the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Sep 2023
An overview of organic aerosols at an urban site in Hong Kong: insights from in-situ measurement of molecular markers
Hongyong Li
Xiaopu Lyu
Likun Xue
Yunxi Huo
Dawen Yao
Haoxian Lu
Abstract. Organic aerosol (OA) is a significant constituent of urban particulate matter (PM), and molecular markers therein provide information on sources and formation mechanisms of OA. With in-situ measurement of over 70 OA molecular markers at a bihourly resolution, this study focused on the temporal variations of representative markers and dynamic source contributions to OA at an urban site in Hong Kong. The levels of secondary OA (SOA) markers were markedly elevated in continental and coastal air, and the primary markers were more of local characteristics. The diurnal patterns of 2-methyltetrols differed between scenarios, and their aqueous formation at night seemed plausible, particularly in the presence of troughs. Seven unambiguous sources were identified for the organic matters in submicron PM (PM1-OM). Despite an urban site, the SOA contribution (49 ± 8 %), primarily anthropogenic, was significant. Anthropogenic SOA dominated in continental and coastal air and in early afternoon. Local cooking and vehicle emissions became predominant in the scenario of marine air without troughs. Even averaged over the study period, cooking emissions contributed up to 40 % to PM1-OM in the early evening. The study highlighted the need to control regional anthropogenic SOA and local cooking emissions to mitigate PM pollution in Hong Kong.
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Hongyong Li et al.
Status: open (until 20 Dec 2023)
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RC1: 'Comment on egusphere-2023-1835', Anonymous Referee #1, 29 Nov 2023
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This study characterized the diel dynamics of various organic markers in aerosols using a TAG system at urban Hong Kong during a summer period. The high time resolution observations of organic markers allowed to identify specific sources or processes that had crucial contributions to organic aerosols. This effort promoted our understandings of the sources of organic aerosols in urban HK, and the manuscript was well written. However, there have been some studies using TAG to apportion the sources of organic aerosols at urban/suburban areas. This study just looks like a supplement of TAG data in urban HK. Therefore, I recommend a major revision on the structure of writing/descriptions to point out some interesting findings. Detailed comments are as follows:
- The title looks inappropriate. First, the study was only conducted in a summer period less than one month. I feel the results could not represent “An overview of organic aerosols” in urban HK. Second, as stated by the authors, the concentrations of organic markers were not quantified, and some important markers such as monoterpene derived SOA tracer were not identified. In addition, nitroaromatics, which are key brown carbon species, were not detected. Nitroaromatics may also help evaluate the contributions of oxidation products from biomass burning and vehicle emissions to organic aerosols. Third, the title did not carry impressive information.
- The manuscript is mainly discussing the influence of different air masses (continental, coastal, and marine) on organic markers and OM sources. However, looking at Figure 3, there was only one day when air mass was from continental regions; only two days when air mass was from coastal regions and there was a one-day long maintenance of instruments when air mass was from coastal regions. One-day long observation is not representative. Therefore, grouping the data by continental, coastal, and marine air masses is not appropriate. For example, it is odd that cooking emissions had an insignificant contribution to OM when air mass was from continental region if the sampling site is surrounded by restaurants. This may be due to cooking plums did not significantly affect the sampling on just that day. It is feasible to regard the periods when continental and coastal winds were prevailing as cases to discuss the influences on organic markers but cannot regards the results as common situations when continental and coastal air mass dominated.
- For the PMF analysis, it is not a good way to keep a factor which is unexplainable. The authors are encouraged to improve the PMF analysis by modifying inputs or others. As highlighted by the authors, cooking emissions should be a major source of OM at the sampling site, while the contribution from cooking oxidation products to OM could not be evaluated. A previous study (Huang et al., Comparative Assessment of Cooking Emission Contributions to Urban Organic Aerosol Using Online Molecular Tracers and Aerosol Mass Spectrometry Measurements, Environ. Sci. Technol. 2021, 55, 14526-14535) used azelaic acid, nonanoic acid, and 9-oxononanoic acid to indicate the oxidation of cooking emissions. I noticed azelaic acid has been detected in this study (No. 20 in Figure 2). The authors may try to conduct an analysis to evaluate how significant of cooking oxidation is in contributing to OM mass.
- It was interesting to identify the periods lasting a few days when trough played a role. The meteorological parameters such as RH, Temperature, solar radiation, wind speed showed significant differences between “trough” days and “non-trough” days. I think it is valuable to focus on discussing the variations of organic tracers, especially SOA tracers, during the two distinct periods. It is good to see that in the manuscript the authors have pointed out the increases of phthalic acid and DCAs during “trough” days, indicating an aqueous formation of the species. The authors are encouraged to find more markers that had significant difference between the periods. Hopefully, the authors can evaluate how significant of aqueous formation is in contributing to OM mass. This would be a very interesting part.
- Some inconsistencies may exist. For example, in Figure 7a, cooking emissions (dark yellow) constituted a significant part of OM sources during June 15th to 17th (continental and coastal air masses dominated according to Figure 3), while in Figure 7b and text, cooking emission was a very minor source of OM during the period. Another example is the X axis of Figure S8 shows a OM range of 0-22 μg/m3 while the Y axis of Figure 7a only reached 15 μg/m3. If X axis of Figure S8 shows the observation result, then there should be an unresolved percentage in Figure S8.
- line 185-188: the explanation may need to be modified. Just like I mentioned above, one-day observation for continental air mass dominated period may not capture the influence from cooking emission. During fall-winter period, most air masses may come from the north, and I guess you may still find the contribution from cooking emissions if a long period of observation is available.
- line 227: why not examine the correlation between pyrene and hopanes? NOx can also be emitted from biomass burning.
- line 290: add “respectively” after PM1-OM.
- line 318: please try to add azelaic acid in PMF analysis to see if cooking oxidation factor can be resolved.
- Figure 1. How about show the locations of restaurants in the map?
- Figure 3. Please add the time series of DCAs.
Citation: https://doi.org/10.5194/egusphere-2023-1835-RC1
Hongyong Li et al.
Hongyong Li et al.
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