3A) varies significantly, showing summer maxima at all sites and no winter peak. Similar to PM_2.5, PM₁₀ concentrations at Auburn are lower than those at all permanent sites as indicated by mean bias (Chullora: -2.26 µgm^-3, Liverpool: -3.24 µgm^-3, Prospect: -1.53 µgm^-3).

- Authors should justify the selection of the Auburn “balcony” site.

The below text has been added at Page 3, Line 44:

This site was chosen purely for pragmatic reasons, as we had connections to allow us access to the roof (and gained permission to locate the retro-reflectors for the open-path measurements on the council building 400m away across the town centre). However, Auburn is a good representative suburban centre with a good mixture of land uses including residential, industrial and transport.

- Measurements at the Auburn site are available from 26 May 2016 until 18 September 2017 (section 2.2, l.44). This correspond to 16-month of data. Please correct throughout the manuscript, or clarify.

Edited to 16-month campaign throughout manuscript.

- Exceedances of the national air quality standards are presented in Table 1 and discussed in Section 4 (page 13, l.6-14). Exceedances are calculated only for ozone (one hour and four hour limit values) and PM2.5 (daily limit value only). Why is the annual limit value for PM2.5 not included? Why are CO and NO2 limit values not included? Even if there are not exceedances to some of these pollutants (e.g., for CO), this should be acknowledged. Moreover, in the discussion, exceedances for different pollutants / different averaging periods are summed all together. In the reviwer's opinion, this is not correct. The exceedances analysis must be clarified (or removed).  

Upon review, the authors believe the exceedance analysis does not contribute significantly to the main conclusions of the manuscript. Considering this, it has been removed.

Minor comments:

- Abstract: PM2.5 appears as 'PM2.5' at l.20, and as 'particulate matter less then 2.5 µm diameter' in l.25. Authors should consider reordering those terms.

Edited as suggested.

- Introduction: Air quality in Sidney is introduced as good when compared to other cities, and O3 (mean concentrations) and PM2.5 concentration values are presented. However, O3 mean concentrations are usually not a problem in urban areas (e.g., close to pollutant sources), due to the O3 degradation by NO. Authors should add a comment on ozone episodes in the study region. In addition, the reviewer suggests to include here a reference on the NOx concentrations, if available.

The below text has been added at Page 1, Line 44:

Ozone exceedances in Sydney are associated with very high summer temperatures, with influence from both synoptic [8] and meso-scale [9] meteorological variables. The Sydney region predominantly experiences a NOx-limited regime during ozone events, with the influence of biogenic emissions highlighted in recent literature [10].

8.             Hart, M.; de Dear, R.; Hyde, R. A synoptic climatology of tropospheric ozone episodes in Sydney, Australia. International Journal of Climatology 2006, 26, 1635-1649, doi:10.1002/joc.1332.

9.             Jiang, N.; Scorgie, Y.; Hart, M.; Riley, M.L.; Crawford, J.; Beggs, P.J.; Edwards, G.C.; Chang, L.; Salter, D.; Virgilio, G.D. Visualising the relationships between synoptic circulation type and air quality in Sydney, a subtropical coastal-basin environment. International Journal of Climatology 2017, 37, 1211-1228, doi:doi:10.1002/joc.4770.

10.          Utembe, S.R.; Rayner, P.J.; Silver, J.D.; Guérette, E.A.; Fisher, J.A.; Emmerson, K.M.; Cope, M.; Paton-Walsh, C.; Griffiths, A.D.; Duc, H., et al. Hot summers: Effect of extreme temperatures on ozone in Sydney, Australia. Atmosphere 2018, 9, doi:10.3390/atmos9120466.

- Introduction (l.35): Please define NSW (New South Wales).

Edited as suggested.

- Section 2.2: Which instrument did you use to measure the PM2.5 concentrations you present in this study? Did you compare results from TEOM and nephelometer?

The TEOM instrument referenced on Page 3, Line 28 was used for reported measurements. This has been clarified at Page 3, Line 28. The nephelometer measures the scattering coefficient of light due to particles in the air and is reported as inverse megametres (Mm^-1). The OEH do not convert this to an estimate of PM2.5 but use it directly in their estimate of overall air quality (as a visibility metric) for each day. Therefore the measurements were not compared between instruments.

- Section 2.3: Title must be formatted.

Edited as suggested.

- Section 2.3: Are the instruments used in the three regulatory air quality monitoring stations the same as the ones from the MAQ station?

No, they are different. The below has been added at Page 4, Line 10 to clarify.

Instrumentation at permanent sites is different to that used in the MAQ station, with specifications for each site publicly available [40].

40.          Office of Environment and Heritage. Air quality monitoring network. Availabe online: https://www.environment.nsw.gov.au/topics/air/monitoring-air-quality (accessed on March 15).

- Section 3: Authors must format time values consistently (see for example differences in Section 3.1, l.31, Section 3.2.2, l.30 and Section 3.2.3, l.6).

Edited as suggested.

- Section 3: There are two figures numbered as Figure 5.

Edited as suggested.

- Section 3: Ozone concentrations are analysed in Section 3.2.3 (traffic as a major source of CO and NOx). Authors should move this analysis to another section / rethink Section 3 subsections.

Analysis of ozone measurements occur in Section 3.2.4: Ozone. There is no mention of ozone in section 3.2.3. Please clarify if this needs further attention.

- Section 3: What is the % of valid data in the measurement campaign (page 10, l.14-15)? Authors should consider including this information in the Methodology.

The following has been added at Page 5, Line 24:

The percentage of valid data collected over the measurement period was as follows for analysed variables: temperature and wind speed: 100%, CO: 94%, NOx: 78%, O3: 60.1%, PM2.5 and PM10: 99%.

- Section 3: Calibration of the ozone instrument occurred at 14:00 or 15:00 each day (page 10, l.16). The reviewer suggests to highlight calibration period in Fig. 4E (e.g., by including text in the figure, instead of the continuous line).

The figure has been edited as suggested to show a measurement gap.

- Section 3: Legend of Figure 7 mentions 95% confidence intervals in the mean. However, these are not presented in the figure. Please correct.

Shaded areas present in manuscript downloaded from Atmospheres: though we have had occasional display issues. If there is need for provision of the raw figures as PDFs, this can be arranged.

- Section 4: The authors say that the fact that Prospect station recorded only one exceedance is likely the result of less localised pollution sources. However, since ozone is included in this analysis, and due to the regional nature of ozone pollution and due to the ozone titration by NO, the review does not agree with this reasoning.  

Exceedance analysis has been removed.

- Section 5: The reviewer suggests to include “outdoor” in the last sentence: “... indication of personal exposure to outdoor air quality pollutants at a simulated balcony site”.

Edited as suggested.

- Reference number 24 should be corrected (typo in the manuscript title).

Edited as suggested.

- Reference number 40 should be completed (pages and doi are missing).

Edited as suggested.